CO2 balance of a secondary tropical peat swamp forest in Sarawak, Malaysia

Tropical peat swamp forest (PSF) has accumulated a huge amount of carbon as peat over millennia, though the carbon rich ecosystem is now threatened with disturbances due to land-use change to industrial plantations of oil palm and pulp woods. Through the land conversion, peat carbon has become vulnerable and potentially changes to a great carbon dioxide (CO2) source to the atmosphere. It is essential to quantify the CO2 balance of the ecosystem and understand how the CO2 balance responds to environmental changes to predict the role of PSF in global carbon cycles. However, filed studies based on the ecosystem-scale monitoring of CO2 flux are quite limited. Thus, we began CO2 flux monitoring over a secondary PSF in Sarawak, Malaysia, by the eddy covariance technique in 2010. Daily NEE and RE were significantly different between the dry and wet periods (p < 0.01), respectively, whereas no significant difference was found in daily GPP. As a result, the seasonal difference in NEE between the two periods (0.52 g C m−2 d−1) was due to that in RE (0.57 g C m−2 d−1). Daily RE was significantly greater in the dry period mainly because of lower groundwater level (GWL). Lower GWL enhances peat aeration and potentially increases oxidative peat decomposition, which results in higher soil CO2 efflux. Annual NEE was −136 ± 51 g C m−2 yr−1 (mean ± 1 standard deviation) during the four years until 2014. The negative annual NEE was equivalent to those of some tropical rain forests on mineral soil, but was more negative than 174 g C m−2 yr−1 for an almost undrained PSF in Central Kalimantan, Indonesia (Hirano et al., 2012). The difference in annual NEE between the two sites is attributable to higher leaf area index and less distinct seasonality in precipitation in this site.