We have estimated the emission of carbon (C) and carbon‐containing trace gases including CO2, CO, CH4, and NMHC (nonmethane hydrocarbons) from forest fires in China for the time period from 1950 to ...2000 by using a combination of remote sensing, forest fire inventory, and terrestrial ecosystem modeling. Our results suggest that mean annual carbon emission from forest fires in China is about 11.31 Tg per year, ranging from a minimum level of 8.55 Tg per year to a maximum level of 13.9 Tg per year. This amount of carbon emission is resulted from the atmospheric emissions of four trace gases as follows: (1) 40.66 Tg CO2 with a range from 29.21 to 47.53 Tg, (2) 2.71 Tg CO with a range from 1.48 to 4.30 Tg, (3) 0.112 Tg CH4 with a range from 0.06 to 0.2 Tg, and (4) 0.113 Tg NMHC with a range from 0.05 to 0.19 Tg. Our study indicates that fire‐induced carbon emissions show substantial interannual and decadal variations before 1980 but have remained relatively low and stable since 1980 because of the application of fire suppression. Large spatial variation in fire‐induced carbon emissions exists due to the spatial variability of climate, forest types, and fire regimes.
The Amazon Basin contains almost one-half of the world's undisturbed tropical evergreen forest as well as large areas of tropical savanna,. The forests account for about 10 per cent of the world's ...terrestrial primary productivity and for a similar fraction of the carbon stored in land ecosystems,, and short-term field measurements suggest that these ecosystems are globally important carbon sinks. But tropical land ecosystems have experienced substantial interannual climate variability owing to frequent El Niño episodes in recent decades. Of particular importance to climate change policy is how such climate variations, coupled with increases in atmospheric CO2 concentration, affect terrestrial carbon storage. Previous model analyses have demonstrated the importance of temperature in controlling carbon storage,. Here we use a transient process-based biogeochemical model of terrestrial ecosystems, to investigate interannual variations of carbon storage in undisturbed Amazonian ecosystems in response to climate variability and increasing atmospheric CO2 concentration during the period 1980 to 1994. In El Niño years, which bring hot, dry weather to much of the Amazon region, the ecosystems act as a source of carbon to the atmosphere (up to 0.2 petagrams of carbon in 1987 and 1992). In other years, these ecosystems act as a carbon sink (up to 0.7 Pg C in 1981 and 1993). These fluxes are large; they compare to a 0.3 Pg C per year source to the atmosphere associated with deforestation inthe Amazon Basin in the early 1990s. Soil moisture, which is affected by both precipitation and temperature, and which affects both plant and soil processes, appears to be an important control on carbon storage.
Reported in this paper are foliar chemistry, tree growth (above- and below-ground), soil chemistry, nitrogen cycling (net mineralization and nitrification) and soil $\mathrm{N}_2\mathrm{O}$ flux ...responses to the first 6 yr of chronic nitrogen amendments at the Harvard Forest (Massachusetts, USA). A 70-yr-old red pine (Pinus resinosa Ait.) stand and a 50-yr-old mixed hardwood stand received control, low nitrogen (50 kg$\cdot$ ha$^{-1}\cdot$ yr$^{-1})$, high nitrogen (150 kg$\cdot$ ha$^{-1}\cdot$ yr$^{-1})$, and low nitrogen plus sulfur treatments, with additions occurring in six equal doses over the growing season as NH$_4$NO$_3$ and Na$_2$SO$_4$. Foliar N concentrations increased up to 25% in the hardwood stand and 67% in the pines, and there was no apparent decrease of N retranslocation due to fertilization. Wood production increased in the hardwood stand in response to fertilization but decreased in the pine stand. Fine-root nitrogen concentrations increased with N additions, and fine roots were a significant sink for added nitrogen. Nitrate leaching losses increased continuously over the 6-yr period in the treated pine stands but remained insignificant in the hardwoods. Annual net N mineralization increased substantially in response to treatments in both stands but declined in the pine high-N plot by the end of year six. Net nitrification increased from 17% of net mineralization in 1988 to 51% in 1993 for the pine high-N plot. Only a slight increase in net nitrification was measured in the hardwood stand, and only in 1993. Extractable $\mathrm{NH}_4$ was consistently higher in treated plots than in controls in both stands, where extractable $\mathrm{NO}_3$ was higher than controls only in the treated pine plots. Soil extracts yielded $<$1.5 kg/ha of $\mathrm{NO}_3$-N for all plots in the hardwood stand throughout the experiment. Effluxes of $\mathrm{N}_2$O were consistently greater in the pine high-N plot than in the pine control plot, but there were no observed large-scale increases in $\mathrm{N}_2\mathrm{O}$ emissions immediately following fertilizer application. Calculated nitrogen budgets for the first 6 yr showed extremely high N retention (85-99%). Of the retained N, 50-83% appears to be in the long-term, recalcitrant soil pool. The relative importance of biotic and abiotic mechanisms of N incorporation into soils remains uncertain. Size, kinetics, and uptake capacity of this soil pool are critical and largely unknown factors determining ecosystem response to increased N loading and may be related to land-use history.
Data on three major determinants of the carbon storage in terrestrial ecosystems are used with the process-based Terrestrial Ecosystem Model (TEM) to simulate the combined effect of climate ...variability, increasing atmospheric CO
2 concentration, and cropland establishment and abandonment on the exchange of CO
2 between the atmosphere and monsoon Asian ecosystems. During 1860–1990, modeled results suggest that monsoon Asia as a whole released 29.0 Pg C, which represents 50% of the global carbon release for this period. Carbon release varied across three subregions: East Asia (4.3 Pg C), South Asia (6.6 Pg C), and Southeast Asia (18.1 Pg C). For the entire region, the simulations indicate that land-use change alone has led to a loss of 42.6 Pg C. However, increasing CO
2 and climate variability have added carbon to terrestrial ecosystems to compensate for 23% and 8% of the losses due to land-use change, respectively. During 1980–1989, monsoon Asia as a whole acted as a source of carbon to the atmosphere, releasing an average of 0.158 Pg C per year. Two of the subregions acted as net carbon source and one acted as a net carbon sink. Southeast Asia and South Asia were sources of 0.288 and 0.02 Pg C per year, respectively, while East Asia was a sink of 0.149 Pg C per year. Substantial interannual and decadal variations occur in the annual net carbon storage estimated by TEM due to comparable variations in summer precipitation and its effect on net primary production (NPP). At longer time scales, land-use change appears to be the important control on carbon dynamics in this region.
ABSTRACT
There is substantial evidence that soil thermal dynamics are changing in terrestrial ecosystems of the Northern Hemisphere and that these dynamics have implications for the exchange of ...carbon between terrestrial ecosystems and the atmosphere. To date, large‐scale biogeochemical models have been slow to incorporate the effects of soil thermal dynamics on processes that affect carbon exchange with the atmosphere. In this study we incorporated a soil thermal module (STM), appropriate to both permafrost and non‐permafrost soils, into a large‐scale ecosystem model, version 5.0 of the Terrestrial Ecosystem Model (TEM). We then compared observed regional and seasonal patterns of atmospheric CO2 to simulations of carbon dynamics for terrestrial ecosystems north of 30°N between TEM 5.0 and an earlier version of TEM (version 4.2) that lacked a STM. The timing of the draw‐down of atmospheric CO2 at the start of the growing season and the degree of draw‐down during the growing season were substantially improved by the consideration of soil thermal dynamics. Both versions of TEM indicate that climate variability and change promoted the loss of carbon from temperate ecosystems during the first half of the 20th century, and promoted carbon storage during the second half of the century. The results of the simulations by TEM suggest that land‐use change in temperate latitudes (30–60°N) plays a stronger role than climate change in driving trends for increased uptake of carbon in extratropical terrestrial ecosystems (30–90°N) during recent decades. In the 1980s the TEM 5.0 simulation estimated that extratropical terrestrial ecosystems stored 0.55 Pg C yr−1, with 0.24 Pg C yr−1 in North America and 0.31 Pg C yr−1 in northern Eurasia. From 1990 through 1995 the model simulated that these ecosystems stored 0.90 Pg C yr−1, with 0.27 Pg C yr−1 stored in North America and 0.63 Pg C yr−1 stored in northern Eurasia. Thus, in comparison to the 1980s, simulated net carbon storage in the 1990s was enhanced by an additional 0.35 Pg C yr−1 in extratropical terrestrial ecosystems, with most of the additional storage in northern Eurasia. The carbon storage simulated by TEM 5.0 in the 1980s and 1990s was lower than estimates based on other methodologies, including estimates by atmospheric inversion models and remote sensing and inventory analyses. This suggests that other issues besides the role of soil thermal dynamics may be responsible, in part, for the temporal and spatial dynamics of carbon storage of extratropical terrestrial ecosystems. In conclusion, the consideration of soil thermal dynamics and terrestrial cryospheric processes in modeling the global carbon cycle has helped to reduce biases in the simulation of the seasonality of carbon dynamics of extratropical terrestrial ecosystems. This progress should lead to an enhanced ability to clarify the role of other issues that influence carbon dynamics in terrestrial regions that experience seasonal freezing and thawing of soil.
Our main objective in the present study was to assess the spatial variation of chemical and physical soil properties and then use this information to select an appropriate area to install a pasture ...rehabilitation experiment in the Amazon region, Brazil. A regular 25 m grid was used for collecting a total of 2955 soil samples (from 985 georeferenced soil pits) at 0 to10, 10 to 20 and 20 to 30 cm layers. Soil samples were analyzed for total carbon and nitrogen,
δ
13C and
δ
15N, pH in H
2O, pH in KCl, clay, and sand contents. Conventional statistical methods and geostatistics were performed in order to analyze soil properties spatial dependence. Mean, standard deviation, skewness, and kurtosis for all measured variables were evaluated. All variograms generally were well structured with a relatively large nugget effect. Total C, total N, pH in H
2O, pH in KCl,
δ
13C and
δ
15N semivariograms were best fitted by spherical models, while clay and sand contents were best fitted by exponential models. Two types of validation (“Jackknife” or cross-validation and external validation) were conducted, indicating a lack of bias for the used prediction models. Block kriging was used to interpolate the values at unmeasured locations, generating maps of spatial variation for each soil property. Those maps were processed using Geographic Information System and restrictive criteria were adopted in order to select the best area in which to install the pasture rehabilitation experiment. The study field was then divided into zones with similar homogeneity. The selected zone can now be subjected to different treatments once the natural initial conditions are well known, and could also be used as a baseline in carbon sequestration projects within the scope of the Kyoto Protocol's Clean Development Mechanism.
Nitrous oxide emissions from tropical forest soils are thought to account for 2.2–3.7 Tg N yr−1 of the total annual global production of 10–17 Tg N yr−1. Recent research suggests that clearing of ...tropical forest for pasture can increase N2O emissions but that the period of elevated emissions may be limited and fluxes from older pastures may be lower than from the original forest. Here we report N2O emissions from two land‐use sequences in the Brazilian Amazon's state of Rondônia. Each sequence includes a forest and a set of pastures of different ages. One sequence contains a newly created pasture that we studied intensively through its first 2 years, including forest cutting, burning, and the planting of forage grasses. Emissions from the newly created pasture were about two and one half times the forest emissions during the first 2 years (5.0 kg N2O‐N ha−1 yr−1 versus 1.9 kg N2O‐N ha−1 yr−1). Nitrous oxide fluxes from pastures older than 3 years were on average about one third lower than fluxes from uncut forest (1.4 kg N2O‐N ha−1 yr−1 versus 1.9 kg N2O‐N ha−1 yr−1). The best predictor of N2O flux across the chronosequences was the magnitude of the NO3 pool in the upper 10 cm of soil measured at the time of gas sampling. Using a simple cohort model combined with deforestation rates estimated from satellite images by Brazil's Instituto de Pesquisas Espaciais (INPE) for the period 1978 through 1997, we estimate that for the Brazilian Amazon the basin‐wide flux of N2O‐N from pasture soils was 0.06 Tg in 1997. This is ∼8% of the combined forest plus pasture flux of 0.78 Tg N2O‐N we estimate for the Brazilian part of the basin in 1997. In the absence of any forest‐to‐pasture conversion in the Brazilian part of the basin, we estimate that the basin‐wide flux of N2O‐N would have been only slightly larger: 0.80 Tg in 1997. Through a second modeling analysis we estimate that for the whole of the Amazon Basin, including parts of the basin outside of Brazil, the N2O‐N emissions from forests averaged 1.3 Tg yr−1 over the period 1978–1995.
• Here, we investigated effects of increased atmospheric CO2 concentration, increased temperatures, and both factors in combination on ericoid mycorrhizal colonization, mycorrhizal functioning and ...below-ground carbon allocation in a subarctic forest understorey, to evaluate the hypothesis that photosynthesis is a primary driver for mycorrhizal colonization. • Treatment effects on ecosystem processes were investigated using 14 C-pulse labelling and photosynthesis measurements in combination with analysis of ergosterol content in roots. The effects on δ 15 N in leaves were also studied. • Ergosterol content in hair roots was positively correlated with ecosystem photosynthesis and was higher in heat- and CO2-treated plots. Leaves from CO2 plots tended to be more depleted in 15 N compared with controls both for Vaccinium myrtillus and V. vitis-idaea. • Our results suggest that changes in ecosystem photosynthesis, plant carbon (C) allocation may give rise to changing mycorrhizal colonization under elevated CO2 and temperature. The role of mycorrhizas in ecosystem N-cycling may change on a long-term basis as inorganic N availability declines with increasing levels of atmospheric CO2.
We are conducting a field study to determine the long-term response of belowground processes to elevated soil temperatures in a mixed deciduous forest. We established 18 experimental plots and ...randomly assigned them to one of three treatments in six blocks. The treatments are: (1) heated plots in which the soil temperature is raised 5@?C above ambient using buried heating cables; (2) disturbance control plots (cables but no heat); and (3) undisturbed control plots (no cables and no heat). In each plot we measured indexes of N availability, the concentration of N in soil solutions leaching below the rooting zone, and trace gas emissions (CO"2, N"2O, and CH"4). In this paper we present results from the first 6 mo of this study. The daily average efflux of CO"2 increased exponentially with increasing soil temperature and decreased linearly with increasing soil moisture. A linear regression of temperature and the natural logarithm of CO"2 flux explained 92% of the variability. A linear regression of soil moisture and CO"2 flux could explain only 44% of the variability. The relationship between soil temperature and CO"2 flux is in good agreement with the Arrhenius equation. For these CO"2 flux data, the activation energy was 63 kJ/mol and the Q"1"0 was 2.5. The daily average uptake of CH"4 increased linearly with increasing soil temperatures and decreased linearly with increasing soil moisture. Linear regression could explain 46% of the variability in the relationship between temperature and CH"4 uptake and 49% of the variability in the relationship between soil moisture and CH"4 uptake. We predicted the annual CO"2 flux from our study site in 1991 using two empirical relationships: the relationship between air temperature and soil temperature, and the relationship between soil temperature and CO"2 flux. We estimate that the annual CO"2-C flux in 1991 was 712 g/m^2 from unheated soil and 1250 g/m^2 from heated soil. By elevating the soil temperature 5@?C above ambient, we estimate that an additional carbon flux of 538 g@?m^-^2@?yr^-^1 was released from the soil as CO"2.
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