Spatio-Temporal Variability of Peat CH4 and N2O Fluxes and Their Contribution to Peat GHG Budgets in Indonesian Forests and Oil Palm Plantations

Abstract

Land-use change in tropical peatlands substantially impacts peat emissions of methane (CH4) and nitrous oxide (N2O) in addition to emissions of carbon dioxide (CO2). However, assessments of full peat greenhouse gas (GHG) budgets are scarce and CH4 and N2O contributions 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 year−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 year−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 year−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.

Comparing GHG Emissions from Drained Oil Palm and Recovering Tropical Peatland Forests in Malaysia

“…Similarly, N 2 O emissions in tropical peat swamp forest are generally low compared to emissions of CH 4 and CO 2 [5,37]. The annualized emission of N 2 O at the undrained NF in this study (0.08 g m −2 yr −1 ) was within the range of the observed emissions from Southeast Asian tropical swamp forests, e.g., Sarawak: 0.002-0.17 g N 2 O m −2 yr −1 [37], Kalimantan: 0.06-0.52 g N 2 O m −2 yr −1 [17,40]. Furthermore, Kandel et al [95] recorded 0.03 N 2 O g m −2 yr −1 from undrained natural bog of northern peatlands.…”

“…The wide range in N 2 O fluxes at the OP compared to the NF (Figure 4b) indicates that nitrogen fertilizer inputs for palm oil production resulted in increased N 2 O emissions [17,97,98]. The annualized N 2 O emissions at the OP (0.42 g N 2 O m −2 yr −1 ) in this study were comparable to the emissions from drained mature oil palm plantations on Southeast Asian peatland (0.12-2.52 g N 2 O m −2 yr −1 [37,40,44]). Additionally, it was similar to the N 2 O emissions from other types of agriculture on drained peatlands, such as Sago plantation: 0.33 g N 2 O m −2 yr −1 [37], mixed-agriculture: 0.02 g N 2 O m −2 yr −1 [17].…”

“…Global Warming Potential (GWP) is an indicator of the radiative forcing of a greenhouse gas over a chosen time horizon, relative to CO 2 . To compare the overall GWP load of the three GHGs, annualized emissions of CH 4 and N 2 O were converted to CO 2 equivalents (CO 2 -eq) over a 20-year horizon because this time period was considered to be the most appropriate for evaluating the impacts of land-use change that typically occur over 20-30-year time periods in tropical regions [14,40]. Annualized emissions were calculated using the cumulative emissions multiplied by a constant of 365/total days of cumulative emission.…”

“…However, previous studies involving closed-chamber methods have generally not been able to produce statistically robust flux values or uncertainty ranges due to inadequate numbers of replicates, and not enough close-interval measurements conducted over long enough periods of time [31,34,35]. In addition, most studies of temporal measurements have only focused on dry or wet seasons (selective time periods) [14,31,35,36] with very few studies conducting measurements for at least one full year [36][37][38][39][40]. Additionally, most studies of the relationships between Southeast Asian tropical peatlands and GHG emissions have been conducted in Indonesia [33,35,36,38,41,42] and east Malaysia, [7,37,[43][44][45][46], with fewer studies in peninsular Malaysia peatland [14,30,34].…”

Seasonal litter contribution to total peat respiration from drained tropical peat under mature oil palm plantation

“…Under the waterlogged and strong reductive condition, peat can emit CH4. However, some researchers pointed out that high lignin content in Southeast Asian peatlands inhibits CH4 production (Hirano et al, 2009;Arai 2014), notably with oxic (Swails et al, 2021) and compacted conditions (Busman et al, 2021) in drained oil palm plantation. Oppositely, a considerable amount of CO2 emanates in the peat surface after clearing and draining the peat for cultivation purposes (Prananto et al, 2020) owing to enhanced aerobic microorganism activity under excessive O2 presence (Xu et al, 2021).…”