How do land use practices affect methane emissions from tropical peat ecosystems?

Abstract

Wetlands in Southeast Asia are thought to be one of the greatest sources of methane (CH4) to the atmosphere. Tropical peatlands are typical in Southeast Asia, and store an enormous amount of soil organic carbon. However, chamber studies of soil CH4 flux have reported that CH4 emissions from tropical peatlands in Southeast Asia are almost negligible. Recently, it was reported that some tree species growing in peat swamp forests emit considerable CH4 from their stems. Thus, ecosystem-scale flux measurement is essential to quantify the CH4 balance of tropical peat ecosystems. In this 3-year study (February 2014 to January 2017), using the eddy covariance technique, we measured the net ecosystem exchange of CH4 (FCH4) above three different tropical peat ecosystems in Sarawak, Malaysia. The three sites were an undrained peat swamp forest (UF), a relatively disturbed secondary peat swamp forest (DF) and an oil palm plantation (OP). The weekly mean FCH4 was positively correlated to the groundwater level (GWL) in the UF and DF. In contrast, the FCH4 was independent of GWL in the OP, in which GWL was lowered by drainage. The monthly mean FCH4 was always positive, even in the drained OP. Mean annual CH4 emissions (± 1 SD) were 8.46 ± 0.51, 4.17 ± 0.69, and 2.19 ± 0.21 g C m–2 year–1 in the UF, DF, and OP, respectively. The inter-site differences in emissions were explained by a significant positive exponential relationship (P < 0.001) with the GWL. This relationship indicates that the conversion of a peat swamp forest to an oil palm plantation decreases CH4 emissions, because the land conversion is accompanied by drainage. The OP, which was drained to –62 cm on average, still functioned as a small CH4 source, probably because of the high CH4 emissions from ditches.