Climate Change and...

Annotated Bibliography

Carbon Dynamics

Floodplain and Riparian

J. A. Aitkenhead, W. H. McDowell (2000). Soil C:N ratio as a predictor of annual riverine DOC flux at local and global scales. Global Biogeochemical Cycles 14 (1): 127-138

ABSTRACT; Dissolved organic carbon (DOC) is important in a wide variety of chemical, physical, and biological processes in surface waters. We examined the relationship between DOC flux and soil C:N ratio on a biome basis. DOC fluxes for 164 rivers were subdivided into 15 biome types including tropical rain forest, coniferous forests, peatland, deciduous forests, mixed forests, and grasslands. A database of soil C:N ratios was constructed and subdivided into biome types. At a global scale, mean soil C:N ratio of a biome accounts for 99.2% of the variance in annual riverine DOC flux among biomes. The relationship between soil C:N ratio and DOC flux at the biome scale was used to predict annual riverine DOC flux at the watershed scale for three test watersheds not included in the original model. Predicted flux of each watershed was within 4.5% of the actual DOC flux. Using the C:N model, we estimated the total export of carbon from land to the oceans to be 3.6×1014 g yr−1 . This empirical model should be useful in predicting changes in DOC flux under changing climatic conditions.

Gurwick, N. P., Groffman, P. M., Yavitt, J. B., Gold, A. J., Blazejewski, G., Stolt, M. (2008). Microbially available carbon in buried riparian soils in a glaciated landscape. Soil Biology and Biochemistry 40 (1): 85-96

ABSTRACT: Buried horizons and lenses in riparian soil profiles harbor large amounts of carbon relative to the surrounding soil horizons. Because these buried soil horizons, as well as deep surface horizons, frequently lie beneath the water table, their impact on nitrogen transport across the terrestrial-aquatic interface depends upon their frequency and spatial distribution, and upon the lability of associated organic matter. We collected samples of 51 soil horizons from 14 riparian zones Rhode Island, USA, where soil profiles are characterized by glacial outwash and alluvial deposits. These soil samples came from as deep as 2 m and ranged in carbon content from <1% to 44% in a buried O horizon 54-74 cm deep. We used these samples to: (1) determine the extent to which carbon in buried horizons, and deep surface horizons, is potentially microbially available; (2) identify spatial patterns of carbon mineralization associated with surface and buried horizons; and (3) evaluate likely relationships between soil horizon types, chemical characteristics and carbon mineralization. Carbon mineralization rates associated with buried horizons during anaerobic incubations ranged from 0.0001 to 0.0175 [mu]mol C kg soil-1 s-1 and correlated positively with microbial biomass (R=0.89, P<0.0001, n=21). Excluding surface O horizons from the analysis, carbon mineralization varied systematically with horizon type (surface A, buried A, buried O, lenses, A/C, B, C) (P<0.05) but not with depth or depth x horizon interaction (overall R2=0.59, P<0.0005, n=47). In contrast to this result and to most published data sets,13 C-to-12 C and15 N-to-14 N ratios of organic matter declined with depth (13C-26.9 to -29.3 per mil, 15N+5.6 to -0.8 per mil). The absence of a relationship between horizon depth and C availability suggests that carbon availability in these buried horizons may be determined by the abundance and quality of organic matter at the time of horizon formation or burial, rather than by duration since burial, and implies that subsurface microbial activity is largely disconnected from surface ecosystems. Our results contribute to the emerging view that buried horizons harbor microbially available C in quantities relevant to ecosystem processes, and suggest that buried C-rich soil horizons need to be incorporated into assessments of the depth of the biologically active zone in near-stream subsurface soils.

Harms, T. K., N. B. Grimm (2008). Hot spots and hot moments of carbon and nitrogen dynamics in a semiarid riparian zone. Journal of Geophysical Research - Biogeosciences 113 (G01020)

ABSTRACT: Riparian ecosystems are characterized by spatial and temporal heterogeneity in physical and biological attributes, with consequences for nutrient cycling. We investigated the responses of carbon (C) and nitrogen (N) cycling processes to the hydrogeomorphic template in the riparian zone of the San Pedro River, Arizona, a large (catchment area ~11,500 km2 ), free-flowing, semiarid river. Over an annual period we documented spatial and temporal patterns in soil, shallow groundwater, and stream nutrient chemistry as well as rates of N-transforming processes in soils of the surface (0–17 cm) and region of seasonal saturation (RoSS). A hot moment of N retention and removal was indicated by elevated rates of microbial processes during the summer monsoon season. At the same time, elevated C was observed in soil microbial biomass for both surface soils and soils in the RoSS. Analyses of C-use profiles for soil microbes, coupled with trends in stream and shallow-groundwater chemistry, further suggest that this hot moment of N removal was fueled by newly available, labile organic material. In a spatial context, patchiness in soil resources, microbial biomass, and potential denitrification were best explained by variation in microtopography; low-elevation landscape positions were hot spots of resource availability and microbial activity. Vertical heterogeneity also corresponded with variation in the factors influencing N transformation rates. Organic matter was more frequently a significant factor explaining N transformation rates in RoSS soils whereas soil water content was more often important in surface soils. Together, these patterns suggest that understanding the points on the hydrogeomorphic template, both in space and in time, that bring together water and labile organic matter will lead to greater predictive capability regarding C and N cycling in semiarid river-riparian corridors.

Hazlett, P. W., Gordon, A. M., Sibley, P. K., Buttle, J. M. (2005). Stand carbon stocks and soil carbon and nitrogen storage for riparian and upland forests of boreal lakes in northeastern Ontario. Forest Ecology and Management 219 (1): 56-68

ABSTRACT: The establishment of shoreline reserves (buffer strips) has guided riparian forest management in Ontario for many years. A riparian area is defined as the transitional zone between the aquatic and terrestrial environments and therefore is also known as the aquatic/terrestrial ecotone. While many functions of riparian forests have been recognized and well studied, less is known about their potential to sequester C and whether this potential differs from other areas in the boreal forest landscape. Increased harvesting pressure due to decreased wood supply in Ontario and debate about the effectiveness of the current reserve guidelines has resulted in a renewed interest in harvesting riparian forests. In this study riparian and upslope forest C and soil C and N storage were quantified for 21 lakes shorelines at the Esker Lakes Research Area, a boreal forest ecosystem in northeastern Ontario, Canada. Objectives were to compare the C and N storage potential of riparian forests with those of adjacent upland forests, and to examine the potential impacts of harvesting on C stocks in riparian zones of the boreal forest.

Riparian forests did not differ from upslope stands in terms of total aboveground overstory C storage although there were significant differences in stocking density and species composition. However, a greater proportion of total site C in riparian areas was stored in the overstory tree layer (>5 cm dbh) compared to upslope areas. Forest floor layers were deeper and stored more C and N in riparian forest stands in comparison to upslope stands. In contrast, mineral soil in upslope stands had greater C and N storage than mineral soil horizons within the riparian forest. As a result, the riparian organic horizons comprise a larger percentage of the overall soil storage of C and N than upslope layers. Currently practiced full-tree harvesting would result in a removal of approximately 76% of total aboveground C (17% of the ecosystem C) in upslope stands compared to 98% of total aboveground C (35% of the ecosystem C) in riparian forests. Selective or modified harvesting in riparian zones could decrease C removal to levels equal to that obtained by full-tree harvesting in upslope areas.

Jacobs, S. M., Bechtold, J. S., Biggs, H. C., Grimm, N. B., Lorentz, S., McClain, M. E., Naiman, R. J., Perakis, S. S., Pinay, G., Scholes, M. C. (2007). Nutrient vectors and riparian processing: a review with special reference to African semiarid savanna ecosystems. Ecosystems 10 (8): 1231-1249

ABSTRACT: This review article describes vectors for nitrogen and phosphorus delivery to riparian zones in semiarid African savannas, the processing of nutrients in the riparian zone and the effect of disturbance on these processes. Semiarid savannas exhibit sharp seasonality, complex hillslope hydrology and high spatial heterogeneity, all of which ultimately impact nutrient fluxes between riparian, upland and aquatic environments. Our review shows that strong environmental drivers such as fire and herbivory enhance nitrogen, phosphorus and sediment transport to lower slope positions by shaping vegetative patterns. These vectors differ significantly from other arid and semiarid ecosystems, and from mesic ecosystems where the impact of fire and herbivory are less pronounced and less predictable. Also unique is the presence of sodic soils in certain hillslopes, which substantially alters hydrological flowpaths and may act as a trap where nitrogen is immobilized while sediment and phosphorus transport is enhanced. Nutrients and sediments are also deposited in the riparian zone during seasonal, intermittent floods while, during the dry season, subsurface movement of water from the stream into riparian soils and vegetation further enrich riparian zones with nutrients. As is found in mesic ecosystems, nutrients are immobilized in semiarid riparian corridors through microbial and plant uptake, whereas dissimilatory processes such as denitrification may be important where labile nitrogen and carbon are in adequate supply and physical conditions are suitable—such as in seeps, wallows created by animals, ephemeral wetlands and stream edges. Interaction between temporal hydrologic connectivity and spatial heterogeneity are disrupted by disturbances such as large floods and extended droughts, which may convert certain riparian patches from sinks to sources for nitrogen and phosphorus. In the face of increasing anthropogenic pressure, the scientific challenges are to provide a basic understanding of riparian biogeochemistry in semiarid African savannas to adequately address the temporal and spatial impact of disturbances, and to apply this knowledge to better regional land and water management. An integrated, multidisciplinary approach applied in protected as well as human-disturbed ecosystems in southern Africa is essential for underpinning a strong environmental basis for sustainable human-related expansion.

McLain, J. E. T., Martens, D. A. (2006). Moisture controls on trace gas fluxes in semiarid riparian soils. Soil Science Society Of America JournalSoil Sci So 70 (2): 367-377

ABSTRACT: Variability in seasonal soil moisture (SM) and temperature (T) can alter ecosystem/atmosphere exchange of the trace gases carbon dioxide (CO2 ), nitrous oxide (N2 O), and methane (CH4 ). This study reports the impact of year-round SM status on trace gas fluxes in three semiarid vegetation zones, mesquite (30 g organic C kg–1 soil), open/forb (6 g organic C kg–1 soil), and sacaton (18 g organic C kg–1 soil) from July 2002–September 2003 in southeastern Arizona. Carbon dioxide and N2 O emissions were highly dependent on available SM and T. During the heavy rains of the 2002 monsoon (238 mm total rainfall), large differences in soil C content did not correlate with variations in CO2 production, as efflux averaged 235.6 ± 39.5 mg CO2 m–2 h–1 over all sites. In 2003, limited monsoon rain (95 mm total rainfall) reduced CO2 emissions by 19% (mesquite), 40% (open), and 30% (sacaton), compared with 2002. Nitrous oxide emissions averaged 21.1 ± 13.4 (mesquite), 2.1 ± 4.4 (open), and 3.9 ± 5.2 µg N2 O m–2 h–1 (sacaton) during the 2002 monsoon. Limited monsoon 2003 rainfall reduced N2 O emissions by 47% in the mesquite, but N2 O production increased in the open (55%) and sacaton (5%) sites. Following a dry winter and spring 2002 (15 mm total rainfall), premonsoon CH4 consumption at all sites was close to zero, but following monsoon moisture input, the CH4 sink averaged 26.1 ± 6.3 µg CH4 m–2 h–1 through April 2003. Laboratory incubations showed potentials for CH4 oxidation from 0 to 45 cm, suggesting that as the soil surface dried, CH4 oxidation activity shifted downward in the sandy soils. Predicted climate change shifts in annual precipitation from one dominated by summer monsoon rainfall to one with higher winter precipitation may reduce soil CO2 and N2 O emissions while promoting CH4 oxidation rates in semiarid riparian soils of the Southwest, potentially acting as a negative feedback for future global warming.

Mladenov, N., Mcknight, D. M., Wolski, P., Murray-Hudson, M. (2007). Simulation of DOM fluxes in a seasonal floodplain of the Okavango Delta, Botswana. Ecological Modelling 205 (1-2): 181-195

ABSTRACT: In order to examine dissolved organic matter (DOM) fluxes in seasonal wetland systems that expand and contract seasonally, a time-variable model of dissolved organic carbon (DOC) was developed for a seasonal floodplain in the Okavango Delta of Botswana. The model simulates DOC concentrations from March 2001 to November 2002, during which time DOC concentrations varied between 8 and 31 mg C L−1 . The model uses a continuously stirred tank reactor (CSTR) approach to describe the hydrologic and biogeochemical controls on DOC leached from litter within the floodplain and transported into the floodplain from upstream. In 2002, a fire burned the floodplain and less litter was available for leaching than in 2001. The model was driven by observations of discharge, water temperature, upstream DOC concentrations, and DOC leaching rates from leaching experiments. Leaching experiments with sedges and grasses indicated that on average 23 mg DOC g−1 were leached during the first day of wetting and 0.6 mg DOC g−1 d−1 were continuously leached afterwards. Leaching experiments also showed a decreased amount of DOC released from burned litter and soils than from unburned litter and soils. A two-pool first-order decay model that represents both rapidly (0.14 d−1 (at 22 °C)) and slowly (0.045 d−1 ) decaying pools of DOC provided the best representation of observed patterns in DOC concentration in 2001. The decay rate of the first pool decreased by nearly half in 2002, when an estimated 78% of litter was removed by fire.

Upstream DOC transport into the floodplain was the dominant source of DOC (representing approximately 70% and 75% of the DOC input in 2001 and 2002, respectively), followed by DOC leaching from litter and DOC originating from microbial sources. In 2001, decomposition (representing approximately 36% of the DOC loss), outflow to an adjacent floodplain (36%) and infiltration (28%) were the major removal mechanisms for DOM from the study floodplain. The large amount of DOC transported by infiltration implies storage of DOC in the subsurface, which may influence subsurface heterotrophic activity. In light of future climate change anticipated for the region, a scenario using a 2 °C increase in average water temperature and 10% reduction in upstream DOC mass was performed and resulted in significant (11%) reduction in annual DOC mass within the study floodplain.

P. N. Nelson, J. A. Baldock, J. M. Oades (1992). Concentration and composition of dissolved organic carbon in streams in relation to catchment soil properties. Biogeochemistry 19 (1): 27-50

ABSTRACT: Two adjacent catchments in the Otway Ranges of Victoria, Australia (Redwater and Clearwater) produce water with markedly different concentrations of dissolved organic carbon (DOC) during summer. Water from Redwater Creek had a DOC concentration of 32 mg L–1 , while water from Clearwater Creek had a DOC concentration of 3.8 mg L–1 . Examination of the catchments revealed that while climate, topography, vegetation and land use were similar, the soils were different. The objective of this study was to examine the relationship between the concentration and chemical composition of DOC in stream waters and the nature of soils in the two catchments. Soil mapping determined that clayey soils formed on Cretaceous sediments (Cretaceous soils) occurred throughout both catchments, but that Redwater Catchment also contained a large area (39%) of sandy soils formed on Tertiary sediments (Tertiary soils). The concentration of DOC in forest floor leachate was high in both the Tertiary and Cretaceous areas; however, the concentration of DOC in water draining areas dominated by Tertiary soils was greater than that in water draining areas dominated by Cretaceous soils. Laboratory experiments showed that the Cretaceous soils had higher adsorption capacities for forest floor leachate DOC than the Tertiary soils. The difference in DOC concentrations of the streams was therefore attributed to the difference in adsorption capacity of catchment soils for DOC. Adsorption capacities of the soils were found to be a function of their clay contents and specific surface areas.

Solid-state3 C nuclear magnetic resonance spectroscopy and pyrolysis-mass spectrometry were used to determine the chemical structure of DOC found in streams and forest floor leachate samples and that remaining in solution after interaction with soil. Chemistry of DOC in forest floor leachate was similar before and after interaction with soil, indicating no preferential adsorption of a particular type of carbon. Thus, differences between the chemical structure of stream DOC and forest floor leachate DOC could be attributed to microbial modifications during its movement through soils and into the streams, rather than losses by adsorption.

Ogden, R., Reid, M., Thoms, M. (2007). Soil fertility in a large dryland floodplain: Patterns, processes and the implications of water resource development. CATENA 70 (2): 114-126

ABSTRACT: 1. Primary production on semiarid floodplains supports a diverse local and regional fauna. Reduced flooding from water resource development (WRD) may affect floodplain production by decreasing water and nutrient supply.

2. We investigated the effects of reduced wetting on soil fertility by performing a regional soil survey across a gradient of flood frequency. Soil nitrogen (N), phosphorus (P) and carbon (C) were recorded over a soil-wetting event where heavy rainfall and flooding coincided.

3. Soil nutrient concentrations indicate N limits plant growth and P does not.

4. No spatial patterns in soil P were detected across the floodplain, suggesting that the principal mechanism controlling P fertility is the concentration of P in floodplain source material, and that flood mediated import and export of P are minor processes.

5. Soil N concentrations rose following rainfall and flooding and the greatest increased occurred in flooded areas. Flood deposition of N accounted for only 9% of the boost in soil N in flooded areas, and N concentrations continued to rise when the floods and rains ceased. Elevated soil N levels do not appear to persist because at the start of a growth cycle, when soils were dry, soil N did not vary significantly with flood frequency. This suggests most of the boost in soil N was due to N-fixation, with the subsequent loss of N likely to have resulted from in situ processes such as denitrification.

6. Agricultural export of nutrients appears to not be a significant process in the context of the high phosphorus fertility of floodplain source material and the apparently high rates of in situ processing of nitrogen.

7. Synthesis and applications. Our data suggest that floodplain soil fertility is controlled by mechanisms other than flood frequency or agricultural export, meaning that WRD is unlikely to affect soil fertility; however, the biological implications of the brief pulse in soil N associated with wetting need further investigation.

Onweremadu, E.U. (2007). Spatial variability of soil carbon in floodplain soils of southeastern Nigeria. International Journal of Agricultural Research 2 (6): 552-558

ABSTRACT: Variability of soil carbon in 5 floodplain soils at landscape scales were studied. Transect technique was used in soil sampling at 0, 25, 50, 75 and 100 m sampling points. Results showed that some measured soil properties varied among floodplain soils although all locations belong to the rainforest agro-ecology. The Oa horizons accounted for the highest variability in thickness (CV = 26%) in some locations. There was significant variation in total soil carbon in 60% of the floodplain soils. It is concluded that intensive soil sampling from landscape delineations will provide more accurate and reliable representation of soil carbon for scaling at regional levels.

Petrone, R.M., Chahil, P., Macrae, M.L., English, M.C. (2008). Spatial variability of CO2 exchange for riparian and open grasslands within a first-order agricultural basin in Southern Ontario. Agriculture, Ecosystems & Environment 125 (1-4): 137-147

ABSTRACT: Agriculture is one of the most widespread landuse types in Ontario, Canada, where cultivated lands are often located within small first-order catchments with extensive grass-dominated riparian areas. These grass-dominated areas may be significant to the overall catchment carbon functioning as they are often high in soil organic matter and elevated biological productivity due to the availability of agricultural nutrients. As such, this study examines midday CO2 exchange between the atmosphere and two grass-dominated riparian areas (one adjacent to a cultivated disk-tilled cornfield, one adjacent to a no-till grassland) and an open area grassland (no-till field) in a predominantly agricultural basin in Southern Ontario using daytime dynamic closed-chamber measurements from May 7 to October 27, 2003. Soils in this catchment are generally comprised of 50% sand, 35% silt and 15% clay, with riparian areas containing an organic layer approximately 0.20–0.40 m deep. The influence of nearby agriculture on riparian areas is apparent, as larger rates of CO2 exchange were correlated with elevated soil nutrients as well as above ground biomass and canopy height, which were larger in the riparian zone influenced by intensive agriculture (Rip-Corn) than the grassland-influenced riparian area (Rip-Grass) and the adjacent grassland field (Grass). Spatial patterns in soil and vegetation respiration (RTot), net ecosystem CO2 exchange (NEE) and gross ecosystem productivity (GEP) for all sites showed strong positive relationships with soil characteristics such as total nitrogen, carbon–nitrogen ratio and above-ground biomass.

Our results show that grass-dominated riparian and non-riparian areas, with similar vegetation that appear to be homogenous, located approximately 250–300 m from one another, may exhibit very different CO2 fluxes. In general, CO2 exchange is much greater in riparian areas than in the adjacent upland grasslands, and this is more exaggerated where those upland sites are tilled and receive inorganic fertilizers. Furthermore, our results indicate that the effects of land-use (i.e. agriculture) over-ride the effects of microclimate in controlling spatial patterns in CO2 exchange in this watershed. This highlights the need to better assess CO2 fluxes from heterogeneous agricultural landscapes, and emphasizes that estimates based on data grouped solely on soil or vegetation units can be rather conservative and may not capture the inherent spatial variability and small scale processes that drive CO2 exchange.

Remington, S. M., Strahm, B. D., Neu, V., Richey, J. E., Brandão da Cunha, H. (2007). The role of sorption in control of riverine dissolved organic carbon concentrations by riparian zone soils in the Amazon basin. Soil Science 172 (4): 279-291

ABSTRACT: Terrestrially derived dissolved organic carbon (DOC) is an important component of biogeochemical cycling in river channels. Despite this, the processes controlling its export from terrestrial ecosystems to river channels are not well known. Sorption is thought to be an important process in controlling riverine DOC concentrations. We describe the sorption of litter-derived DOC by soils of the Barreiras sediment formation in the Amazon basin. Soils were collected along a single transect of a soil toposequence. Clay-rich soils dominate on plateaus and slopes, whereas sandy soils dominate in valleys that compose riparian zones of the region. Soils from each topographic position were subjected to sorption experiments, and soil properties were analyzed. Based on our results, the toposequence was divided into two sorption regions. Plateau and slope soils sorbed 60 ± 5% of initial DOC, whereas valley soils sorbed 34 ± 4%. Plateau and slope soils sorbed DOC twice as quickly (t½ ≤ 1440 min) as valley soils (t½ = 2880 min). A regression of sorption experiment results and soil properties showed that sorption correlates with both soil organic C content and mineral surface area. Our results suggest that control of riverine DOC concentrations by riparian zones is the result of the sorption mechanism operating in soils of this region of the Amazon River basin. In conjunction with hydrologic models and more detailed soil data, it may be possible to apply results from similar replicated studies to the landscapes of the Amazon basin in an effort to better understand C dynamics in tropical river basins.

R. L. Scott, E. A. Edwards, W. J. Shuttleworth, T. E. Huxman, C. Watts, D. C. Goodrich (2004). Interannual and seasonal variation in fluxes of water and carbon dioxide from a riparian woodland ecosystem. Agricultural and Forest Meteorology 122 (1-2): 63-84

ABSTRACT: Fluxes of water, energy and carbon dioxide (CO2 ) were measured using the eddy covariance technique over a mesquite (Prosopis velutina) woodland along the San Pedro River in southeastern Arizona for the entire growing seasons of 2001 and 2002, between the last freeze event of spring and the first of fall. Although the general pattern of ecosystem response to climate forcing was similar in both years, latent heat and CO2 fluxes showed significant variations between and within the growing seasons. The main differences between the two years were a consequence of an extended drought that lasted from October 2001 to July 2002. Most of the within season variability was attributable to the timing and magnitude of mid-summer precipitation associated with the North American Monsoon. Following new tree leaf production and prior to the monsoon onset, there was little precipitation; daytime air temperatures were high and relative humidity low. Evapotranspiration and water level data indicated that the mesquite trees always had ready access to groundwater, though they were likely supplementing this with vadose zone soil water when abundant. Nonetheless, decreases in afternoon transpiration and CO2 uptake suggest stomatal regulation of leaf gas exchange, possibly in response to the high vapor pressure deficit. Because near-surface soil moisture was limited prior to the summer rains, ecosystem respiration was low and there was little evapotranspiration from understory plants and soil. With the arrival of the monsoon rains, understory vegetation activity and, consequently, total ecosystem evapotranspiration increased. Total ecosystem photosynthesis also increased, but the net uptake of carbon decreased, due to enhanced respiration from the abundant carbon sources, stimulated by the precipitation and warm temperatures. The nighttime measurements of CO2 fluxes, although of questionable accuracy, imply the ecosystem was a net sink of CO2 for most of the two growing seasons.

Scott, R. L., Huxman, T. E., Williams, D. G., Goodrich, D. C. (2006). Ecohydrological impacts of woody-plant encroachment: seasonal patterns of water and carbon dioxide exchange within a semiarid riparian environment. Global Change Biology 12 (2): 311-324

ABSTRACT; Across many dryland regions, historically grass-dominated ecosystems have been encroached upon by woody-plant species. In this paper, we compare ecosystem water and carbon dioxide (CO2 ) fluxes over a grassland, a grassland–shrubland mosaic, and a fully developed woodland to evaluate potential consequences of woody-plant encroachment on important ecosystem processes. All three sites were located in the riparian corridor of a river in the southwest US. As such, plants in these ecosystems may have access to moisture at the capillary fringe of the near-surface water table. Using fluxes measured by eddy covariance in 2003 we found that ecosystem evapotranspiration (ET) and net ecosystem exchange of carbon dioxide (NEE) increased with increasing woody-plant dominance. Growing season ET totals were 407, 450, and 639 mm in the grassland, shrubland, and woodland, respectively, and in excess of precipitation by 227, 265, and 473 mm. This excess was derived from groundwater, especially during the extremely dry premonsoon period when this was the only source of moisture available to plants. Access to groundwater by the deep-rooted woody plants apparently decouples ecosystem ET from gross ecosystem production (GEP) with respect to precipitation. Compared with grasses, the woody plants were better able to use the stable groundwater source and had an increased net CO2 gain during the dry periods. This enhanced plant activity resulted in substantial accumulation of leaf litter on the soil surface that, during rainy periods, may lead to high microbial respiration rates that offset these photosynthetic fluxes. March–December (primary growing season) totals of NEE were −63, −212, and −233 g C m−2 in the grassland, shrubland, and woodland, respectively. Thus, there was a greater disparity between ecosystem water use and the strength of the CO2 sink as woody plants increased across the encroachment gradient. Despite a higher density of woody plants and a greater plant productivity in the woodland than in the shrubland, the woodland produced a larger respiration response to rainfall that largely offset its higher photosynthetic potential. These data suggest that the capacity for woody plants to exploit water resources in riparian areas results in enhanced carbon sequestration at the expense of increased groundwater use under current climate conditions, but the potential does not scale specifically as a function of woody-plant abundance. These results highlight the important roles of water sources and ecosystem structure on the control of water and carbon balances in dryland areas.

Sommer, M. (2006). Influence of soil pattern on matter transport in and from terrestrial biogeosystems--A new concept for landscape pedology. Geoderma 133 (1-2): 107-123

ABSTRACT: A better understanding of spatial soil variability, its development over time (pedogenesis) and its functional relationships to recent processes in soil landscapes is one of the biggest challenge in soil science. This paper presents three case studies on the influence of soil pattern—developed in geological time scales—on actual matter transport: (i) solid phase transport in agricultural landscapes, (ii) solute transport from forested catchments, and (iii) gas fluxes from agricultural landscapes. In case study I the exclusion of sedimentation zones as well as a segmentation of soil landscapes by digital terrain analysis leads to a more realistic picture of measured erosion rates compared to area-wide modeling. Soil landscape analysis in forested catchments (case study II) identifies riparian soils to be most sensitive areas for DOC- and Fe-fluxes between terrestrial and fluvial biogeosystems. Regardless of the absolute or relative acreages, the existence of riparian soils as pedochemical barriers or zones of high element mobility determine catchment outputs. In case study III the influence of soil pattern development on the emergence of biogeochemical hot spots in grassland systems is demonstrated. Past solid phase transport (soil erosion) into wet parts of agricultural landscapes led to small fringes of very high CH4 fluxes. The latter are comparable to paddy soils in respect to unit area emissions. From the results a generalized concept for soil landscape research is developed—the so-called multiscale soil landscape analysis. Special emphasis is given to the role of “sensitive areas” in soil landscapes.

J. Steiger, A. M. Gurnell (2003). Spatial hydrogeomorphological influences on sediment and nutrient deposition in riparian zones: observations from the Garonne River, France. Geomorphology 49 (1-2): 1-23

ABSTRACT: This paper investigates the influence of geomorphological setting on riparian zone sedimentation within a reach of the River Garonne, France, during three major floods. The sampling design was stratified to reflect landforms constructed by fluvial processes (e.g. floodplain, lateral benches, islands, side channels and point bars). Observed sedimentation varied significantly with flood event, planform context, landform type and associated vegetation cover and, in some cases, with sample location within the landform. Lowest sedimentation was associated with the flood with the smallest peak discharge, peak sediment concentration and sediment load. Sites under natural riparian vegetation experienced higher sedimentation than poplar plantations. Sites on concave (outer) banks received less sedimentation than those on convex (inner banks). Sedimentation on floodplain sites and higher benches was lower than on low benches, point bars and side channels. There was considerable interdependence among these patterns, reflecting the underlying geomorphological forms and processes. Meandering rivers tend to evolve through erosion of concave banks and deposition on convex banks. Point bar features tend to be built along convex banks, whilst concave banks are eroded into higher floodplain and bench features. As a result, concave banks tend to be bordered by higher riparian margins that are less frequently flooded than convex banks. Where river margins are developed for agriculture, the higher, less frequently flooded sites are preferentially selected.

Analyses of the quantity, calibre, nutrient and carbon content of the deposited sediment reveal further significant relationships, which reflect the geomorphological structure of the riparian zone. Sediment particle size coarsens in locations with higher amounts of sedimentation. The quantities of total organic carbon (TOC), total organic nitrogen (TON) and total phosphorus (TP) all increase as the quantity of deposited sediment increases. The concentration of TOC and TON also increased significantly with an increase in the percentage of silt plus clay in the deposited sediments.

Based upon the above observations, a conceptual model is proposed, which considers the spatial pattern in riparian zone sedimentation according to riparian morphology and flood magnitude. The implications of channel incision for the functioning of the model are also discussed.

bottom right