CH4 and N2O emissions from smallholder agricultural systems on tropical peatlands in SE Asia

Few studies have measured GHG emissions from smallholder agricultural systems in tropical peatlands, or non-CO₂ emissions from human-influenced tropical peatlands more generally.  The aim of this study was to quantify CH₄ and N₂O fluxes from agricultural landscapes on tropical peatlands in SE Asia and assess their environmental controls. The study was carried out in four peatland areas in Malaysia and Indonesia. At each site CH₄ and N₂O fluxes and environmental parameters was measured in four land use types, short rotation agricultural crops, oil palm plantation, tree plantation, and adjacent secondary/degraded forest.  annual CH₄ emissions were 1.8 ± 1.2, 2.1 ± 0.8, 2.3 ± 0.4, 6.1 ± 1.2 and 105.6 ± 18.1 kg CH₄ ha^-1 year^-1 at the degraded forest, tree plantation, oil palm, cropland and intact forest land use classes, respectively, while annual N₂O emissions were 0.6 ± 0.3, 3.3 ± 0.9, 12.5 ± 3.0, 18.0 ± 7.3 and 32.7 ± 5.8 kg N₂O ha^-1 year^-1 at the intact forest, tree plantation, degraded forest, oil palm and cropland land use classes, respectively. CH₄ emissions were strongly determined by WTD following an exponential relationship with production of CH₄ starting when annual WTD was above –25 cm. By contrast, N₂O emissions were strongly correlated with TDN, following a log-normal relationship. The optimum TDN concentration for N₂O production was 10 mg N L^-1 and beyond this threshold, the availability of mineral N was no longer limiting the N₂O production, with other environmental variables such as WTD, soil water content, and temperature becoming more important. The new emission factors for CH₄ and N₂O presented here should be included in country level GHG inventories to improve their accuracy. The strong impact of substrate supply on N₂O emissions shows that fertilisation practices strongly impact net emissions suggesting that policies that result in reduced fertilisation rates can directly cut emissions.