Canopy soil of oil palm plantations emits methane and nitrous oxide

Due to an increasing global demand in cheap oils and biofuels, forest conversion to oil palm plantations is rapidly increasing in Indonesia. Oil palm canopy soil, or the soil lodged between the stems and leaf axils of oil palms, is one ecosystem compartment that has yet to be investigated as to its importance on the soil greenhouse gas budget. Our objectives were 1) to quantify nitrous oxide (N2O) and methane (CH4) fluxes from oil palm canopy soil, and 2) to determine the factors controlling these fluxes. Our study was conducted in Jambi Province, Indonesia, an area of rapid oil palm expansion. Trace gas fluxes were measured in eight smallholder oil palm plantations (9-16 years old). In each plantation, oil palms were delineated into three 1-m height sections (low, middle, and top) to represent possible gradients of canopy soil conditions that influence N2O and CH4 fluxes. Trace gas fluxes were measured by collecting canopy soil from each stem section and immediately incubating in the field in an air-tight glass jar. Canopy soils from all oil palm stem sections emitted N2O and CH4. The top stem section had higher N2O and CH4 emissions than the lower sections, and this pattern was paralleled by nitrogen availability and water content, which strongly influenced these fluxes. Greenhouse gas emissions per unit dry mass of canopy soil were considerable, but on a hectare basis these emissions were small due to the low amount of canopy soil per hectare (170 kg ha−1). Annual canopy soil N2O and CH4 emissions were 10.7 ± 3.3 g N2O-N ha−1 yr−1 and 1.9 ± 0.5 g CH4-C ha−1 yr−1, respectively, which contributed only 1% of the total soil (canopy soil + ground soil) N2O fluxes and 0.2% of the ground soil net CH4 consumption. Our estimate of oil palm canopy soil emissions in Jambi Province were 7.7 Mg N2O-N yr−1 and 1.3 Mg CH4-C yr−1. Considering the increasing areal coverage of oil palm plantations in Southeast Asia, these fluxes may substantially contribute to soil greenhouse gas budgets.