Contributions of methane and nitrous oxide to peat greenhouse gas emissions from forests and oil palm plantations in an Indonesian peatland

Land-use change in tropical peatlands substantially impacts emissions of methane (CH₄) and nitrous oxide (N₂O) in addition to emissions of carbon dioxide (CO₂). However, assessments of peat GHG budgets are scarce and the contributions of CH₄ and N₂O remain highly uncertain. The objective of our research was to assess changes in peat GHG flux and budget associated with peat swamp forest disturbance and conversion to oil palm plantation and to evaluate drivers of variation in trace gas fluxes. Over a period of one and a half year, we monitored monthly CH₄ and N₂O fluxes together with environmental variables in three undrained peat swamp forests and three oil palm plantations on peat in Central Kalimantan. The forests included two primary forests and one 30-year-old secondary forest. We calculated the peat GHG budget in both ecosystems using soil respiration and litterfall rates measured concurrently with CH₄ and N₂O fluxes, site-specific soil respiration partitioning ratios, and literature-based values of root inputs and dissolved organic carbon export. Peat CH₄ fluxes (kg CH₄ ha^-1 yr^-1) were insignificant in oil palm (0.3 ± 0.4) while emissions in forest were high (14.0 ± 2.8), and larger in wet than in dry months. N₂O emissions (kg N₂O ha^-1 yr^-1) were highly variable spatially and temporally and similar across land-uses (5.0 ± 3.9 and 5.2 ± 3.7 in oil palm and forest). Temporal variation of CH₄ was controlled by water table level and soil water-filled pore space in forest and oil palm, respectively. Monthly fluctuations of N₂O were linked to water table level in forest. The peat GHG budget (Mg CO₂ equivalent ha^-1 yr^-1) in oil palm (31.7 ± 8.6) was nearly eight times the budget in forest (4.0 ± 4.8) owing mainly to decreased peat C inputs and increased peat C outputs. The GHG budget was also ten times higher in the secondary forest (10.2 ± 4.5) than in the primary forests (0.9 ± 3.9) on the account of a larger peat C budget and N₂O emission rate. In oil palm 96% of emissions were released as CO₂ whereas in forest CH₄ and N₂O together contributed 65% to the budget. Our study highlights the disastrous atmospheric impact associated with forest degradation and conversion to oil palm in tropical peatlands and stresses the need to investigate GHG fluxes in disturbed undrained lands.