Soil carbon stocks of the Brazilian Amazon Basin Moraes, J.L. (Centro de Energia Nuclear na Agricultura, Piracicaba, Brazil); Cerri, C.C; Melillo, J.M ...
Soil Science Society of America journal,
January-February 1995, Volume:
59, Issue:
1
Journal Article
Peer reviewed
We determined stocks of C and N for soils under undisturbed vegetation across the Brazilian Amazon Basin based on 1162 soil profiles of the RADAMBRASIL survey and a digitized Brazilian soil survey ...map. Mean basin soil C density was 10.3 kg C m -2. Forty-seven petagrams C and 4.4 Pg N were contained in the top 1 m of soil. Forty-five percent of total basin soil C (21 Pg C) and 41% of total soil N (1.8 Pg N) were contained in the top 20 cm across an approximately 5 000 000-km2 area. Mean C/N ratio for the basin to a depth of 1 m was 10.7. Because these data represent sites with forest vegetation in the absence of significant disturbances, they represent a valuable baseline for evaluating the effects of land-use changes on soil C stocks in the Amazon Basin
Decay processes in an ecosystem can be thought of as a continuum beginning with the input of plant litter and leading to the formation of soil organic matter. As an example of this continuum, we ...review a 77-month study of the decay of red pine (Pinus resinosa Ait.) needle litter. We tracked the changes in C chemistry and the N pool in red pine (Pinus resinosa Ait.) needle litter during the 77-month period using standard chemical techniques and stable isotope analyses of C and N. Mass loss is best described by a two-phase model: an initial phase of constant mass loss and a phase of very slow loss dominated by degradation of 'lignocellulose' (acid soluble sugars plus acid insoluble C compounds). As the decaying litter enters the second phase, the ratio of lignin to lignin and cellulose (the lignocellulose index, LCI) approaches 0.7. Thereafter, the LCI increases only slightly throughout the decay continuum indicating that acid insoluble materials ('lignin') dominate decay in the latter part of the continuum. Nitrogen dynamics are also best described by a two-phase model: a phase of N net immobilization followed by a phase of N net mineralization. Small changes in C and N isotopie composition were observed during litter decay. Larger changes were observed with depth in the soil profile. An understanding of factors that control 'lignin' degradation is key to predicting the patterns of mass loss and Í dynamics late in decay. The hypothesis that labile C is needed for iignin' degradation must be evaluated and the sources of this C must be identified. Also, the hypothesis that the availability of inorganic N slows 'lignin' decay must be evaluated in soil systems.
The availability of labile organic C for microbial metabolic processes could be an important factor regulating N^sub 2^O emissions from tropical soils. We explored the effects of labile C on the ...emissions of N^sub 2^O from a forest soil in the State of Rondônia in the southwestern quadrant of the Brazilian Amazon. We measured emissions of N^sub 2^O from a forest soil after amendments with solutions containing glucose, water only or NO^sub 3^^sup -^. Addition of glucose to the forest soil resulted in very large increases in N^sub 2^O emissions whereas the water only and NO^sub 3^^sup -^ additions did not. These results suggest a strong C limitation on N^sub 2^O production in this forest soil in the southwestern Amazon.PUBLICATION ABSTRACT
Although there is a great deal of information concerning responses to increases in atmospheric CO₂ at the tissue and plant levels, there are substantially fewer studies that have investigated ...ecosystem-level responses in the context of integrated carbon, water, and nutrient cycles. Because our understanding of ecosystem responses to elevated CO₂ is incomplete, modeling is a tool that can be used to investigate the role of plant and soil interactions in the response of terrestrial ecosystems to elevated CO₂. In this study, we analyze the responses of net primary production (NPP) to doubled CO₂ from 355 to 710 ppmv among three biogeochemistry models in the Vegetation/Ecosystem Modeling and Analysis Project (VEMAP): BIOME-BGC (BioGeochemical Cycles), Century, and the Terrestrial Ecosystem Model (TEM). For the conterminous United States, doubled atmospheric CO₂ causes NPP to increase by 5% in Century, 8% in TEM, and 11% in BIOME-BGC. Multiple regression analyses between the NPP response to doubled CO₂ and the mean annual temperature and annual precipitation of biomes or grid cells indicate that there are negative relationships between precipitation and the response of NPP to doubled CO₂ for all three models. In contrast, there are different relationships between temperature and the response of NPP to doubled CO₂ for the three models: there is a negative relationship in the responses of BIOME-BGC, no relationship in the responses of Century, and a positive relationship in the responses of TEM. In BIOME-BGC, the NPP response to doubled CO₂ is controlled by the change in transpiration associated with reduced leaf conductance to water vapor. This change affects soil water, then leaf area development and, finally, NPP. In Century, the response of NPP to doubled CO₂ is controlled by changes in decomposition rates associated with increased soil moisture that results from reduced evapotranspiration. This change affects nitrogen availability for plants, which influences NPP. In TEM, the NPP response to doubled CO₂ is controlled by increased carboxylation which is modified by canopy conductance and the degree to which nitrogen constraints cause down-regulation of photosynthesis. The implementation of these different mechanisms has consequences for the spatial pattern of NPP responses, and represents, in part, conceptual uncertainty about controls over NPP responses. Progress in reducing these uncertainties requires research focused at the ecosystem level to understand how interactions between the carbon, nitrogen, and water cycles influence the response of NPP to elevated atmospheric CO₂.
Summary
We are developing a process‐based modelling approach to investigate how carbon (C) storage of tundra across the entire Arctic will respond to projected climate change. To implement the ...approach, the processes that are least understood, and thus have the most uncertainty, need to be identified and studied. In this paper, we identified a key uncertainty by comparing the responses of C storage in tussock tundra at one site between the simulations of two models – one a global‐scale ecosystem model (Terrestrial Ecosystem Model, TEM) and one a plot‐scale ecosystem model (General Ecosystem Model, GEM). The simulations spanned the historical period (1921–94) and the projected period (1995–2100). In the historical period, the model simulations of net primary production (NPP) differed in their sensitivity to variability in climate. However, the long‐term changes in C storage were similar in both simulations, because the dynamics of heterotrophic respiration (RH) were similar in both models. In contrast, the responses of C storage in the two model simulations diverged during the projected period. In the GEM simulation for this period, increases in RH tracked increases in NPP, whereas in the TEM simulation increases in RH lagged increases in NPP. We were able to make the long‐term C dynamics of the two simulations agree by parameterizing TEM to the fast soil C pools of GEM. We concluded that the differences between the long‐term C dynamics of the two simulations lay in modelling the role of the recalcitrant soil C. These differences, which reflect an incomplete understanding of soil processes, lead to quite different projections of the response of pan‐Arctic C storage to global change. For example, the reference parameterization of TEM resulted in an estimate of cumulative C storage of 2032 g C m−2 for moist tundra north of 50°N, which was substantially higher than the 463 g C m−2 estimated for a parameterization of fast soil C dynamics. This uncertainty in the depiction of the role of recalcitrant soil C in long‐term ecosystem C dynamics resulted from our incomplete understanding of controls over C and N transformations in Arctic soils. Mechanistic studies of these issues are needed to improve our ability to model the response of Arctic ecosystems to global change.
An atmospheric transport model and observations of atmospheric CO2 are used to evaluate the performance of four Terrestrial Carbon Models (TCMs) in simulating the seasonal dynamics and interannual ...variability of atmospheric CO2 between 1980 and 1991. The TCMs were forced with time varying atmospheric CO2 concentrations, climate, and land use to simulate the net exchange of carbon between the terrestrial biosphere and the atmosphere. The monthly surface CO2 fluxes from the TCMs were used to drive the Model of Atmospheric Transport and Chemistry and the simulated seasonal cycles and concentration anomalies are compared with observations from several stations in the CMDL network. The TCMs underestimate the amplitude of the seasonal cycle and tend to simulate too early an uptake of CO2 during the spring by approximately one to two months. The model fluxes show an increase in amplitude as a result of land‐use change, but that pattern is not so evident in the simulated atmospheric amplitudes, and the different models suggest different causes for the amplitude increase (i.e., CO2 fertilization, climate variability or land use change). The comparison of the modeled concentration anomalies with the observed anomalies indicates that either the TCMs underestimate interannual variability in the exchange of CO2 between the terrestrial biosphere and the atmosphere, or that either the variability in the ocean fluxes or the atmospheric transport may be key factors in the atmospheric interannual variability.
The role of carbon (C) and nitrogen (N) interactions on sequestration of atmospheric CO2 in black spruce ecosystems across North America was evaluated with the Terrestrial Ecosystem Model (TEM) by ...applying parameterizations of the model in which C–N dynamics were either coupled or uncoupled. First, the performance of the parameterizations, which were developed for the dynamics of black spruce ecosystems at the Bonanza Creek Long-Term Ecological Research site in Alaska, were evaluated by simulating C dynamics at eddy correlation tower sites in the Boreal Ecosystem Atmosphere Study (BOREAS) for black spruce ecosystems in the northern study area (northern site) and the southern study area (southern site) with local climate data. We compared simulated monthly growing season (May to September) estimates of gross primary production (GPP), total ecosystem respiration (RESP), and net ecosystem production (NEP) from 1994 to 1997 to available field-based estimates at both sites. At the northern site, monthly growing season estimates of GPP and RESP for the coupled and uncoupled simulations were highly correlated with the field-based estimates (coupled: R2= 0.77, 0.88 for GPP and RESP; uncoupled: R2 = 0.67, 0.92 for GPP and RESP). Although the simulated seasonal pattern of NEP generally matched the field-based data, the correlations between field-based and simulated monthly growing season NEP were lower (R2 = 0.40, 0.00 for coupled and uncoupled simulations, respectively) in comparison to the correlations between field-based and simulated GPP and RESP. The annual NEP simulated by the coupled parameterization fell within the uncertainty of field-based estimates in two of three years. On the other hand, annual NEP simulated by the uncoupled parameterization only fell within the field-based uncertainty in one of three years. At the southern site, simulated NEP generally matched field-based NEP estimates, and the correlation between monthly growing season field-based and simulated NEP (R2 = 0.36, 0.20 for coupled and uncoupled simulations, respectively) was similar to the correlations at the northern site. To evaluate the role of N dynamics in C balance of black spruce ecosystems across North America, we simulated historical and projected C dynamics from 1900 to 2100 with a global-based climatology at 0.5° resolution (latitude × longitude) with both the coupled and uncoupled parameterizations of TEM. From analyses at the northern site, several consistent patterns emerge. There was greater inter-annual variability in net primary production (NPP) simulated by the uncoupled parameterization as compared to the coupled parameterization, which led to substantial differences in inter-annual variability in NEP between the parameterizations. The divergence between NPP and heterotrophic respiration was greater in the uncoupled simulation, resulting in more C sequestration during the projected period. These responses were the result of fundamentally different responses of the coupled and uncoupled parameterizations to changes in CO2 and climate. Across North American black spruce ecosystems, the range of simulated decadal changes in C storage was substantially greater for the uncoupled parameterization than for the coupled parameterization. Analysis of the spatial variability in decadal responses of C dynamics revealed that C fluxes simulated by the coupled and uncoupled parameterizations have different sensitivities to climate and that the climate sensitivities of the fluxes change over the temporal scope of the simulations. The results of this study suggest that uncertainties can be reduced through (1) factorial studies focused on elucidating the role of C and N interactions in the response of mature black spruce ecosystems to manipulations of atmospheric CO2 and climate, (2) establishment of a network of continuous, long-term measurements of C dynamics across the range of mature black spruce ecosystems in North America, and (3) ancillary measurements in the network to elucidate the role of C and N interactions in exchange of CO2 with the atmosphere.
To investigate the influence of forest conversion to pasture on soil N transformations, we compared soil inorganic-N pools and net mineralization and nitrification rates along two chronosequences of ...upland (
terra firme) forest and pastures ranging in age from 4 to 82 years in the state of Rondônia in the western Brazilian Amazon Basin. Forest and pasture soils had similar total extractable inorganic-N pools at 0–5 and 5–10 cm depths. Ammonium-N and NO
3
−N pools were of similar magnitude in forest soils (2–10 μg N g
−1 dry soil), while NH
4
+N dominated pasture soil inorganic-N pools. Annual average net N mineralization rates for the two chronosequences at 0–5 cm depth in the forests were 1.31–1.88 μg N g
−1 d.s. d
−1 and exceeded the annual average net N mineralization rates measured in pastures of −0.11-0.02 μg N g
−1 d.s. d
−1. Annual average net nitrification rates at 0–5 cm depth in forest (1.09–1.46 μg N g
−1 d.s. d
−1) were also higher than in pastures (0.24–0.25 μg N g
−1 d.s. d
−1). Pasture soils had lower net N mineralization and net nitrification rates than forest soils even though they had approximately equal or higher total C and total N content. Pasture age did not affect NH
4
+N pools or net nitrification rates, but decreased NO
3
−N pools and net N mineralization rates. Net N mineralization rate was unaffected by soil moisture, but net nitrification rate decreased at higher soil moisture. Higher net mineralization and nitrification rates in forest soils suggest a higher potential for NO
3
−N losses either through leaching or gaseous emissions from intact forests compared with established pastures.
We measured the responses of nitric oxide (NO), nitrous oxide (N2O) and carbon dioxide (CO2) to nitrogen (N) and/or phosphorus (P) fertilization in a mature moist tropical forest and an 11‐year‐old ...pasture in the Brazilian Amazon. Nitrogen was applied in two forms, ammonium (NH4+) and nitrate (NO3−). In the forest, NO emissions increased by 4 to 9 times the controls in the NH4+ amended plots. Nitrous oxide emissions showed a small response only in the NH4+ amended plots. In the pasture, NO emissions during the first 7 days after fertilization with either form of N were about twice those in the control plots. Nitrous oxide emissions increased more than 18 times the controls in the NO3− amended plots 1 day after fertilization. The estimated yields of total nitrogen oxide loss from the forest were between 0.2 and 1.6% of the applied nitrogen, predominately as NO. Pasture yields were greater, up to 2.8% of the applied nitrogen, predominately as N2O. In the context of Rondônia and other regions in the Amazon Basin, pasture management practices are changing to include increased use of fertilizer, particularly in older pastures that have lower NO and N2O emissions than the original intact forests. This may lead to large short‐term releases of N2O and alter the future N2O emissions from the Basin.
Management of ecosystems at large regional or continental scales and determination of the vulnerability of ecosystems to large-scale changes in climate or atmospheric chemistry require understanding ...how ecosystem processes are governed at large spatial scales. A collaborative project, the Vegetation and Ecosystem Modeling and Analysis Project (VEMAP), addressed modeling of multiple resource limitation at the scale of the conterminous United States, and the responses of ecosystems to environmental change. In this paper, we evaluate the model-generated patterns of spatial variability within and between ecosystems using Century, TEM, and Biome-BGC, and the relationships between modeled water balance, nutrients, and carbon dynamics. We present evaluations of models against mapped and site-specific data. In this analysis, we compare model-generated patterns of variability in net primary productivity (NPP) and soil organic carbon (SOC) to, respectively, a satellite proxy and mapped SOC from the VEMAP soils database (derived from USDA-NRCS Natural Resources Conservation Service information) and also compare modeled results to site-specific data from forests and grasslands. The VEMAP models simulated spatial variability in ecosystem processes in substantially different ways, reflecting the models' differing implementations of multiple resource limitation of NPP. The models had substantially higher correlations across vegetation types compared to within vegetation types. All three models showed correlation among water use, nitrogen availability, and primary production, indicating that water and nutrient limitations of NPP were equilibrated with each other at steady state. This model result may explain a number of seemingly contradictory observations and provides a series of testable predictions. The VEMAP ecosystem models were implicitly or explicitly sensitive to disturbance in their simulation of NPP and carbon storage. Knowledge of the effects of disturbance (human and natural) and spatial data describing disturbance regimes are needed for spatial modeling of ecosystems. Improved consideration of disturbance is a key "next step" for spatial ecosystem models.
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