Abstract
Heterotrophic respiration is a major component of the soil C balance however we critically lack understanding of its variation upon conversion of peat swamp forests in tropical areas. Our research focused on a primary peat swamp forest and two oil palm plantations aged 1 (OP2012) and 6 years (OP2007). Total and heterotrophic soil respiration were monitored over 13 months in paired control and trenched plots. Spatial variability was taken into account by differentiating hummocks from hollows in the forest; close to palm from far from palm positions in the plantations. Annual total soil respiration was the highest in the oldest plantation (13.8 ± 0.3 Mg C ha−1 year−1) followed by the forest and youngest plantation (12.9 ± 0.3 and 11.7 ± 0.4 Mg C ha−1 year−1, respectively). In contrast, the contribution of heterotrophic to total respiration and annual heterotrophic respiration were lower in the forest (55.1 ± 2.8%; 7.1 ± 0.4 Mg C ha−1 year−1) than in the plantations (82.5 ± 5.8 and 61.0 ± 2.3%; 9.6 ± 0.8 and 8.4 ± 0.3 Mg C ha−1 year−1 in the OP2012 and OP2007, respectively). The use of total soil respiration rates measured far from palms as an indicator of heterotrophic respiration, as proposed in the literature, overestimates peat and litter mineralization by around 21%. Preliminary budget estimates suggest that over the monitoring period, the peat was a net C source in all land uses; C loss in the plantations was more than twice the loss observed in the forest.
Highlights
Indonesia is one of the main holders of peatlands in the tropics with an estimated area of 225,000 km2 (Gumbricht et al 2017), of which about 14–18% is located in Central Kalimantan (Warren et al 2017)
Indonesian peat swamp forests have experienced continued deforestation and conversion over the past decades (Miettinen et al 2016).The proportion of pristine and degraded peat swamp forest in Sumatra and Kalimantan fell from 76% in 1990, to 40% in 2007 and 29% in 2015
Peat net CO2 emission estimates resulting from forest to oil palm plantation conversion are based on limited data
Summary
Indonesia is one of the main holders of peatlands in the tropics with an estimated area of 225,000 km (Gumbricht et al 2017), of which about 14–18% is located in Central Kalimantan (Warren et al 2017). Apart from being the habitat of rare species and providing hydrological regulation services, Indonesian peat swamp forests play a prominent role as a global carbon (C) pool and sink. In their pristine status, they are estimated to store on average 220 Mg C ha-1 in the phytomass (Hergoualc’h and Verchot 2011) and as much as 670 Mg C ha-1 m-1 depth of peat (Warren et al 2012). There is no information on land cover type to which smallholders converted forests, but oil palm plantations are likely to represent the most dominant land use
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