Implications of land management practices for greenhouse gas fluxes from tropical peatland oil palm plantations

English Abstract:

Tropical peatlands are one the largest terrestrial reserves of C and are experiencing some of the most rapid rates of land-use change globally. Conversion of tropical peatlands to oil palm are one of the dominant forms of land-use change in the Southeast Asian tropics, with potentially significant implications for regional and global atmospheric budgets of biogenic trace gases. Here we report preliminary findings from a long-term, multi-scale project in Sarawak, Malaysian Borneo that aims to evaluate the impacts of oil palm conversion on ecosystem greenhouse gas budgets, and investigate how alternate management practices could mitigate the negative environmental impacts of land-use change. Flux chamber measurements indicate that soil CO₂, CH₄ and N₂O fluxes averaged 20.0 ± 16.0 Mg CO₂-C ha^-1 yr^-1, 37.4 ± 29.9 kg CH₄-C ha^-1 yr^-1 and 4.7 ± 4.2 g N₂O-N ha^-1 yr^-1, respectively. Fluxes of soil CO₂ and N₂O were spatially stratified, and linked to the distribution of palms, harvest residues and soil moisture. Soil CO₂ fluxes were most strongly influenced by the distribution of palms. On average, autotrophic respiration accounted for approximately 78 % of total soil CO₂ flux, and total soil respiration declined steeply away from palms; e.g. soil respiration in the immediate 1 m radius around palms were up to 6 times greater than fluxes in inter-palm spaces. Harvest residue placement also played an important role in modulating soil CO₂ fluxes; soil respiration rates doubled in areas where harvest residues were deposited, reflecting an enhanced input of labile organic matter for decomposition. In contrast, N₂O fluxes were best-predicted by the distribution of harvest residues, and were only weakly related to plant distributions or soil moisture. N₂O fluxes from harvest residue piles were up to twice of the overall plot-average, suggesting that N₂O fluxes were at least partially constrained by the availability of labile substrates for denitrification. In contrast, N₂O fluxes showed no clear pattern around palms; this is surprising because N fertilizers are applied near palm stems, and we predicted that N₂O fluxes would be greatest in areas of high fertilizer input. This suggests that palms may be a strong competitor for inorganic N in these ecosystems, and that fertilizer application may closely match overall plant demand.