Abstract
Tropical peatlands are currently being rapidly cleared and drained for the establishment of oil palm plantations, which threatens their globally significant carbon sequestration capacity. Large-scale land conversion of tropical peatlands is important in the context of greenhouse gas emission factors and sustainable land management. At present, quantification of carbon dioxide losses from tropical peatlands is limited by our understanding of the relative contribution of heterotrophic and autotrophic respiration to net peat surface CO2 emissions. In this study we separated heterotrophic and autotrophic components of peat CO2 losses from two oil palm plantations (one established in ‘2000’ and the other in 1978, then replanted in ‘2006’) using chamber-based emissions sampling along a transect from the rooting to non-rooting zones on a peatland in Selangor, Peninsular Malaysia over the course of 3 months (June–August, 2014). Collar CO2 measurements were compared with soil temperature and moisture at site and also accompanied by depth profiles assessing peat C and bulk density. The soil respiration decreased exponentially with distance from the palm trunks with the sharpest decline found for the plantation with the younger palms with overall fluxes of 1341 and 988 mg CO2 m−2 h−1, respectively, at the 2000 and 2006 plantations, respectively. The mean heterotrophic flux was 909 ± SE 136 and 716 ± SE 201 mg m−2 h−1 at the 2000 and 2006 plantations, respectively. Autotrophic emissions adjacent to the palm trunks were 845 ± SE 135 and 1558 ± SE 341 mg m−2 h−1 at the 2000 and 2006 plantations, respectively. Heterotrophic CO2 flux was positively related to peat soil moisture, but not temperature. Total peat C stocks were 60 kg m−2 (down to 1 m depth) and did not vary among plantations of different ages but SOC concentrations declined significantly with depth at both plantations but the decline was sharper in the second generation 2006 plantation. The CO2 flux values reported in this study suggest a potential for very high carbon (C) loss from drained tropical peats during the dry season. This is particularly concerning given that more intense dry periods related to climate change are predicted for SE Asia. Taken together, this study highlights the need for careful management of tropical peatlands, and the vulnerability of their carbon storage capability under conditions of drainage.
Highlights
Tropical peatlands are estimated to occupy 441,025 km2 globally, with more than half of the total area (247,778 km2) being located in South-East Asia (Page et al 2011a), and provide the largest longterm sink of terrestrial carbon (Page et al 2011b)
Average annual rainfall in the area is 2419 mm with the dry season normally occurring from May to September and, to a lesser extent, December to February
It is likely that the lower overall Soil Organic Carbon (SOC) in the 2006 plantation was caused by long-term high heterotrophic C losses depleting the SOC (Figs. 2, 5b) in line with
Summary
Tropical peatlands are estimated to occupy 441,025 km globally, with more than half of the total area (247,778 km2) being located in South-East Asia (Page et al 2011a), and provide the largest longterm sink of terrestrial carbon (Page et al 2011b). Peatlands are especially attractive as areas for plantation establishment due to the capacity for water retention of organic soils and high nutrient release from decomposing drained peat soils (Corley and Tinker 2003). Since oil palm trees do not grow well on waterlogged soil due to poor anchorage and anoxic conditions, the establishment of oil palm plantation requires drainage of peat. This greatly increases the risk of high levels of organic matter decomposition, as the presence of oxygen enables the activity of aerobic microorganisms (Husnain et al 2014)
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