Climate Change and...
- Climate Variability
- Climate Models
Effects of Climate Change
Semi-arid and Arid Deserts
ABSTRACT: We studied the spatial patterns and temporal dynamics of vegetation structural responses to precipitation variation in grassland, transitional, and desertified-shrubland ecosystems in an 800 km2 region of Northern Chihuahua, USA. Airborne high-fidelity imaging spectroscopy data collected from 1997 to 2001 provided spatially detailed measurements of photosynthetic and senescent canopy cover and bare soil extent. The observations were made following wintertime and summer monsoonal rains, which varied in magnitude by >300% over the study period, allowing an assessment of ecosystem responses to climate variation in the context of desertification.
Desertification caused a persistent increase in both photosynthetic vegetation (PV) and bare soil cover, and a lasting decrease in nonphotosynthetic vegetation (NPV). We did not observe a change in the spatial variability of PV cover, but its temporal variation decreased substantially. In contrast, desertification caused the spatial variability of NPV to increase markedly, while its temporal variation did not change. Both the spatial and temporal variation of exposed bare surfaces decreased with desertification. Desertification appeared to be linked to a shift in seasonal precipitation use by vegetation from mainly summer to winter inputs, resulting in an apparent decoupling of vegetation responses to inter-annual monsoonal variation. Higher winter rainfall led to decreased springtime spatial variability in the PV cover of desertified areas. Higher summer rainfall resulted in decreased PV cover variation in grassland, transition and desertified-shrubland regions. The effects of desertification on NPV dynamics were more than three times greater than on PV or bare soil dynamics. Using remotely sensed PV and NPV as proxies for net primary production (NPP) and litter dynamics, respectively, we estimated that desertification decreases the temporal variability of NPP and increases spatial variation of litter production and loss. Quantitative studies of surface biological materials and ecosystem processes can now be measured with high 'structural' detail using imaging spectroscopy and shortwave-infrared spectral mixture analysis.
Austin, A.T., Yahdjian, L., Stark, J. M., Belnap, J., Porporato, A., Norton, U., Ravetta, D. A., Schaeffer, S. M. (2004). Water pulses and biogeochemical cycles in arid and semiarid ecosystems. Oecologia 141 (2): 221-235
ABSTRACT: The episodic nature of water availability in arid and semiarid ecosystems has significant consequences on belowground carbon and nutrient cycling. Pulsed water events directly control belowground processes through soil wet-dry cycles. Rapid soil microbial response to incident moisture availability often results in almost instantaneous C and N mineralization, followed by shifts in C/N of microbially available substrate, and an offset in the balance between nutrient immobilization and mineralization. Nitrogen inputs from biological soil crusts are also highly sensitive to pulsed rain events, and nitrogen losses, particularly gaseous losses due to denitrification and nitrate leaching, are tightly linked to pulses of water availability. The magnitude of the effect of water pulses on carbon and nutrient pools, however, depends on the distribution of resource availability and soil organisms, both of which are strongly affected by the spatial and temporal heterogeneity of vegetation cover, topographic position and soil texture. The inverse texture hypothesis for net primary production in water-limited ecosystems suggests that coarse-textured soils have higher NPP than fine-textured soils in very arid zones due to reduced evaporative losses, while NPP is greater in fine-textured soils in higher rainfall ecosystems due to increased water-holding capacity. With respect to belowground processes, fine-textured soils tend to have higher water-holding capacity and labile C and N pools than coarse-textured soils, and often show a much greater flush of N mineralization. The result of the interaction of texture and pulsed rainfall events suggests a corollary hypothesis for nutrient turnover in arid and semiarid ecosystems with a linear increase of N mineralization in coarse-textured soils, but a saturating response for fine-textured soils due to the importance of soil C and N pools. Seasonal distribution of water pulses can lead to the accumulation of mineral N in the dry season, decoupling resource supply and microbial and plant demand, and resulting in increased losses via other pathways and reduction in overall soil nutrient pools. The asynchrony of resource availability, particularly nitrogen versus water due to pulsed water events, may be central to understanding the consequences for ecosystem nutrient retention and long-term effects on carbon and nutrient pools. Finally, global change effects due to changes in the nature and size of pulsed water events and increased asynchrony of water availability and growing season will likely have impacts on biogeochemical cycling in water-limited ecosystems.
ABSTRACT: Little is known about the contribution of arid and semiarid regions to the carbon balance at a global scale. The lack of information is especially noticeable for the Gran Chaco, which covers an area of about 1,200,000 km2 in South America. This study quantified carbon pools and their changes along a land-use gradient in the Dry Chaco, the driest portion of the Gran Chaco, measured in the aboveground biomass and in soils (20 cm depth). The work was conducted in the Chancani reserve, where the best preserved forests of the region are found, and in surrounding areas, including a primary forest, a secondary forest and shrubby grasslands. Previous works indicate that the entire area was originally covered by forests similar to those found at the Chancani reserve, and that the land-use changes occurred at least 30 years prior to this study. Total aboveground carbon stock, which comprises the total amount of living organic matter in trees and shrubs, was 30.31 Mg C ha-1 in the primary forest, which was reduced to 8.38 Mg C ha-1 in the secondary forest and to 1.37 Mg C ha-1 in shrubby grasslands. Carbon stock in the tree component decreased drastically between the primary and the secondary forests from 25.40 to 5.11 Mg C ha-1 . The component described as saplings of trees and shrubs also decreased significantly among the three communities from 4.91 Mg C ha-1 in the primary forest to 3.27 Mg C ha-1 in the secondary forest and to only 1.37 Mg C ha-1 in the shrubby grasslands. No significant differences were detected in the carbon content per unit area of soil, although it decreased from 34.59 Mg C ha-1 in the primary forest to 28.04 Mg C ha-1 in the secondary forest and to 22.93 Mg C ha-1 in the shrubby grassland, with a significant increase in soil bulk density in the disturbed communities. Therefore, differences in carbon stocks between communities were primarily the result of differences in vegetation biomass, whereas changes in the land-use gradient analyzed had a lower impact on soils. Nevertheless, soil constitutes the largest pool, and more severe ecological disturbances could lead to important changes in net carbon storage.
Q. Feng, W. Liu, Z. Yanwu, S. Jianhua, S. Yonghong, C. Z. Qiang, X. Haiyang (2006). Effect of climatic changes and human activity on soil carbon in desertified regions of China. Tellus: Series B 58 (2): 117-128
ABSTRACT: A total of 340 soil samples from 17 different types of sandy land regions in China were analyzed for soil organic carbon (SOC). Stepwise regression served to identify correlations between SOC and a number of climatic factors measured at the sampling sites: winter, summer, nighttime and daytime air temperatures (AT) and precipitation. In desertified lands SOC showed a direct correlation with precipitation, but no significant relationship with the mean air temperature. However, in northwest and northeast China decreases in SOC were linked to rises in day/night and seasonal temperatures, while in north China they were linked to a rise in mean temperature. Over the entire study period (1950–1990), precipitation in the region decreased by 25 mm decade−1 , while the mean air temperature increased by 0.14°, 0.28° and 0.15° decade−1 in the northwest, northeast and north central regions, respectively. Driven by rises in day/night and seasonal temperatures, long-term alterations to global ecosystem processes, particularly the carbon cycle, may significantly alter the number and diversity of plants in desertified ecosystems. Based on multiple regression analysis, over the past 40 years, about 20% of emissions from SOC in the northeast were attributable to air temperature; in the north central region 31 and 16% of emissions were attributable to precipitation and mean AT, respectively; while in the northwest 28, 18 and 32% of the emissions were attributable to mean AT, nighttime temperature, and decreasing precipitation, respectively. It is obvious that human activities are the main single causative factor in the release of SOC-derived greenhouse gases to the atmosphere from desertified areas.
Housman, D.C., Powers, H.H., Collins, A.D., Belnap, J. (2006). Carbon and nitrogen fixation differ between successional stages of biological soil crusts in the Colorado Plateau and Chihuahuan Desert. Journal of Arid Environments 66 (4): 620-634
ABSTRACT: Biological soil crusts (cyanobacteria, mosses and lichens collectively) perform essential ecosystem services, including carbon (C) and nitrogen (N) fixation. Climate and land-use change are converting later successional soil crusts to early successional soil crusts with lower C and N fixation rates. To quantify the effect of such conversions on C and N dynamics in desert ecosystems we seasonally measured diurnal fixation rates in different biological soil crusts. We classified plots on the Colorado Plateau (Canyonlands) and Chihuahuan Desert (Jornada) as early (Microcoleus ) or later successional (Nostoc /Scytonema orPlacidium /Collema ) and measured photosynthesis (Pn ), nitrogenase activity (NA), and chlorophyll fluorescence (Fv /Fm ) on metabolically active (moist) soil crusts. Later successional crusts typically had greater Pn, averaging 1.2–1.3-fold higher daily C fixation in Canyonlands and 2.4–2.8-fold higher in the Jornada. Later successional crusts also had greater NA, averaging 1.3–7.5-fold higher daily N fixation in Canyonlands and 1.3–25.0-fold higher in the Jornada. Mean daily Fv /Fm was also greater in later successional Canyonlands crusts during winter, and Jornada crusts during all seasons except summer. Together these findings indicate conversion of soil crusts back to early successional stages results in large reductions of C and N inputs into these ecosystems.
Ivans, S., Hipps, L., Leffler, A. J., Ivans, C. Y. (2006). Response of water vapor and CO2 fluxes in semiarid lands to seasonal and intermittent precipitation pulses. Journal of Hydrometeorology 7 (5): 995-1010
ABSTRACT: Precipitation pulses are important in controlling ecological processes in semiarid ecosystems. The effects of seasonal and intermittent precipitation events on net water vapor and CO2 fluxes were determined for crested wheatgrass (Agropyron desertorum ), juniper (Juniperus osteosperma ), and sagebrush (Artemisia tridentata ) ecosystems using eddy covariance measurements. The measurements were made at Rush Valley, Utah, in the northern Great Basin of the United States. Data were evaluated during the growing seasons of 2002 and 2003. Each of these communities responds to precipitation pulses in all seasons, but these responses vary among season and ecosystem, and differ for water vapor and CO2 . The degree and direction of response (i.e., net uptake or efflux) depended upon the timing and amount of precipitation. In early spring, both evapotranspiration (ET) and CO2 fluxes responded only slightly to precipitation pulses because soils were already moist from snowmelt and spring rains. As soils dried later in the spring, ET response to rainfall increased. The summer season was very warm and dry in both years, and both water and CO2 fluxes were generally reduced as compared to fluxes in the spring. Water vapor fluxes increased during and immediately after periodic summer rain events at all sites, especially at juniper, followed by the sagebrush and crested wheatgrass sites. Net CO2 exchange changed significantly at the juniper and sagebrush sites but changed very little at the crested wheatgrass site due to senescence of this grass. However, in the wetter summer of 2003, the grass species maintained physiological activity and responded to rain events. In the fall of both years, responses of ET and CO2 fluxes to precipitation were very similar for all three communities, with only small changes, presumably due to significantly lower temperatures in the fall. This research documents the importance of the temporal distribution of rainfall on patterns of ET and CO2 fluxes and suggests that soil moisture and stand-level leaf area index (LAI) are critical factors governing ET and CO2 responses to precipitation in these communities.
Jasoni, R. L., Smith, S. D., Arnone, J. A., III (2005). Net ecosystem CO2 exchange in Mojave Desert shrublands during the eighth year of exposure to elevated CO2 . Global Change Biology 11 (5): 749-756
ABSTRACT: Arid ecosystems, which occupy about 35% of the Earth's terrestrial surface area, are believed to be among the most responsive to elevated [CO2 ]. Net ecosystem CO2 exchange (NEE) was measured in the eighth year of CO2 enrichment at the Nevada Desert Free-Air CO2 Enrichment (FACE) Facility between the months of December 2003–December 2004. On most dates mean daily NEE (24 h) (μmol CO2 m−2 s−1 ) of ecosystems exposed to elevated atmospheric CO2 were similar to those maintained at current ambient CO2 levels. However, on sampling dates following rains, mean daily NEEs of ecosystems exposed to elevated [CO2 ] averaged 23 to 56% lower than mean daily NEEs of ecosystems maintained at ambient [CO2 ]. Mean daily NEE varied seasonally across both CO2 treatments, increasing from about 0.1μmol CO2 m−2 s−1 in December to a maximum of 0.5–0.6μmol CO2 m−2 s−1 in early spring. Maximum NEE in ecosystems exposed to elevated CO2 occurred 1 month earlier than it did in ecosystems exposed to ambient CO2 , with declines in both treatments to lowest seasonal levels by early October (0.09±0.03μmol CO2 m−2 s−1 ), but then increasing to near peak levels in late October (0.36±0.08μmol CO2 m−2 s−1 ), November (0.28±0.03μmol CO2 m−2 s−1 ), and December (0.54±0.06μmol CO2 m−2 s−1 ). Seasonal patterns of mean daily NEE primarily resulted from larger seasonal fluctuations in rates of daytime net ecosystem CO2 uptake which were closely tied to plant community phenology and precipitation. Photosynthesis in the autotrophic crust community (lichens, mosses, and free-living cyanobacteria) following rains were probably responsible for the high NEEs observed in January, February, and late October 2004 when vascular plant photosynthesis was low. Both CO2 treatments were net CO2 sinks in 2004, but exposure to elevated CO2 reduced CO2 sink strength by 30% (positive net ecosystem productivity=127±17 g Cm−2 s−1 ambient CO2 and 90±11 g Cm−2 s−1 elevated CO2 , P=0.011). This level of net C uptake rivals or exceeds levels observed in some forested and grassland ecosystems. Thus, the decrease in C sequestration seen in our study under elevated CO2 – along with the extensive coverage of arid and semi-arid ecosystems globally – points to a significant drop in global C sequestration potential in the next several decades because of responses of heretofore overlooked dryland ecosystems.
ABSTRACT: Identifying soil microbial responses to anthropogenically driven environmental changes is critically important as concerns intensify over the potential degradation of ecosystem function. We assessed the effects of elevated atmospheric CO2 on microbial carbon (C) and nitrogen (N) cycling in Mojave Desert soils using extracellular enzyme activities (EEAs), community-level physiological profiles (CLPPs), and gross N transformation rates. Soils were collected from unvegetated interspaces between plants and under the dominant shrub (Larrea tridentata ) during the 2004–2005 growing season, an above-average rainfall year. Because most measured variables responded strongly to soil water availability, all significant effects of soil water content were used as covariates to remove potential confounding effects of water availability on microbial responses to experimental treatment effects of cover type, CO2 , and sampling date. Microbial C and N activities were lower in interspace soils compared with soils underLarrea , and responses to date and CO2 treatments were cover specific. Over the growing season, EEAs involved in cellulose (cellobiohydrolase) and orthophosphate (alkaline phosphatase) degradation decreased under ambient CO2 , but increased under elevated CO2 . Microbial C use and substrate use diversity in CLPPs decreased over time, and elevated CO2 positively affected both. Elevated CO2 also altered microbial C use patterns, suggesting changes in the quantity and/or quality of soil C inputs. In contrast, microbial biomass N was higher in interspace soils than soils underLarrea , and was lower in soils exposed to elevated CO2 . Gross rates of NH4 + transformations increased over the growing season, and late-season NH4 + fluxes were negatively affected by elevated CO2. Gross NO3 − fluxes decreased over time, with early season interspace soils positively affected by elevated CO2 . General increases in microbial activities under elevated CO2 are likely attributable to greater microbial biomass in interspace soils, and to increased microbial turnover rates and/or metabolic levels rather than pool size in soils underLarrea . Because soil water content and plant cover type dominates microbial C and N responses to CO2 , the ability of desert landscapes to mitigate or intensify the impacts of global change will ultimately depend on how changes in precipitation and increasing atmospheric CO2 shift the spatial distribution of Mojave Desert plant communities.
ABSTRACT: Problems of frequent drought stress, low soil organic carbon (SOC) concentration, low aggregation, susceptibility to compaction, salinization and accelerated soil erosion in dry regions are accentuated by removal of crop residues, mechanical methods of seedbed preparation, summer clean fallowing and overgrazing, and excessive irrigation. The attendant soil degradation and desertification lead to depletion of SOC, decline in biomass production, eutrophication/pollution of waters and emission of greenhouse gases. Adoption of conservation agriculture, based on the use of crop residue mulch and no till farming, can conserve water, reduce soil erosion, improve soil structure, enhance SOC concentration, and reduce the rate of enrichment of atmospheric CO2 . The rate of SOC sequestration with conversion to conservation agriculture, elimination of summer fallowing and growing forages/cover crops may be 100 to 200 kg ha-1 y-1 in coarse-textured soils of semiarid regions and 150 to 300 kg ha-1 y-1 in heavy-textured soils of the subhumid regions. The potential of soil C sequestration in central Asia is 10 to 22 Tg C y-1 (16±8 Tg C y-1 ) for about 50 years, and it represents 20 per cent of the CO2 emissions by fossil fuel combustion.
Leffler, A. J., Caldwell, M. M., Ryel, R. J., Hipps, L., Ivans, S. (2002). Carbon acquisition and water use in a northern Utah Juniperus osteosperma (Utah juniper ) population. Tree Physiology 22 (17): 1221-1230
ABSTRACT: Water use and carbon acquisition were examined in a northern Utah population ofJuniperus osteosperma (Torr.) Little. Leaf-level carbon assimilation, which was greatest in the spring and autumn, was limited by soil water availability. Gas exchange, plant water potential and tissue hydrogen stable isotopic ratio (D) data suggested that plants responded rapidly to summer rain events. Based on a leaf area index of 1.4, leaf-level water use and carbon acquisition scaled to canopy-level means of 0.59 mm day–1 and 0.13 mol m–2 ground surface day–1 , respectively. Patterns of soil water potential indicated thatJ. osteosperma dries the soil from the surface downward to a depth of about 1 m. Hydraulic redistribution is a significant process in soil water dynamics. Eddy covariance data indicated a mean evapotranspiration rate of 0.85 mm day–1 from March to October 2001, during which period the juniper population at the eddy flux site was a net source of CO2 (3.9 mol m–2 ground area). We discuss these results in relation to the rapid range expansion of juniper species during the past century.
ABSTRACT: Termites and soil calcium carbonate are major factors in the global carbon cycle: termites by their role in decomposition of organic matter and methane production, and soil calcium carbonate by its storage of atmospheric carbon dioxide. In arid and semiarid soils, these two factors potentially come together by means of biomineralization of calcium carbonate by termites. In this study, we evaluated this possibility by testing two hypotheses. Hypothesis 1 states that termites biomineralize calcium carbonate internally and use it as a cementing agent for building aboveground galleries. Hypothesis 2 states that termites transport calcium carbonate particles from subsoil horizons to aboveground termite galleries where the carbonate detritus becomes part of the gallery construction. These hypotheses were tested by using (1) field documentation that determined if carbonate-containing galleries only occurred on soils containing calcic horizons, (2)13 C/12 C ratios, (3) X-ray diffraction, (4) petrographic thin sections, (5) scanning electron microscopy, and (6) X-ray mapping. Four study sites were evaluated: a C4-grassland site with no calcic horizons in the underlying soil, a C4 -grassland site with calcic horizons, a C3 -shrubland site with no calcic horizons, and a C3 -shrubland site with calcic horizons. The results revealed that carbonate is not ubiquitously present in termite galleries. It only occurs in galleries if subsoil carbonate exists within a depth of 100 cm.13 C/12 C ratios of carbonate in termite galleries typically matched13 C/12 C ratios of subsoil carbonate. X-ray diffraction revealed that the carbonate mineralogy is calcite in all galleries, in all soils, and in the termites themselves. Thin sections, scanning electron microscopy, and X-ray mapping revealed that carbonate exists in the termite gut along with other soil particles and plant opal. Each test argued against the biomineralization hypothesis and for the upward-transport hypothesis. We conclude, therefore, that the gallery carbonate originated from upward transport and that this CaCO3 plays a less active role in short-term carbon sequestration than it would have otherwise played if it had been biomineralized directly by the termites.
SUMMARY: Semi-arid temperate steppes comprise approximately 30% of the world’s temperate grassland, and consequently, are a significant component of the global carbon cycle. To better understand how precipitation affects soil carbon fluxes in semi-arid steppes, we examined the effects of irrigation (simulated rainfall) on CO2 and CH4 fluxes from Mongolian semi-arid steppe soil on 10–12 August 2002 and 19–22 August 2003. Meteorological data revealed that the soil was dry in 2002 and wet in 2003. Summer flux measurements in both years showed that the soil emitted CO2 at 75–250 mg m−2 h−1 and consumed atmospheric CH4 at 30–90μg m−2 h−1 . In 2002, the CO2 flux of the irrigated soil showed an increase of 50% over one day following irrigation compared to the non-irrigated soil, and thereafter, no increase. This enhancing effect of irrigation was found only immediately following irrigation in 2003. Soil CH4 fluxes showed little difference between the irrigated and non-irrigated soils in 2002 and 2003. There was also little difference in soil temperatures (at the surface and 5 cm depth) between the soils in 2002 and 2003. The water content of the irrigated soil increased following irrigation then rapidly decreased with time. These results demonstrate that rainfall events enhance carbon loss from semi-arid steppe soil at least within the day following irrigation. However, long-term meteorological observations of precipitation and soil water content in 2003 and 2004 suggest that usual rainfall pulses throughout the growing season (June–September) do not markedly enhance CO2 emission from such soils.
ABSTRACT: The rates of CaCO3 formation in desert soils of the American Southwest are highly variable and potentially useful in estimating the age of soil profiles. The objective of this study was to examine the correlation between the long-term rate of CaCO3 formation (LTR) and modern mean annual precipitation (MAP) and temperature (MAT). LTR is not significantly related to temperature but is linearly related to MAP [LTR = 0.0150(MAP − 37), r2 = 0.924]. The slope of the regression is significantly different from 0.0 and is equivalent to a calcium concentration of 6.0 mg liter−1 in rainfall. However, the intercept term, which suggests a lower limit of precipitation for CaCO3 formation of 37 mm, is not significantly different from 0.0. The LTR-MAP relationship may be useful in estimating the age of soil profiles by the CaCO3 method in the desert Southwest.
ABSTRACT: Our objective was to develop and validate a regional model for CaCO3 deposition in desert soils of the southwestern United States. There were five major components in the simulation model: a stochastic precipitation model, an evapotranspiration model, chemical thermodynamic relationships, soil parameterization, and a soil water and CaCO3 flux model.
For the present climate, a cold-dry Pleistocene climate, and a cool-wet (summer) Pleistocene climate, the model predicted a shallower depth for the calcic horizon than was found in field soils. However, the model was compatible with field soils if one assumed that most pedogenic carbonate formed during a cool-wet (winter) Pleistocene climate. The model was highly sensitive to the frequency of extreme precipitation events and to soil water-holding capacity. The biotic factor played an important role in CaCO3 deposition through its control of soil CO2 concentrations and evapotranspiration rates. The range in predicted CaCO3 deposition rates agreed with the rates for most field studies (1 to 5 g/m2 /yr); also, the model predicted an increasing rate of CaCO3 deposition with increasing precipitation, which agreed with field studies. The model is a valuable research tool for evaluating the role of state factors on soil CaCO3 deposition.
McFadden, L. D., McDonald, E. V., Wells, S. G., Anderson, K., Quade, J., Forman, S. L. (1998). The vesicular layer and carbonate collars of desert soils and pavements: formation, age and relation to climate change. Geomorphology 24 (2): 101-145
ABSTRACT: The vesicular, fine-grained A horizon (Av) is the widespread, ubiquitous surficial horizon of desert soils in diverse landforms and parent materials of varying ages. Now known to form mostly through accumulation of eolian dust, recent studies show that dust accumulation and concomitant soil development are genetically linked to stone pavement formation. Changes in the magnitude of eolian activity and effective leaching related to Quaternary climatic changes are also hypothesized to have influenced the evolution of the Av horizon. Numerical modeling, geochronologic, and field/laboratory studies elucidate the nature of pedogenic processes controlling compositional evolution of Av, how the changing Av horizon increasingly influences soil infiltration and carbonate translocation and accumulation, and the control that clasts of the evolving pavement exert on pedogenic processes. Results of a model that determines soil bulk chemical composition based on mixing of estimated proportions of externally derived (eolian) material and parent materials imply that the evolution of the soil bulk composition is strongly influenced by Av horizon formation. The early development of a weakly to moderately developed Av horizon directly over gravelly parent material in late and middle Holocene soils moderately influences soil infiltration, but significant leaching of very soluble materials and some carbonate in dust are permitted. In older, Pleistocene soils, however, the texturally more mature Av and underlying, cumulic nongravelly horizons more strongly limit the rate and depth of leaching, and soil bulk composition therefore more closely approximates a simple mixture of dust and parent material. Other aspects of Av horizon development and its relations to the pavement are evaluated through studies of pavement clasts with coatings of soil carbonate, referred to as carbonate collars. Development of a numerical model that integrates soil hydrology, a CO2 production–diffusion model, calcite kinetics and thermodynamic considerations, composition and thermal characteristics of pavement clasts and the textural and structural properties of the surface horizon provides the basis for testing a hypothesis of collar formation. Model results, combined with results ofd13 C andd18 O analyses of collar carbonate, demonstrate how precipitation of calcite on pavement clasts and within the Av is favored at a depth much shallower than that indicated by the classic carbonate depth–climate relationship of Jenny and Leonard [Jenny, H.J., Leonard, C.D., 1935. Functional relationships between soil properties and rainfall. Soil Science 38, 363–381] and Arkley [Arkley, R.J., 1963. Calculations of carbonate and water movement in soil from climatic data. Soil Science 96, 239–248], or simulated by numerical models of carbonate accumulation. Simultaneous development of thick carbonate collars and the Av horizon requires the sustained pavement clast–Av horizon coupling for at least centuries to possibly millennia. New thermoluminescence ages also indicate that much of the Av horizon formed in the Holocene, and that it is certainly much younger than the older Pleistocene pavements. This supports the previously proposed hypothesis that increased dust flux during the Pleistocene-to-Holocene transition triggered and/or greatly accelerated Av horizon development. An understanding of the genesis of collars provides not just an understanding of how carbonate can accumulate in surface environments, but it also provides important clues into processes of pavement evolution and preservation of Av horizons during long glacial periods. The Av horizon is not merely an insignificant surficial zone of recent dust accretion; instead, its development profoundly influences the genesis of desert soils and pavements.
Nadeau, J. A., Qualls, R. G., Nowak, R. S., Blank, R. R. (2007). The potential bioavailability of organic C, N, and P through enzyme hydrolysis in soils of the Mojave Desert. Biogeochemistry 82 (3): 305-320
ABSTRACT: Increases in the growth rate of plants and microbes in the Mojave Desert in response to predicted increases in precipitation and CO2 due to global climate change may induce nutrient limitations. This study was designed to measure the pool of potentially bioavailable nutrients in soils of the Mojave Desert. Soils were collected from shrub and interspace microsites and then subjected to amendment with buffered solutions of an excess of various enzymes. The products of each enzyme reaction were then measured and the maximum quantity of hydrolyzable substrates was calculated. In interspace and shrub microsite soils, respectively, 14.5 and 9.7% of the organic C in the form cellulose, 60.0–97.8% and 61.2–100.0% of the organic N in the form protein, and 44.0 and 57.5% of the organic P was hydrolyzable. There were significant differences between microsites for hydrolyzable substrate using all enzyme amendments, except protease. We propose that accumulations of hydrolyzable organic C, N, and P in the Mojave Desert could be a result of the persistently dry soil conditions often found in desert ecosystems and the immobilization of enzymes, which may result in low diffusivity of soil substrates and enzymes and, accordingly, little degradation of organic C, N, and P. Alternatively, rapid nutrient cycling and immobilization by soil microorganisms could account for accumulations of organic C, N, and P. Further refinement of the methods used in this study could lead to a valuable tool for the assessment of potential bioavailability of nutrients in a variety of soils.
ABSTRACT: There is much interest in predicting future carbon-soil degradation and that occurring today. We have National Soil Survey Laboratory data to assess some of the soil carbon degradation in the Great Basin and western Utah. For this we included data on 32 Nevada and Utah soils on Pleistocene geomorphic surfaces at elevations of 973 to 3172 m. Their mean annual precipitation (MAP) ranges from 20 to 55 cm and mean annual soil temperatures (MAST) from 5 to 12 °C. The MAP and MAST closely correlate with elevation (E) (r = 0.96 and − 0.97 respectively). Mountain big sagebrush (Artemisia tridentata Nutt. ssp.vaseyana (Rydb.) Beetle) dominates vegetation at the higher, colder elevations. Wyoming big sagebrush (Artemisia tridentata Nutt. ssp.wyomingensis Beetle and Young) and juniper (Juniperus L.) dominate at intermediate elevations. Little sagebrush (Artemisia arbuscula Nutt.) and related desert species dominate at the lower, warmer elevations. We used acid dichromate digestion and FeSO4 titration to analyze for soil organic carbon (SOC) and bulk density and coarse fragments in the soils to put the data on a volume basis. The soils are well drained and uncultivated. Accumulation of organic carbon in each pedon (OCp) is correlated to MAP and MAST (r = 0.81, and − 0.78 respectively). We predicted OCp from the relationship,
OCp = -0.942+ 2.546* sqrt(MAP/MAST) (1)
r2 = 0.64, S.E. = 1.30, n = 32. The soil OC degradation that may have occurred through the Holocene ranges from 35% at sites of the present Aridisols and Vertisols to 22% for the sites of the Mollisols and Alfisols. Eq. (1) shows that today, MAST rises of 1 to 3 °C would produce further OCp degradation from today's levels of 1% to 13% in Aridisols and Vertisols and 12% to 25% in the Mollisols and Alfisols respectively. It also shows that if the MAST drop of 6 °C predicted for the Pleistocene occurred, many of the Aridisols and Vertisols likely would have been Mollisols or Alfisols during the Pleistocene. A temperature rise of 1 °C in a century would likely move the Mollisol–Aridisol boundary from its present 2300 m elevation to an elevation of about 2900 m. A temperature rise of 3 °C in a century would likely move the Mollisol–Aridisol boundary from its present 2300 m elevation to elevations of the highest elevations in Nevada and to the middle of Idaho. Increasing the temperature by 3 °C will likely also increase the area affected by severe desertification in the southern Great Basin north by about 20%.
ABSTRACT: A lower limit for nitrogen loss from desert ecosystems in the southwestern United States was estimated by comparing nitrogen inputs to the amount of nitrogen stored in desert soils and vegetation. Atmospheric input of nitrogen for the last 10 000 years was conservatively estimated to be 2.99 kg N/m2 . The amount of nitrogen stored in desert soils was calculated to be 0.604 kg N/m3 using extant data from 212 profiles located in Arizona, California, Nevada, and Utah. The average amount of nitrogen stored in desert vegetation is approximately 0.036 kg N/m2 .
Desert conditions have existed in the southwestern United States throughout the last 10 000 years. Under such conditions, vertical leaching of nitrogen below a depth of 1 m is small (ca. 0.028 kg N/m2 over 10 000 years) and streamflow losses of nitrogen from the desert landscape are negligible. Thus, the discrepancy found between nitrogen input and storage represents the amount of nitrogen lost to the atmosphere during the last 10 000 years. Loss of nitrogen to the atmosphere was calculated to be 2.32 kg N/m2 , which is 77% of the atmospheric inputs.
Processes resulting in nitrogen loss to the atmosphere from desert ecosystems include wind erosion, ammonia volatilization, nitrification, and denitrification. Our analysis cannot assess the relative importance of these processes, but each is worthy of future research efforts.
Phillips, D. L., Johnson, M. G., Tingey, D. T., Catricala, C. E., Hoyman, T. L., Nowak, R. S. (2006). Effects of elevated CO2 on fine root dynamics in a Mojave Desert community: a FACE study. Global Change Biology 12 (1): 61-73
ABSTRACT: Fine roots (≤1 mm diameter) are critical in plant water and nutrient absorption, and it is important to understand how rising atmospheric CO2 will affect them as part of terrestrial ecosystem responses to global change. This study's objective was to determine the effects of elevated CO2 on production, mortality, and standing crops of fine root length over 2 years in a free-air CO2 enrichment (FACE) facility in the Mojave Desert of southern Nevada, USA. Three replicate 25 m diameter FACE rings were maintained at ambient (~370μmol mol−1 ) and elevated CO2 (~550μmol mol−1 ) atmospheric concentrations. Twenty-eight minirhizotron tubes were placed in each ring to sample three microsite locations: evergreenLarrea shrubs, drought-deciduousAmbrosia shrubs, and along systematic community transects (primarily in shrub interspaces which account for ~85% of the area). Seasonal dynamics were similar for ambient and elevated CO2 : fine root production peaked in April–June, with peak standing crop occurring about 1 month later, and peak mortality occurring during the hot summer months, with higher values for all three measures in a wet year compared with a dry year. Fine root standing crop, production, and mortality were not significantly different between treatments except standing crop along community transects, where fine root length was significantly lower in elevated CO2 . Fine root turnover (annual cumulative mortality/mean standing crop) ranged from 2.33 to 3.17 year−1 , and was not significantly different among CO2 treatments, except for community transect tubes where it was significantly lower for elevated CO2 . There were no differences in fine root responses to CO2 between evergreen (Larrea ) and drought-deciduous (Ambrosia ) shrubs. Combined with observations of increased leaf-level water-use efficiency and lack of soil moisture differences, these results suggest that under elevated CO2 conditions, reduced root systems (compared with ambient CO2 ) appear sufficient to provide resources for modest aboveground production increases across the community, but in more fertile shrub microsites, fine root systems of comparable size with those in ambient CO2 were required to support the greater aboveground production increases. For community transects, development of the difference in fine root standing crops occurred primarily through lower stimulation of fine root production in the elevated CO2 treatment during periods of high water availability.
Quade, J., Rech, J.A., Latorre, C., Betancourt, J.L., Gleeson, E., Kalin, M.T.K. (2007). Soils at the hyperarid margin: The isotopic composition of soil carbonate from the Atacama Desert, northern Chile. Geochimica Et Cosmochimica Acta 71 (15): 3772-3795
ABSTRACT: We evaluate the impact of exceptionally sparse plant cover (0–20%) and rainfall (2–114 mm/yr) on the stable carbon and oxygen composition of soil carbonate along elevation transects in what is among the driest places on the planet, the Atacama Desert in northern Chile.d13 C andd18 O values of carbonates from the Atacama are the highest of any desert in the world.d13 C (VPDB) values from soil carbonate range from−8.2‰ at the wettest sites to +7.9‰ at the driest. We measured plant composition and modeled respiration rates required to form these carbonate isotopic values using a modified version of the soil diffusion model of [Cerling (1984) Earth Planet. Sci. Lett. 71, 229–240], in which we assumed an exponential form of the soil CO2 production function, and relatively shallow (20–30 cm) average production depths. Overall, we find that respiration rates are the main predictor of thed13 C value of soil carbonate in the Atacama, whereas the fraction C3 to C4 biomass at individual sites has a subordinate influence. The high averaged13 C value (+4.1‰) of carbonate from the driest study sites indicates it formed—perhaps abiotically—in the presence of pure atmospheric CO2 .d18 O (VPDB) values from soil carbonate range from−5.9‰ at the wettest sites to +7.3‰ at the driest and show much less regular variation with elevation change thand13 C values.d18 O values for soil carbonate predicted from local temperature andd18 O values of rainfall values suggest that extreme (>80% in some cases) soil dewatering by evaporation occurs at most sites prior to carbonate formation. The effects of evaporation compromise the use ofd18 O values from ancient soil carbonate to reconstruct paleoelevation in such arid settings.
Reynolds, J. F., Kemp, P. R., Ogle, K., Fernandez, R. J. (2004). Modifying the 'pulse-reserve' paradigm for deserts of North America: precipitation pulses, soil water, and plant responses. Oecologia 141 (2): 194-210
ABSTRACT: The pulse–reserve conceptual model—arguably one of the most-cited paradigms in aridland ecology—depicts a simple, direct relationship between rainfall, which triggers pulses of plant growth, and reserves of carbon and energy. While the heuristics of pulses, triggers and reserves are intuitive and thus appealing, the value of the paradigm is limited, both as a conceptual model of how pulsed water inputs are translated into primary production and as a framework for developing quantitative models. To overcome these limitations, we propose a revision of the pulse–reserve model that emphasizes the following: (1) what explicitly constitutes a biologically significant rainfall pulse, (2) how do rainfall pulses translate into usable soil moisture pulses, and (3) how are soil moisture pulses differentially utilized by various plant functional types (FTs) in terms of growth? We explore these questions using the patch arid lands simulation (PALS) model for sites in the Mojave, Sonoran, and Chihuahuan deserts of North America. Our analyses indicate that rainfall variability is best understood in terms of sequences of rainfall events that produce biologically-significant pulses of soil moisture recharge, as opposed to individual rain events. In the desert regions investigated, biologically significant pulses of soil moisture occur in either winter (October–March) or summer (July–September), as determined by the period of activity of the plant FTs. Nevertheless, it is difficult to make generalizations regarding specific growth responses to moisture pulses, because of the strong effects of and interactions between precipitation, antecedent soil moisture, and plant FT responses, all of which vary among deserts and seasons. Our results further suggest that, in most soil types and in most seasons, there is little separation of soil water with depth. Thus, coexistence of plant FTs in a single patch as examined in this PALS study is likely to be fostered by factors that promote: (1) separation of water use over time (seasonal differences in growth), (2) relative differences in the utilization of water in the upper soil layers, or (3) separation in the responses of plant FTs as a function of preceding conditions, i.e., the physiological and morphological readiness of the plant for water-uptake and growth. Finally, the high seasonal and annual variability in soil water recharge and plant growth, which result from the complex interactions that occur as a result of rainfall variability, antecedent soil moisture conditions, nutrient availability, and plant FT composition and cover, call into question the use of simplified vegetation models in forecasting potential impacts of climate change in the arid zones in North America.
Schaeffer, S. M., Billings, S. A., R. D. Evans (2007). Laboratory incubations reveal potential responses of soil nitrogen cycling to changes in soil C and N availability in Mojave Desert soils exposed to elevated atmospheric CO2 . Global Change Biology 13 (4): 854-865
ABSTRACT: Elevated atmospheric carbon dioxide (CO2 ) has the potential to alter soil carbon (C) and nitrogen (N) cycling in arid ecosystems through changes in net primary productivity. However, an associated feedback exists because any sustained increases in plant productivity will depend upon the continued availability of soil N. We took soils from under the canopies of major shrubs, grasses, and plant interspaces in a Mojave Desert ecosystem exposed to elevated atmospheric CO2 and incubated them in the laboratory with amendments of labile C and N to determine if elevated CO2 altered the mechanistic controls of soil C and N on microbial N cycling. Net ammonification increased under shrubs exposed to elevated CO2 , while net nitrification decreased. Elevated CO2 treatments exhibited greater fluxes of N2 O–N underLycium spp., but not other microsites. The proportion of microbial/extractable organic N increased under shrubs exposed to elevated CO2 . Heterotrophic N2 -fixation and C mineralization increased with C addition, while denitrification enzyme activity and N2 O–N fluxes increased when C and N were added in combination. Laboratory results demonstrated the potential for elevated CO2 to affect soil N cycling under shrubs and supports the hypothesis that energy limited microbes may increase net inorganic N cycling rates as the amount of soil-available C increases under elevated CO2 . The effect of CO2 enrichment on N-cycling processes is mediated by its effect on the plants, particularly shrubs. The potential for elevated atmospheric CO2 to lead to accumulation of NH4 + under shrubs and the subsequent volatilization of NH3 may result in greater losses of N from this system, leading to changes in the form and amount of plant-available inorganic N. This introduces the potential for a negative feedback mechanism that could act to constrain the degree to which plants can increase productivity in the face of elevated atmospheric CO2 .
ABSTRACT: Radiocarbon and230 Th-234 U dates of calcic horizons from calciorthid soil profiles in the Mojave Desert were used to calculate the rate of deposition of pedogenic CaCO3 . A major period of CaCO3 deposition appears to have occurred about 20000 yBP forming calcic horizons below 100-cm depth during a climatic regime with greater effective rainfall than in the present. The overall rate of deposition has been 1.0 to 3.5 g CaCO3 /m2 /yr during soil formation. This rate is consistent with present-day rates, assuming that the atmospheric deposition of Ca limits the process. Stable isotope ratios in calcic horizons indicate that CaCO3 precipitated from a soil environment with CO2 of − 15.5%.13 C/12 C (vs. PDB) and H2 O of + 2.0%.18 O/16 O (vs. SMOW). These values suggest that CaCO3 precipitates when seasonal drought simultaneously lowers soil porep CO2 and enriches soil water18 O by evaporation. The role of soil calcic horizons in the global geochemical cycle of carbon is discussed.
Scott, R.L., Huxman, T.E., Cable, W.L., Emmerich, W.E. (2006). Partitioning of evapotranspiration and its relation to carbon dioxide exchange in a Chihuahuan Desert shrubland. Hydrological Processes 20 (15): 3227-3243
ABSTRACT: Key to evaluating the consequences of woody plant encroachment on water and carbon cycling in semiarid ecosystems is a mechanistic understanding of how biological and non-biological processes influence water loss to the atmosphere. To better understand how precipitation is partitioned into the components of evapotranspiration (bare-soil evaporation and plant transpiration) and their relationship to plant uptake of carbon dioxide (CO2 ) as well as ecosystem respiratory efflux, we measured whole plant transpiration, evapotranspiration, and CO2 fluxes over the course of a growing season at a semiarid Chihuahuan Desert shrubland site in south-eastern Arizona. Whole plant transpiration was measured using the heat balance sap-flow method, while evapotranspiration and net ecosystem exchange (NEE) of CO2 were quantified using the Bowen ratio technique.
Before the summer rainy season began, all water and CO2 fluxes were small. At the onset of the rainy season, evapotranspiration was dominated by evaporation and CO2 fluxes were dominated by respiration as it took approximately 10 days for the shrubs to respond to the higher soil moisture content. During the growing season, periods immediately following rain events (<2 days) were dominated by evaporation and respiration while transpiration and CO2 uptake peaked during the interstorm periods. The surface of the coarse, well-drained soils dried quickly, rapidly reducing evaporation. Overall, the ratio of total transpiration to evapotranspiration was 58%, but it was around 70% during the months when the plants were active. Peak respiration responses following rain events generally lagged after the evaporation peak by a couple of days and were better correlated with transpiration. Transpiration and CO2 uptake also decayed rather quickly during interstorm periods, indicating that optimal plant soil moisture conditions were rarely encountered. NEE of CO2 was increasingly more negative as the growing season progressed, indicating a greater net uptake of CO2 and greater water use efficiency due mainly to decreases in respiration.
ABSTRACT: The second largest pool of terrestrial carbon is soil CaCO3 . In addition to being an important sink of atmospheric CO2 , soil carbonate is potentially an important source of atmospheric CO2 . The cemented form of soil carbonate—the petrocalcic horizon—develops in geomorphically stable soil in arid, semiarid, and some subhumid climates. In many of these dryland areas, such as the Chihuahuan Desert of North America, erosion has stripped away overlying soil and exhumed the petrocalcic horizon, thereby exposing it to a weathering zone above the calcification zone where it normally forms. This research tested the hypothesis that soil type 1 (eroded Aridisols with exhumed petrocalcic horizons) will emit more CO2 than soil type 2 (noneroded Aridisols with petrocalcic horizons) or soil type 3 (Entisols formed in sandy, noncalcareous sediments). We tested this hypothesis by comparing the amount of CO2 and thed13 C of CO2 released from the three soil types. Using a randomized complete block design, CO2 emissions were measured using NaOH and soda lime traps from June 2002 to October 2003. Neither the NaOH traps nor soda lime traps detected any statistical difference in cumulative CO2 emissions from the three soil types at thea= 0.05 level. Moreover, the isotopic analysis of CO2 did not match the isotopic values of pedogenic carbonate, nor were there any statistical differences (a= 0.05) ind13 C of CO2 among the three soil types. We conclude, therefore, that exhumed petrocalcic horizons are not actively emitting CO2 at a rate significantly greater than adjacent soils, and thus carbon stored in petrocalcic horizons can be considered a recalcitrant reservoir within the decadal timeframe pertinent to carbon sequestration policies.
ABSTRACT: Deserts are known to mankind, but the term desertification has always been an elusive concept. It is now defined in the United Nations Convention to Combat Desertification (UNCCD) as land degradation in the drylands (land falling within arid, semi-arid and dry sub-humid areas) resulting from various factors, including climatic variations and human activities. This definition, which is now being used worldwide to describe desertification and its impacts, leads to the need to consider carefully the two-way interactions between climate and desertification. Dramatic changes in agricultural practices during the last several decades are one of the main driving forces for land degradation in the drylands and examples of land degradation are given for several regions around the world. The effects of desertification on climate have been described mainly in terms of changes in land use and land cover leading to land degradation; overgrazing; biomass burning and atmospheric emissions; agriculture's contribution to air pollution; forest and woodland clearing and accelerated wind erosion; anthropogenic land disturbances and wind erosion; and the impact of irrigated agriculture on surface conditions in drylands. It is equally important to consider the impact of dryland climates on soils and vegetation and the impact of climate change on desertification. It is important to adopt uniform criteria and methods to assess desertification and encourage monitoring of dryland degradation in all the regions around the world. To better understand the interactions between climate and desertification, it is also important to identify the sources and sinks of dryland carbon, aerosols and trace gases in drylands.
ABSTRACT: Precipitation is a major driver of biological processes in arid and semiarid ecosystems. Soil biogeochemical processes in these water-limited systems are closely linked to episodic rainfall events, and the relationship between microbial activity and the amount and timing of rainfall has implications for the whole-system carbon (C) balance. Here, the influences of storm size and time between events on pulses of soil respiration were explored in an upper Sonoran Desert ecosystem. Immediately following experimental rewetting in the field, CO2 efflux increased up to 30-fold, but generally returned to background levels within 48 h. CO2 production integrated over 48 h ranged from 2.5 to 19.3 g C m−2 and was greater beneath shrubs than in interplant spaces. When water was applied on sequential days, postwetting losses of CO2 were only half a large as initial fluxes, and the size of the second pulse increased with time between consecutive events. Soil respiration was more closely linked to the organic matter content of surface soils than to rainfall amount. Beneath shrubs, rates increased nonlinearly with storm size, reaching an asymptote at approximately 0.5 cm simulated storms. This nonlinear relationship stems from (1) resource limitation of microbial activity that is manifest at small time scales, and (2) greatly reduced process rates in deeper soil strata. Thus, beyond some threshold in storm size, increasing the duration or depth of soil moisture has little consequence for short-term losses of CO2 . In addition, laboratory rewetting across a broad range in soil water content suggest that microbial activity and CO2 efflux following rainfall may be further modified by the routing and redistribution of water along hillslopes. Finally, analysis of long-term precipitation data suggests that half the monsoon storms in this system are sufficient to induce soil heterotrophic activity and C losses, but are not large enough to elicit autotrophic activity and C accrual by desert shrubs.
ABSTRACT: Soil inorganic carbon (C) represents a substantial C pool in arid ecosystems, yet little data exist on the contribution of this pool to ecosystem C fluxes. A closed jar incubation study was carried out to test the hypothesis that CO2 -13 C production and response to sterilization would differ in a calcareous (Mojave Desert) soil and a non-calcareous (Oklahoma Prairie) soil due to contributions of carbonate-derived CO2 . In addition to non-sterilized controls, soils were subjected to sterilization treatments (unbuffered HgCl2 addition for Oklahoma soil and unbuffered HgCl2 addition, buffered HgCl2 addition, and autoclaving for Mojave Desert soil) to decrease biotic respiration and more readily measure abiotic CO2 flux. Temperature and moisture treatments were also included with sterilization treatments in a factorial design.
The rate of CO2 production in both soils was significantly decreased (36–87%) by sterilization, but sterilization treatments differed in effectiveness. Sterilization had no significant effect on effluxed CO2 -13 C values in the non-calcareous Oklahoma Prairie soil and autoclaved Mojave Desert soil as compared to their respective non-sterilized controls. However, sterilization significantly altered CO2 -13 C values in Mojave Desert soil HgCl2 sterilization treatments (both buffered and non-buffered). Plots of 1/CO2 versus CO2 -d13 C (similar to Keeling plots) indicated that the sourceCO2 -d13 C value of the Oklahoma Prairie soil treatments was similar to thed13 C value of soil organic matter [(SOM);−17.76‰ VPDB] whereas the source for the (acidic) unbuffered-HgCl2 sterilized Mojave Desert soil was similar to thed13 C value of carbonates (−0.93‰ VPDB). The source CO2-d13 Cvalue of non-sterilized and autoclaved (−18.4‰ VPDB) Mojave Desert soil treatments was intermediate between SOM (−21.43‰ VPDB) and carbonates and indicates up to 13% of total C efflux may be from abiotic sources in calcareous soils.
ABSTRACT: Phenol oxidase and peroxidase activities in desert grassland soils at the Sevilleta Long Term Ecological Research site in central New Mexico (USA) are far greater than those of temperate soils. Activity is uniformly distributed across particles ranging from >1 mm to <38μm and is unaffected by autoclaving, in contrast to hydrolase activities. The sorbed enzymes are readily extractable and inactivated by boiling. High soil pH, high stabilized oxidative enzyme activity, and carbonates create optimal conditions for degradation of phenols which increase decomposition potentials and limit soil organic matter accumulation.
ABSTRACT: Knowledge of carbon dioxide concentration in desert soils is required in theoretical models of the development of pedogenic and ground-water calcium carbonate. Most studies have concentrated on medium- to fine-textured soils in temperate to humid environments. Very little data exist for CO2 concentrations in stony, coarse-textured deposits such as those making up alluvial fans and fluvial terraces. The purpose of this study was to obtain CO2 concentration data in stony, coarse-textured, uncultivated soils in a desert environment.
Soil gas samples were collected from two sites-Yucca Wash and Rock Valley-at the Nevada Test Site in southern Nevada between June 1985 and June 1986 to determine soil CO2 concentrations and their seasonal and depth variations in the soil profile. Soil CO2 concentrations ranged from 0.03 volume percent in winter and summer to 0.25 volume percent in spring. During the summer, fall, and winter, CO2 concentrations were close to atmospheric levels and varied randomly with depth. In spring, CO2 varied with root density, highest concentrations occurring between 40 and 80 cm of depth. Maximum CO2 values decrease and occur at progressively deeper levels toward the end of spring. Biological activity appears to be limited throughout the year because of the arid climate. During the winter and spring, soil CO2 appears to be more dependent on temperature than moisture, whereas in the summer and fall it appears to be dependent on moisture.
Soil gas samples were also collected during the spring from three sites in the Kyle Canyon area in southern Nevada to determine the variation of soil CO2 levels with changes in elevation and vegetation. Soil CO2 values increased from 0.04 to 0.24 volume percent with increasing elevation.
ABSTRACT: Encroachment of woody plants into grasslands, and subsequent brush management, are among the most prominent changes to occur in arid and semiarid systems over the past century. Despite the resulting widespread changes in landcover, substantial uncertainty about the biogeochemical impacts of woody proliferation and brush management exists. We explored the role of shrub encroachment and brush management on leaf litter decomposition in a semidesert grassland where velvet mesquite (Prosopis velutina ) abundance has increased over the past 100 years. This change in physiognomy may affect decomposition directly, through altered litter quality or quantity, and indirectly through altered canopy structure. To assess the direct and indirect impacts of shrubs on decomposition, we quantified changes in mass, nitrogen, and carbon in litterbags deployed under mesquite canopies and in intercanopy zones. Litterbags contained foliage from mesquite and Lehmann lovegrass (Eragrostis lehmanniana ), a widespread, nonnative grass in southern Arizona. To explore short- and long-term influences of brush management on the initial stages of decomposition, litterbags were deployed at sites where mesquite canopies were removed three weeks, 45 years, or 70 years prior to study initiation.
Mesquite litter decomposed more rapidly than lovegrass, but negative indirect influences of mesquite canopies counteracted positive direct effects. Decomposition was positively correlated with soil infiltration into litterbags, which varied with microsite placement, and was lowest under canopies. Low under-canopy decomposition was ostensibly due to decreased soil movement associated with high under-canopy herbaceous biomass. Decomposition rates where canopies were removed three weeks prior to study initiation were comparable to those beneath intact canopies, suggesting that decomposition was driven by mesquite legacy effects on herbaceous cover-soil movement linkages. Decomposition rates where shrubs were removed 45 and 70 years prior to study initiation were comparable to intercanopy rates, suggesting that legacy effects persist less than 45 years.
Accurate decomposition modeling has proved challenging in arid and semiarid systems but is critical to understanding biogeochemical responses to woody encroachment and brush management. Predicting brush-management effects on decomposition will require information on shrub–grass interactions and herbaceous biomass influences on soil movement at decadal timescales. Inclusion of microsite factors controlling soil accumulation on litter would improve the predictive capability of decomposition models.
ABSTRACT: Restoration of degraded soils is a development strategy to reduce desertification, soil erosion and environmental degradation, and alleviate chronic food shortages with great potential in sub-Saharan Africa (SSA). Further, it has the potential to provide terrestrial sinks of carbon (C) and reduce the rate of enrichment of atmospheric CO2 . Soil organic carbon (SOC) contents decrease by 0 to 63 per cent following deforestation. There exists a high potential for increasing SOC through establishment of natural or improved fallow systems (agroforestry) with attainable rates of C sequestration in the range of 0·1 to 5·3 Mg C ha-1 yr-1 . Biomass burning significantly reduces SOC in the upper few centimeters of soil, but has little impact below 10 to 20 cm depth. The timing of burning is also important, and periods with large amounts of biomass available generally have the largest losses of SOC. In cultivated areas, the addition of manure in combination with crop residues and no-till show similar rates of attainable C sequestration (0 to 0·36 Mg C ha-1 yr-1 ). Attainable rates of SOC sequestration on permanent cropland in SSA under improved cultivation systems (e.g. no-till) range from 0·2 to 1·5 Tg C yr-1 , while attainable rates under fallow systems are 0·4 to 18·5 Tg C yr-1 . Fallow systems generally have the highest potential for SOC sequestration in SSA with rates up to 28·5 Tg C yr-1 .
M. V. K. Sivakumar, H. P. Das, O. Brunini (2005). Impacts of present and future climate variability and change on agriculture and forestry in the arid and semi-arid tropics. Climatic Change 70 (1-2): 31-72
ABSTRACT: The arid and semi-arid regions account for approximately 30% of the world total area and are inhabited by approximately 20% of the total world population. Issues of present and future climate variability and change on agriculture and forestry in the arid and semi-arid tropics of the world were examined and discussion under each of these issues had been presented separately for Asia, Africa and Latin America. Several countries in tropical Asia have reported increasing surface temperature trends in recent decades. Although, there is no definite trend discernible in the long-term mean for precipitation for the tropical Asian region, many countries have shown a decreasing trend in rainfall in the past three decades. African rainfall has changed substantially over the last 60 yr and a number of theoretical, modelling and empirical analyses have suggested that noticeable changes in the frequency and intensity of extreme events, including floods may occur when there are only small changes in climate. Climate in Latin America is affected by the El Niño-southern oscillation (ENSO) phases and there is a close relationship between the increase and decrease of rainfall depending upon the warm or cold phases of the phenomenon.
Over land regions of Asia, the projected area-averaged annual mean warming is likely to be 1.6 ± 0.2 °C in the 2020s, 3.1 ± 0.3 °C in the 2050s, and 4.6 ± 0.4 °C in the 2080s and the models show high uncertainty in projections of future winter and summer precipitation. Future annual warming across Africa is projected to range from 0.2 °C per decade to more than 0.5 °C per decade, while future changes in mean seasonal rainfall in Africa are less well defined. In Latin America, projections indicate a slight increase in temperature and changes in precipitation. Impacts of climate variability and changes are discussed with suitable examples. Agricultural productivity in tropical Asia is sensitive not only to temperature increases, but also to changes in the nature and characteristics of monsoon. Simulations of the impacts of climate change using crop simulation models show that crop yield decreases due to climate change could have serious impacts on food security in tropical Asia. Climate change is likely to cause environmental and social stress in many of Asia’s rangelands and drylands. In the arid and semi-arid tropics of Africa, which are already having difficulty coping with environmental stress, climate change resulting in increased frequencies of drought poses the greatest risk to agriculture. Impacts were described as those related to projected temperature increases, the possible consequences to water balance of the combination of enhanced temperatures and changes in precipitation and sensitivity of different crops/cropping systems to projected changes. In Latin America, agriculture and water resources are most affected through the impact of extreme temperatures (excessive heat, frost) and the changes in rainfall (droughts, flooding). Adaptation potential in the arid and semi-arid tropics of Asia, Africa and Latin America was described using suitable examples. It is emphasized that approaches need to be prescriptive and dynamic, rather than descriptive and static.
W. H. Schlesinger, J. F. Reynolds, G. L. Cunningham, L. F. Huenneke, W. M. Jarrell, R. A. Virginia, W. G. Whitford (1990). Biological feedbacks in global desertification. Science 247 (4946): 1043-1048
ABSTRACT: Studies of ecosystem processes on the Jornada Experimental Range in southern New Mexico suggest that longterm grazing of semiarid grasslands leads to an increase in the spatial and temporal heterogeneity of water, nitrogen, and other soil resources. Heterogeneity of soil resources promotes invasion by desert shrubs, which leads to a further localization of soil resources under shrub canopies. In the barren area between shrubs, soil fertility is lost by erosion and gaseous emissions. This positive feedback leads to the desertification of formerly productive land in southern New Mexico and in other regions, such as the Sahel. Future desertification is likely to be exacerbated by global climate warming and to cause significant changes in global biogeochemical cycles.
ABSTRACT: Geostatistical analyses show that the distribution of soil N, P and K is strongly associated with the presence of shrubs in desert habitats. Shrubs concentrate the biogeochemical cycle of these elements in islands of fertility that are localized beneath their canopies, while adjacent barren, intershrub spaces are comparatively devoid of biotic activity. Both physical and biological processes are involved in the formation of shrub islands. Losses of semiarid grassland in favor of invading shrubs initiate these changes in the distribution of soil nutrients, which may promote the further invasion and persistence of shrubs and cause potential feedbacks between desertification and the Earth's climate system.
ABSTRACT: Recent reports of net ecosysytem production >100 g Cm−2 yr−1 in deserts are incompatible with existing measurements of net primary production and carbon pools in deserts. The comparisions suggest that gas exchange measurements should be used with caution and better validation if they are expected to indicate the magnitude of carbon sink in these ecosysytems.