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Extensive draining at tropical ombrotrophic peatlands in Southeast Asia has made these landscapes a global ‘hot spots’ for greenhouse gas emissions. Management practices and fires have changed substrate status, which affects microbial processes. Here, we present data on how change in management practices affect carbon (C) mineralization processes at these soils. We compared the C mineralization potentials of undrained swamp forest peat to those of abandoned peat (deforested, drained and burned peatlands in degraded condition) at various depths, with and without additional substrates (glucose, glutamate and nitrate), under oxic and anoxic conditions through ex situ experiments. Carbon mineralization (CO2 and CH4 production) rates were higher in the forest peat, with higher litter deposition and C availability. Production rates decreased with peat depth coinciding with decreasing availability of labile C. Consequently, the increase in production rates after labile substrate addition was relatively modest in forest peat as compared to the abandoned site and from the top layers as compared to deeper layers. Methanogenesis had little importance in total C loss. Adding labile C and nitrogen (N) enhanced heterotrophic CO2 production more than only addition of N. Surprisingly, oxygen availability did not limit CO2 production rates, but anoxic respiration also yielded substantial rates, especially at the forest peat. Flooding of these sites will therefore reduce, but not completely cease, peat C-loss. Reintroduced vegetation and fertilization in abandoned peatlands can enrich the peat with labile C and N compounds and thus lead to increased microbiological activity.

Tropical peat carbon compound composition (CCC) is a highly understudied subject. Advanced understanding of peat CCC and carbon dynamics in differing conditions is desperately needed due to large-scale utilization of these peatlands. We studied the CCC—i.e. the hemicellulosic carbohydrate and uronic acid composition and concentrations of extractives, cellulose, acid-soluble lignin and acid-insoluble lignin—in association with peat profile depth and physical structure of peat, under representative, common land uses. Samples were gathered from an undrained forest and three sites altered 20–30 years prior to the study, which in aggregate form a continuum of increasing land-use intensity (drainage-affected forest; drained and deforested degraded open site; drained and deforested site under cultivation) in Central Kalimantan, Indonesia. Peat samples were taken from depths between 10 and 115 cm that covered mostly oxic, frequently waterlogged and permanently waterlogged, anoxic conditions. Our results demonstrated greater modification of peat properties when both vegetation and hydrological conditions were altered. The differences between sites were mainly present in the topmost peat and decreased with depth. Peat located at the surface contained more labile compounds (hemicelluloses, extractives, uronic acids, cellulose) on forest sites than at the most intensively altered open sites, where peat was enriched with recalcitrant acid insoluble lignin. The effect of drainage was evident in the drained forest site, where at the approximate median water table depth peat more closely resembled open sites in terms of the peat properties. The increased recalcitrance of peat in reclaimed areas has been a result of enhanced decomposition, reduced litter input rates and, at open sites also by repeated fires.

Tropical peatlands are one of the most important terrestrial ecosystems in terms of impact on the atmospheric greenhouse gas composition. Currently, greenhouse gas emissions from tropical peatlands following disturbances due to deforestation, drainage or wildfire are substantial. We quantified in situ nitrous oxide (N2O) fluxes during both dry and wet seasons using a closed chamber method at sites that represented differing land uses and land use change intensities in Central Kalimantan, Indonesia. Cumulative N2O fluxes were compared with carbon dioxide (CO2) and methane (CH4) fluxes. The mean N2O flux rates (N2O-N &plusmn: SD, mg m−2 h−1) varied as follows: drained forest (0.112 ± 0.293) > agricultural peat at the Kalampangan site (0.012 ± 0.026) > drained burned peat (0.011 ± 0.018) > agricultural peat at the Marang site (0.0072 ± 0.028) > undrained forest (0.0025 ± 0.053) > clear-felled, drained, recovering forest (0.0022 ± 0.021). The widest N2O flux range was detected in the drained forest (max. 2.312 and min. −0.043 mg N2O-N m−2 h−1). At the other flux monitoring sites the flux ranges remained at about one tenth that of the drained forest site. The highest N2O emission rates were observed at water tables close to the peat surface where also the flux range was widest. Annual cumulative peat surface N2O emissions (expressed in CO2 equivalents as a percentage of the total greenhouse gas (N2O, CO2 and CH4) emissions) were 9.2 % at highest, but typically ~1 %. Average N2O fluxes and also the total of monitored GHG emissions were highest in drainage-affected forest which is characterized by continuous labile nitrogen availability from vegetation, and water tables typically below the surface.

Methane emissions from natural wetlands and rice paddies constitute a large proportion of atmospheric methane, but the magnitude and year-to-year variation of these methane sources are still unpredictable. Here we describe and evaluate the integration of a methane biogeochemical model (CLM4Me; Riley et al., 2011) into the Community Land Model 4.0 (CLM4CN) in order to better explain spatial and temporal variations in methane emissions. We test new functions for soil pH and redox potential that impact microbial methane production in soils. We also constrain aerenchyma in plants in always-inundated areas in order to better represent wetland vegetation. Satellite inundated fraction is explicitly prescribed in the model, because there are large differences between simulated fractional inundation and satellite observations, and thus we do not use CLM4-simulated hydrology to predict inundated areas. A rice paddy module is also incorporated into the model, where the fraction of land used for rice production is explicitly prescribed. The model is evaluated at the site level with vegetation cover and water table prescribed from measurements. Explicit site level evaluations of simulated methane emissions are quite different than evaluating the grid-cell averaged emissions against available measurements. Using a baseline set of parameter values, our model-estimated average global wetland emissions for the period 1993–2004 were 256 Tg CH4 yr−1 (including the soil sink) and rice paddy emissions in the year 2000 were 42 Tg CH4 yr−1. Tropical wetlands contributed 201 Tg CH4 yr−1, or 78% of the global wetland flux. Northern latitude (>50 N) systems contributed 12 Tg CH4 yr−1. However, sensitivity studies show a large range (150–346 Tg CH4 yr−1) in predicted global methane emissions (excluding emissions from rice paddies). The large range is sensitive to (1) the amount of methane transported through aerenchyma, (2) soil pH (±100 Tg CH4 yr−1), and (3) redox inhibition (±45 Tg CH4 yr−1). Results are sensitive to biases in the CLMCN and to errors in the satellite inundation fraction. In particular, the high latitude methane emission estimate may be biased low due to both underestimates in the high-latitude inundated area captured by satellites and unrealistically low high-latitude productivity and soil carbon predicted by CLM4.

Conversion of tropical peatlands to agriculture leads to a release of carbon from previously stable, long-term storage, resulting in land subsidence that can be a surrogate measure of CO2 emissions to the atmosphere. We present an analysis of recent large-scale subsidence monitoring studies in Acacia and oil palm plantations on peatland in SE Asia, and compare the findings with previous studies. Subsidence in the first 5 yr after drainage was found to be 142 cm, of which 75 cm occurred in the first year. After 5 yr, the subsidence rate in both plantation types, at average water table depths of 0.7 m, remained constant at around 5 cm yr−1. The results confirm that primary consolidation contributed substantially to total subsidence only in the first year after drainage, that secondary consolidation was negligible, and that the amount of compaction was also much reduced within 5 yr. Over 5 yr after drainage, 75 % of cumulative subsidence was caused by peat oxidation, and after 18 yr this was 92 %. The average rate of carbon loss over the first 5 yr was 178 t CO2eq ha−1 yr−1, which reduced to 73 t CO2eq ha−1 yr−1 over subsequent years, potentially resulting in an average loss of 100 t CO2eq ha−1 yr−1 over 25 yr. Part of the observed range in subsidence and carbon loss values is explained by differences in water table depth, but vegetation cover and other factors such as addition of fertilizers also influence peat oxidation. A relationship with groundwater table depth shows that subsidence and carbon loss are still considerable even at the highest water levels theoretically possible in plantations. This implies that improved plantation water management will reduce these impacts by 20 % at most, relative to current conditions, and that high rates of carbon loss and land subsidence are inevitable consequences of conversion of forested tropical peatlands to other land uses.

Peat surface CO2 emission, groundwater table depth and peat temperature were monitored for two years along transects in an Acacia plantation on thick tropical peat (>4 m) in Sumatra, Indonesia. A total of 2300 emission measurements were taken at 144 locations, over a 2 year period. The autotrophic root respiration component of CO2 emission was separated from heterotrophic emission caused by peat oxidation in three ways: (i) by comparing CO2 emissions within and beyond the tree rooting zone, (ii) by comparing CO2 emissions with and without peat trenching (i.e. cutting any roots remaining in the peat beyond the tree rooting zone), and (iii) by comparing CO2 emissions before and after Acacia tree harvesting. On average, the contribution of autotrophic respiration to daytime CO2 emission was 21% along transects in mature tree stands. At locations 0.5 m from trees this was up to 80% of the total emissions, but it was negligible at locations more than 1.3 m away. This means that CO2 emission measurements well away from trees were free of any autotrophic respiration contribution and thus represent only heterotrophic emissions. We found daytime mean annual CO2 emission from peat oxidation alone of 94 t ha−1 y−1 at a mean water table depth of 0.8 m, and a minimum emission value of 80 t ha−1 y−1 after correction for the effect of diurnal temperature fluctuations, which may result in a 14.5% reduction of the daytime emission. There is a positive correlation between mean long-term water table depth and peat oxidation CO2 emission. However, no such relation is found for instantaneous emission/water table depth within transects and it is clear that factors other than water table depth also affect peat oxidation and total CO2 emissions. The increase in the temperature of the surface peat due to plantation establishment may explain over 50% of peat oxidation emissions. Our study sets a standard for greenhouse gas flux studies from tropical peatlands under different forms of agricultural land management. It is the first to purposefully quantify heterotrophic CO2 emissions resulting from tropical peat decomposition by separating these from autotrophic emissions. It also provides the most scientifically- and statistically-rigorous study to date of CO2 emissions resulting from anthropogenic modification of this globally significant carbon rich ecosystem. Our findings indicate that past studies have underestimated emissions from peatland plantations, with important implications for the scale of greenhouse gas emissions arising from land use change, particularly in the light of current, rapid agricultural conversion of peatlands in the Southeast Asian region.

Nitrous oxide (N2O) is a powerful greenhouse gas and the main driver of stratospheric ozone depletion. Since soils are the largest source of N2O, predicting soil response to changes in climate or land use is central to understanding and managing N2O. Here we find that N2O flux can be predicted by models incorporating soil nitrate concentration (NO3−), water content and temperature using a global field survey of N2O emissions and potential driving factors across a wide range of organic soils. N2O emissions increase with NO3− and follow a bell-shaped distribution with water content. Combining the two functions explains 72% of N2O emission from all organic soils. Above 5 mg NO3−-N kg−1, either draining wet soils or irrigating well-drained soils increases N2O emission by orders of magnitude. As soil temperature together with NO3− explains 69% of N2O emission, tropical wetlands should be a priority for N2O management., In a global field survey across a wide range of organic soils, the authors find that N2O flux can be predicted by models incorporating soil nitrate concentration (NO3–), water content and temperature. N2O emission increases with NO3– and temperature and follows a bell-shaped distribution with water content.

The carbon balance of tropical peatlands was investigated using measurements of gaseous fluxes of carbon dioxide (CO2) and methane (CH4) at several land-use types, including nondrained forest (NDF), drained forest (DF), drained regenerating forest (DRF) after clear cutting and agricultural land (AL) in Central Kalimantan, Indonesia. Soil greenhouse gas fluxes depended on land-use, water level (WL), microtopography, temperature and vegetation physiology, among which WL was the strongest driver. All sites were CH4 sources on an annual basis and the emissions were higher in sites providing fresh litter deposition and water logged conditions. Soil CO2 flux increased exponentially with soil temperature (T s) even within an amplitude of 4–5°C. In the NDF soil CO2 flux sharply decreased when WLs rose above −0.2 and 0.1 m for hollows and hummocks, respectively. The sharp decrease suggests that the contribution of surface soil respiration (RS) to total soil CO2 flux is large. In the DF soil CO2 flux increased as WL decreased below −0.7 m probably because the fast aerobic decomposition continued in lower peat. Such an increase in CO2 flux at low WLs was also found at the stand level of the DF. Soil CO2 flux showed diurnal variation with a peak in the daytime, which would be caused by the circadian rhythm of root respiration. Among the land-use types, annual soil CO2 flux was the largest in the DRF and the smallest in the AL. Overall, the global warming potential (GWP) of CO2 emissions in these land-use types was much larger than that of CH4 fluxes.

Vast areas of deforested tropical peatlands do not receive noteworthy shading by vegetation, which increases the amount of solar radiation reaching the peat surface. Peat temperature dynamics and heterotrophic carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) fluxes were monitored under four shading conditions, i.e. unshaded, 28%, 51% and 90% shading at experiment sites established on reclaimed fallow agricultural- and degraded sites in Central Kalimantan, Indonesia. Groundwater tables on the sites were at about 50 cm depth, the sites were maintained vegetation free and root ingrowth to gas flux monitoring locations was prevented. Half of the four shading areas received NPK-fertilization 50 kg ha−1 for each of N, P and K during the experiment and the other half was unfertilized. Increases in shading created a lasting decrease in peat temperatures, and decreased diurnal temperature fluctuations, in comparison to less shaded plots. The largest peat temperature difference in the topmost 50 cm peat profile was between the unshaded and 90% shaded surface, where the average temperatures at 5 cm depth differed up to 3.7 °C, and diurnal temperatures at 5 cm depth varied up to 4.2 °C in the unshaded and 0.4 °C in the 90% shaded conditions. Highest impacts on the heterotrophic CO2 fluxes caused by the treatments were on agricultural land, where 90% shading from the full exposure resulted in a 33% lower CO2 emission average on the unfertilized plots and a 66% lower emission average on the fertilized plots. Correlation between peat temperature and CO2 flux suggested an approximately 8% (unfertilized) and 25% (fertilized) emissions change for each 1 °C temperature change at 5 cm depth on the agricultural land. CO2 flux responses to the treatments remained low on degraded peatland. Fertilized conditions negatively correlated with N2O efflux with increases in temperature, suggesting a 12–36% lower efflux for each 1 °C increase in peat temperature (at 5 cm depth) at the sites. Despite the apparently similar landscapes of fallow agricultural land and degraded peatland sites, the differences in greenhouse gas dynamics are expected to be an outcome of the long-term management differences.

The ombrotrophic Sphagnum balticum (Russ.) C. Jens. and the oligo-mesotrophic Sphagnum papillosum Lindb. were grown at ambient (360 mu l l(-1)) and at elevated (720 mu l l(-1)) atmospheric CO2 concentrations and at different nitrogen deposition rates, varying between 0 and 30kg N ha(-1) yr(-1), The growth response to elevated atmospheric CO2 differed between species and this difference also varied with the measured growth parameters. Structural biomass of S. papillosum was significantly stimulated by elevated CO2, whereas S. balticum did not respond. In both species, soluble sugar content in the capitula and stems was significantly increased by elevated CO2 in the absence of nitrogen deposition, but not at elevated CO2 and high nitrogen deposition. The ability of both Sphagnum species to respond to elevated CO2 by enhancement of growth was independent of nitrogen deposition level and plant nitrogen status. The response to increased nitrogen addition was in line with the response to elevated CO2; the oligo-mesotrophic S. papillosum showed an increased growth, while the ombrotrophic S. balticum again did not respond. The species-dependent growth response to elevated CO2 and increased nitrogen deposition, may have considerable implications for interspecific competition between these species.Doubling atmospheric CO2 reduced total nitrogen content in both Sphagnum species. Elevated CO2 did not promote secondary metabolite production, such as soluble phenols. An increase in soluble phenol content in S. papillosum was observed when plants were grown with increased nitrogen deposition.

Nitrogen additions as NH4NO3 corresponding to 0 (N0), 1 (N1), 3 (N3) and 10 (N10) g N m-2 yr-1 were made to Sphagnum magellanicum cores at two-week intervals in situ at four sites across Europe, i.e. Lakkasuo (Finland), Mannikjarve (Estonia), Moidach More (UK) and Cote de Braveix (France). The same treatments were applied in a glasshouse experiment in Neuchâtel (Switzerland) in which the water table depth was artificially maintained at 7, 17 and 37 cm below the moss surface. In the field, N assimilation in excess of values in wet deposition occurred in the absence of growth, but varied widely between sites, being absent in Lakkasuo (moss N:P ratio 68) and greatest in Moidach More (N:P 21). In the glasshouse, growth was reduced by lowering the water table without any apparent effect on N assimilation. Total N content of the moss in field sites increased as the mean depth of water table increased indicating growth limitation leading to increased N concentrations which could reduce the capacity for N retention. Greater contents of NH4+ in the underlying peat at 30 cm depth, both in response to NH4NO3 addition and in the unamended cores confirmed poor retention of inorganic N by the moss at Lakkasuo. Nitrate contents in the profiles at Lakkasuo, Moidach More, and Cote de Braveix were extremely low, even in the N10 treatment, but in Mannikjarve, where the mean depth of water table was greatest and retention absent, appreciable amounts of NO3- were detected in all cores. It is concluded that peatland drainage would reduce the capture of inorganic N in atmospheric deposition by Sphagnum mosses.

The rate of nitrogen uptake by seven Sphagnum species, which from a gradient from hummock to hollow and from ombrotrophic to minerotrophic conditions, was measured as the decrease in the concentrations of NH 4 + and NO 2 - from solutions in which capitula were grown under laboratory conditions. The highest uptake rate was by individuals of each species with large apitula and a high number of ion exchange sites, i.e. lawn species (S. pulchrum, S. fallax, S. papillosum and S. magellanicum). On a dry-mass basis, the most effective species were the hummock species (S. fuscum and S. rubellum), even though these species have a low dry mass. Hummock species, which occur in high densities and have high potential N-uptake rates on a dry-mass basis, were the most effective species in retaining available nitrogen.

Sphagnum fuscum, S. magellanicum, S. angustifolium and S. warnstorfii were treated with N deposition rates (0, 10, 30 and 100 kg ha-1 a-1) and with four atmospheric CO2 concentrations (350, 700, 1000 and 2000 ppm) in greenhouse for 71–120 days. Thereafter, concentrations of total N, P, K, Ca and Mg in the capitulae of the Sphagna were determined. The response of each species to N deposition was related to ecological differences. With increasing N deposition treatments, moss N concentrations increased and higher N:P-ratios were found, the increase being especially clear at the highest N load. Sphagnum fuscum, which occupies ombrotrophic habitats, was the most affected by the increased nitrogen load and as a consequence the other elements were decreased. Oligotrophic S. magellanicum, wide nutrient status tolerant S. angustifolium and meso-eutrophic S. warnstorfii tolerated better increased N deposition, though there were increased concentrations of Ca and Mg in S. warnstorfii and Mg in S. magellanicum. Nitrogen and P concentrations decreased with raised CO2 concentrations, except for S. magellanicum. This seems to be the first time this kind of response in nutrient concentrations to enhanced CO2 concentration has been shown to exist in bryophytes. The concentration of K clearly decreased in S. fuscum as did the concentration of Mg in the other Sphagna with increasing CO2. Sphagnum angustifolium and S. magellanicum, which are the less specialized species, were the least affected by the CO2 treatments.