Climate Change and...
- Climate Variability
- Climate Models
Effects of Climate Change
Tropical Forest Soils
ABSTRACT: We investigated the effects of oxygen (O2 ) concentration on methane (CH4 ) production and oxidation in two humid tropical forests that differ in long-term, time-averaged soil O2 concentrations. We identified sources and sinks of CH4 through the analysis of soil gas concentrations, surface emissions, and carbon isotope measurements. Isotope mass balance models were used to calculate the fraction of CH4 oxidized in situ. Complementary laboratory experiments were conducted to determine the effects of O2 concentration on gross and net rates of methanogenesis. Field and laboratory experiments indicated that high levels of CH4 production occurred in soils that contained between 9±1.1% and 19±0.2% O2 . For example, we observed CH4 concentrations in excess of 3% in soils with 9±1.1% O2 . CH4 emissions from the lower O2 sites were high (22–101 nmol CH4 m−2 s−1 ), and were equal in magnitude to CH4 emissions from natural wetlands. During peak periods of CH4 efflux, carbon dioxide (CO2 ) emissions became enriched in13 C because of high methanogenic activity. Gross CH4 production was probably greater than flux measurements indicated, as isotope mass balance calculations suggested that 48–78% of the CH4 produced was oxidized prior to atmospheric egress. O2 availability influenced CH4 oxidation more strongly than methanogenesis. Gross CH4 production was relatively insensitive to O2 concentrations in laboratory experiments. In contrast, methanotrophic bacteria oxidized a greater fraction of total CH4 production with increasing O2 concentration, shifting theδ13 C composition of CH4 to values that were more positive. Isotopic measurements suggested that CO2 was an important source of carbon for methanogenesis in humid forests. The δ13 C value of methanogenesis was between−84‰ and −98‰, which is well within the range of CH4 produced from CO2 reduction, and considerably more depleted in13 C than CH4 formed from acetate.
ABSTRACT: In natural ecosystems, soil organic carbon (C) is derived almost exclusively from the residues of plants growing in situ. In agroecosystems, it has at least two origins: one is the remains from the previous native vegetation, and the other is the remains of the crop and the decomposition of its residues. Where vegetation has changed from plants with the C3 photosynthetic pathway to C4 pathway or vice versa, changes in the natural abundance of13 C in soil organic matter (SOM) over time can be used to identify sources of organic C in soil and to determine the turnover rate of SOM. For example, large areas of C3 tropical forest have been replaced with C4 pasture or cropland. Changes in theδ13 C values of soil organic C in these areas reflect soil organic matter turnover rate, and provide insight regarding the functional role of tropical ecosystems in the global C cycle. This paper illustrates how the stable isotope13 C can be used to estimate SOM turnover rates and the sensitivity of different models and different model parameters, using a chronosequence of forest and pastures of different ages from the Brazilian Amazon. A single-compartment exponential decay model and a two-compartment model in which SOM was divided into stable and labile components yielded similar estimates of soil C turnover time at the surface but divergent estimates at depth. The one-compartment model gave the least variable estimates of model parameters and turnover times and was also relatively insensitive to individual C stocks in single pastures of a particular age. Estimates of soil stable and labile C pools obtained using changes in forest soil δ13 C with depth differed from estimates obtained using the chronosequence. This suggests that upon burning and pasture creation, a portion of the previously stable soil C pool is rendered less stable. Model r2 was a poor criterion for selecting an appropriate soil C turnover model to apply to chronosequence data. In the absence of substantial justification for segregating SOM into different compartments based on lability, modeling should be done with the simplest models possible.
Cerri, C.E.P., Easter, M., Paustian, K., Killian, K., Coleman, K., Bernoux, M., Falloon, P., Powlson, D.S., Batjes, N.H., Milne, E., Cerri, C.C. (2007). Predicted soil organic carbon stocks and changes in the Brazilian Amazon between 2000 and 2030. Agriculture, Ecosystems & Environment 122 (1): 58-72
ABSTRACT: Currently we have little understanding of the impacts of land use change on soil C stocks in the Brazilian Amazon. Such information is needed to determine impacts on the global C cycle and the sustainability of agricultural systems that are replacing native forest. The aim of this study was to predict soil carbon stocks and changes in the Brazilian Amazon during the period between 2000 and 2030, using the GEFSOC soil carbon (C) modelling system. In order to do so, we devised current and future land use scenarios for the Brazilian Amazon, taking into account: (i) deforestation rates from the past three decades, (ii) census data on land use from 1940 to 2000, including the expansion and intensification of agriculture in the region, (iii) available information on management practices, primarily related to well managed pasture versus degraded pasture and conventional systems versus no-tillage systems for soybean (Glycine max) and (iv) FAO predictions on agricultural land use and land use changes for the years 2015 and 2030. The land use scenarios were integrated with spatially explicit soils data (SOTER database), climate, potential natural vegetation and land management units using the recently developed GEFSOC soil C modelling system. Results are presented in map, table and graph form for the entire Brazilian Amazon for the current situation (1990 and 2000) and the future (2015 and 2030). Results include soil organic C (SOC) stocks and SOC stock change rates estimated by three methods: (i) the Century ecosystem model, (ii) the Rothamsted C model and (iii) the intergovernmental panel on climate change (IPCC) method for assessing soil C at regional scale. In addition, we show estimated values of above and belowground biomass for native vegetation, pasture and soybean. The results on regional SOC stocks compare reasonably well with those based on mapping approaches. The GEFSOC system provided a means of efficiently handling complex interactions among biotic-edapho-climatic conditions (>363,000 combinations) in a very large area (500 Mha) such as the Brazilian Amazon. All of the methods used showed a decline in SOC stock for the period studied; Century and RothC simulated values for 2030 being about 7% lower than those in 1990. Values from Century and RothC (30,430 and 25,000 Tg for the 0–20 cm layer for the Brazilian Amazon region were higher than those obtained from the IPCC system (23,400 Tg in the 0–30 cm layer). Finally, our results can help understand the major biogeochemical cycles that influence soil fertility and help devise management strategies that enhance the sustainability of these areas and thus slow further deforestation.
ABSTRACT: The assumption that the organic matter content of tropical forest soils is oxidized to atmospheric carbon dioxide when these soils are converted to agricultural use was tested using results of soil surveys in Puerto Rico (1940's, 1960's, and 1980's). Results showed that under intensive agricultural use, soil carbon in the top 18 cm of soil was about 30–37 Mg/ha, regardless of climatic conditions. Reduced intensity of agricultural use resulted in an increase of soil carbon in the order of 0.3–0.5 Mg ha−1 yr−1 over a 40-yr period. Rates of soil carbon accumulation were inversely related to the sand content of soils. The relation between rates of soil carbon accumulation and climate or soil texture were better defined at higher soil carbon content. Soils under pasture accumulated soil carbon and often contained similar or greater amounts than adjacent mature forest soils (60–150 Mg/ha in the top 25 or 50 cm). Soils in moist climates exhibited greater variations in soil carbon content with changes in land use (both in terms of loss and recovery) than did soils in dry climates. However, in all life zones studied, the recovery of soil carbon after abandonment of agriculture was faster than generally assumed. Low carbon-to-nitrogen ratios suggested that intensively used soils may be stable in their nutrient retention capacity. The observed resiliency of these soils suggested that their role as atmospheric carbon sources has been overestimated, while their potential role as atmospheric carbon sinks has been underestimated.
ABSTRACT: The prevailing paradigm for anticipating changes in soil organic carbon (SOC) with changes in land use postulates reductions in SOC in managed systems (agriculture and tree plantations) relative to mature tropical forests. Variations of this notion are used in carbon models to predict the role of tropical soils in the global carbon cycle. Invariably these models show tropical soils as sources of atmospheric carbon. We present data from a variety of studies that show that SOC in managed systems can be lower, the same as, or greater than mature tropical forests and that SOC can increase rapidly after the abandonment of agricultural fields. History of land use affects the comparison of SOC in managed and natural ecosystems. Our review of the literature also highlights the need for greater precautions when comparing SOC in mature tropical forests with that of managed ecosystems. Information on previous land use, bulk density, and consistency in sampling depth are some of the most common omissions in published studies. From comparable SOC data from a variety of tropical land uses we estimate that tropical soils can accumulate between 168 and 553 Tg C/yr. The greatest potential for carbon sequestration in tropical soils is in the forest fallows which cover some 250 million hectares. Increased attention to SOC by land managers can result in greater rates of carbon sequestration than predicted by current SOC models.
SUMMARY: Tropical peatlands are important sources and sinks of atmospheric methane (CH4 ) and major sources of carbon dioxide (CO2 ) and nitrous oxide (N2 O). Recently, large areas of tropical peatland have been developed for agriculture plantations in Southeast Asia whereby drainage is a prerequisite, which can increase greenhouse gas (GHG) emissions substantially and therefore, global warming potential (GWP). Despite this, there is still a paucity of knowledge on GHG emissions from different ecosystems on tropical peatland and their roles and contribution to the global gas budget. Thus, three ecosystems from tropical peatland of Sarawak, Malaysia, mixed peatswamp forest, oil palm (Elaeis guineensis ) plantation and sago (Metroxylon sagu ) plantation, were chosen for the study of GHG emissions from the soils to determine their contribution towards GWP. The GHG emissions were measured monthly over 12 months using a closed chamber technique.
GWP from forest soils was higher (7850 g CO2 m-2 y-1 ) compared with oil palm ecosystem (5706 g CO2 m-2 y-1 ) and sago ecosystem (4233 g CO2 m-2 y-1 ). A high GWP in forest ecosystem was due to its high soil respiration rate of 7817 g CO2 m-2 y-1 . Soil respiration rates for sago and oil palm were 4074 g CO2 m-2 y-1 and 5652 g CO2 m-2 y-1 respectively. About 4 % of GWP from peat soils in sago ecosystem was due to CH4 (5.5 g CO2 m-2 y-1 ) and N2 O (153.4 g CO2 m-2 y-1 ) emissions, which were negligible in forest and oil palm ecosystems. Thus, the GWP of the soils in the three ecosystems on tropical peatland were mainly dominated by CO2 fluxes from the soil implying that tropical peatlands may function as a source for atmospheric CO2 on a global scale.
ABSTRACT: Not Available
Sotta, E.D., Veldkamp, E., Guimaraes, B.R., Paixao, R.K., Ruivo, M.L.P., Almeida, S.S. (2006). Landscape and climatic controls on spatial and temporal variation in soil CO2 efflux in an Eastern Amazonian Rainforest, Caxiuanã, Brazil. Forest Ecology and Management 237 (1-3): 57-64
ABSTRACT: Quantification of temporal and spatial variation of soil CO2 emissions is essential for an accurate interpretation of tower-based measurements of net ecosystem exchange. Here, we measured in the old-growth forest of Caxiuana, Eastern Amazonia soil CO2 efflux and its environmental controls from two Oxisol sites with contrasting soil texture and at different landscape positions. Average CO2 efflux was 21% higher for sand (3.93 +/- 0.06 [mu]mol CO2 m-2 s-1 ) than for the clay (3.08 +/- 0.07 [mu]mol CO2 m-2 s-1 ). No difference was detected for soil temperature between sites, while soil water content in sandy soil (23.2 +/- 0.33%) was much lower than the clay soil (34.5 +/- 0.98%), for the 2-year period. Soil CO2 efflux did not differ between dry and wet season, but we detected a significant interaction between season and topographic position. The variation caused by topography was in the same order of magnitude as temporal variation. Mean contribution of the litter layer to the CO2 efflux rates was 20% and varied from 25% during the wet season to close to 0% during the dry season. The relation between soil water content and soil CO2 efflux showed an optimum for both soil textures but the shape and optimum of the curves were different. The results of our study illustrate that soil moisture is an important driver of temporal variations in soil CO2 efflux in this old-growth forest. When extrapolating soil CO2 efflux to larger areas, the significant influences of soil texture, litter, and the interaction of topographical position and time illustrate that it is necessary to include some of the complexity of landscapes.
Trumbore, Susan, Da Costa, Enir Salazar, Nepstad, Daniel C., Barbosa De Camargo, Pl+¡nio, Martinelli, Luiz A., Ray, David, Restom, Teresa, Silver, Whendee (2006). Dynamics of fine root carbon in Amazonian tropical ecosystems and the contribution of roots to soil respiration.. Global Change Biology 12 (2): 217-229
ABSTRACT: Radiocarbon (14 C) provides a measure of the mean age of carbon (C) in roots, or the time elapsed since the C making up root tissues was fixed from the atmosphere. Radiocarbon signatures of live and dead fine (<2 mm diameter) roots in two mature Amazon tropical forests are consistent with average ages of 4–11 years (ranging from <1 to >40 years). Measurements of14 C in the structural tissues of roots known to have grown during 2002 demonstrate that new roots are constructed from recent (<2-year-old) photosynthetic products. HighΔ14 C values in live roots most likely indicate the mean lifetime of the root rather than the isotopic signature of inherited C or C taken up from the soil.Estimates of the mean residence time of C in forest fine roots (inventory divided by loss rate) are substantially shorter (1–3 years) than the age of standing fine root C stocks obtained from radiocarbon (4–11 years). By assuming positively skewed distributions for root ages, we can effectively decouple the mean age of C in live fine roots (measured using14 C) from the rate of C flow through the live root pool, and resolve these apparently disparate estimates of root C dynamics. Explaining the14 C values in soil pore space CO2 , in addition, requires that a portion of the decomposing roots be cycled through soil organic matter pools with decadal turnover time.
Ullah, S., Frasier, R., King, L., Picotte-Anderson, N., Moore, T.R. (2008). Potential fluxes of N2 O and CH4 from soils of three forest types in Eastern Canada. Soil Biology and Biochemistry 40 (4): 986-994
ABSTRACT: We conducted laboratory incubation experiments to elucidate the influence of forest type and topographic position on emission and/or consumption potentials of nitrous oxide (N2 O) and methane (CH4 ) from soils of three forest types in Eastern Canada. Soil samples collected from deciduous, black spruce and white pine forests were incubated under a control, an NH4 NO3 amendment and an elevated headspace CH4 concentration at 70% water-filled pore space (WFPS), except the poorly drained wetland soils which were incubated at 100% WFPS. Deciduous and boreal forest soils exhibited greater potential of N2 O and CH4 fluxes than did white pine forest soils. Mineral N addition resulted in significant increases in N2 O emissions from wetland forest soils compared to the unamended soils, whereas well-drained soils exhibited no significant increase in N2 O emissions in-response to mineral N additions. Soils in deciduous, boreal and white pine forests consumed CH4 when incubated under an elevated headspace CH4 concentration, except the poorly drained soils in the deciduous forest, which emitted CH4 . CH4 consumption rates in deciduous and boreal forest soils were twice the amount consumed by the white pine forest soils. The results suggest that an episodic increase in reactive N input in these forests is not likely to increase N2 O emissions, except from the poorly drained wetland soils; however, long-term in situ N fertilization studies are required to validate the observed results. Moreover, wetland soils in the deciduous forest are net sources of CH4 unlike the well-drained soils, which are net sinks of atmospheric CH4 . Because wetland soils can produce a substantial amount of CH4 and N2 O, the contribution of these wetlands to the total trace gas fluxes need to be accounted for when modeling fluxes from forest soils in Eastern Canada.
ABSTRACT: Conversion of natural forests to agriculture in the humid tropics leads to a reduction in ecosystem carbon storage due to the immediate removal of aboveground biomass and a gradual subsequent reduction in soil organic carbon. A considerable part of soil carbon is protected from microbial attack by a range of physical and chemical mechanisms and is not sensitive to landuse change. We analyzed the soils data base for Sumatra (Indonesia) developed by the Center for Soil and Agroclimate Research (CSAR) to estimate effects of landuse on soil C content. Sumatra has a considerable diversity of soils ranging from those of recent origin in the highlands, to older sedimentary and heavily leached soils in the pedimont peneplain and large areas of wetland soils along the coast. Peat soils (Histosols) and other wetland soils (Aquic and Fluvic suborders) contain the greatest soil C reserves, followed by young volcanic soils (Andisols). Agricultural use of these soils can have a disproportionately large effect on C release to the atmosphere. On the major part of the upland soils the difference in (top) soil C content between natural forest and agricultural land is in the range 0.5–1.0% C, equivalent to a change in total C stock of 10–20 Mg ha−1 . These results agree with data collected in S. Sumatra in the 1930s. Corg of forest soils is related to soil pH, and is lowest in the pH range 5.0–6.0. Wetland conditions, lower pH, higher altititude (lower temperature) and higher clay and silt content all contributed to higher soil C contents in a multiple regression analysis of the whole data set. Existing models and data sets are insufficient to predict changes in soil C contents under various landuse practices. Carbon isotope studies, and especially thed13 C method may be used to study the effects of landuse change, especially when the vegetation was changed from one dominated by C3 plants (most forest species) to one dominated by grasses and crops with a C4 photosynthetic pathway. Results from Brazil documented a gradual decline of organic matter originating from the forest system and its partial replacement by organic matter derived from inputs of sugarcane during the first fifty years of cultivation. Forest conversion to well managed grasslands may lead to an increased soil C storage, after an initial decline. The consequences of erosion on losses of soil C depend on the scale at which these losses are considered, because of sedimentation processes. When net erosion losses are not expressed per unit area, but per length scale to the power l.6, erosion losses appear to be largely independent of scale. The'fractal dimension of erosion’ (on average around 1.6) probably is a landscape characteristic and estimates of its value are needed for extrapolation. Better understanding of soil C deposition sites is needed to evaluate overall erosion effects and test whether or not erosion can contribute to net C sequestration.
ABSTRACT: Tropical forests are responsible for a large proportion of the global terrestrial C flux annually for natural ecosystems. Increased atmospheric CO2 and changes in climate are likely to affect the distribution of C pools in the tropics and the rate of cycling through vegetation and soils. In this paper, I review the literature on the pools and fluxes of carbon in tropical forests, and the relationship of these to nutrient cycling and climate. Tropical moist and humid forests have the highest rates of annual net primary productivity and the greatest carbon flux from soil respiration globally. Tropical dry forests have lower rates of carbon circulation, but may have greater soil organic carbon storage, especially at depths below 1 meter. Data from tropical elevation gradients were used to examine the sensitivity of biogeochemical cycling to incremental changes in temperature and rainfall. These data show significant positive correlations of litterfall N concentrations with temperature and decomposition rates. Increased atmospheric CO2 and changes in climate are expected to alter carbon and nutrient allocation patterns and storage in tropical forest. Modeling and experimental studies suggest that even a small increase in temperature and CO2 concentrations results in more rapid decomposition rates, and a large initial CO2 efflux from moist tropical soils. Soil P limitation or reductions in C:N and C:P ratios of litterfall could eventually limit the size of this flux. Increased frequency of fires in dry forest and hurricanes in moist and humid forests are expected to reduce the ecosystem carbon storage capacity over longer time periods
ABSTRACT: Tropical forest ecosystems are large stores of carbon which supply millions of people with life support requirements. Currently tropical forests are undergoing massive deforestation. Here, I address the possible impact of global change conditions, including elevated CO2 , temperature rise, and nitrogen deposition on forest structure and dynamics. Tropical forests may be particularly susceptible to climate change for the following reasons: (1) Phenological events (such as flowering and fruiting) are highly tuned to climatic conditions. Thus a small change in climate can have a major impact on the forest, its biological diversity and its role in the carbon cycle. (2) There are strong coevolutionary interactions, such as pollination seed dispersal, with a high degree of specialization, i.e., only certain animals can effect these activities for certain species. Global change can decouple these tight coevolutionary interactions. (3) Because of high species diversity per unit area, species of the tropical rain forest must have narrow niches. Thus changes in global climate can eliminate species and therefore reduce biological diversity. (4) Deforestation and other forms of disturbance may have significant feedback on hydrology both regionally and globally. The predicted decline in the rainfall in the Amazon Basin and the intensification of the Indian monsoon can have a large effect on water availability and floods which are already devastating low-lying areas. It is concluded that tropical forests may be very sensitive to climate change. Under climatic change conditions their structure and function may greatly change, their integrity may be violated and their services to people may be greatly modified. Because they are large stores of great biological diversity, they require immediate study before it is too late. The study requires the collaboration of scientists with a wide range of backgrounds and experiences including biologists, climate modellers, atmospheric scientists, economists, human demographers and sociologists in order to carry out holistic and urgently needed work. Global climatic change brings a great challenge to science and to policy makers.
D. A. Clark (2004). Sources or sinks? The responses of tropical forests to current and future climate and atmospheric composition. Philosophical Transactions of the Royal Society Series B: Biological Sciences 359 (1443): 477-491
ABSTRACT: How tropical rainforests are responding to the ongoing global changes in atmospheric composition and climate is little studied and poorly understood. Although rising atmospheric carbon dioxide (CO2 ) could enhance forest productivity, increased temperatures and drought are likely to diminish it. The limited field data have produced conflicting views of the net impacts of these changes so far. One set of studies has seemed to point to enhanced carbon uptake; however, questions have arisen about these findings, and recent experiments with tropical forest trees indicate carbon saturation of canopy leaves and no biomass increase under enhanced CO2 . Other field observations indicate decreased forest productivity and increased tree mortality in recent years of peak temperatures and drought (strong El Niño episodes). To determine current climatic responses of forests around the world tropics will require careful annual monitoring of ecosystem performance in representative forests. To develop the necessary process-level understanding of these responses will require intensified experimentation at the whole-tree and stand levels. Finally, a more complete understanding of tropical rainforest carbon cycling is needed for determining whether these ecosystems are carbon sinks or sources now, and how this status might change during the next century.