Comment on bg-2021-13

Abstract

Southeast Asian peatlands represent a globally significant carbon store that is destabilized by deforestation and the transformation into plantations, causing high carbon dioxide (CO 2 ) emissions from peat soils and increased leaching rates of peat carbon into rivers. While global model studies assumed that CO 2 emissions from peat-draining rivers would be high, estimates based on field data suggest they are only moderate. In this study we offer an explanation for this phenomenon and show that carbon decomposition is hampered by the low pH in peat-draining rivers, which limits CO 2 production in and emissions from these rivers. We find an exponential pH limitation that shows good agreement with laboratory measurements from high latitude peat soils. Additionally, our results suggest that enhanced input of carbonate minerals increase CO 2 emissions from peat-draining rivers by counteracting the pH limitation. As such inputs of carbonate minerals occur due to human activities like deforestation of river catchments, liming in plantations and enhanced weathering projects, our study points out an important feedback mechanism of those practices. IntroductionRivers and streams emit high amounts of carbon dioxide (CO 2 ) to the atmosphere (Cole et al., 2007), but estimates of these emissions (0.6 − 1.8 PgC yr −1 ) are highly uncertain (Aufdenkampe et al., 2011;Raymond et al., 2013). Studies agree that more than three-quarters of the global river CO 2 emissions occur in the tropics (Raymond et al., 2013;Lauerwald et al., 2015).River CO 2 emissions are controlled by the partial pressure difference between CO 2 in the atmosphere and in the river water (Raymond et al., 2012), whereby riverine CO 2 is fed by decomposition of organic matter that is leached from soils (Wit et al., 2015). Model-based studies suggest Southeast Asia as a hotspot for river CO 2 emissions (Lauerwald et al., 2015;Raymond et al., 2013) due to the presence and degradation of carbon-rich peat soils. About half of the known tropical peatlands are located in Southeast Asia, whereby 84 % of these are Indonesian peatlands, mainly on the islands of Sumatra, Borneo and Irian Jaya (Page et al., 2011). Already in 2010, land use changes affected 90% of the peatlands located on Sumatra and Borneo (Miettinen and Liew, 2010) and turned them from CO 2 sinks to CO 2 sources (Hooijer et al., 2010). Enhanced decomposition in disturbed peatlands additionally increases the leaching of organic matter from soils into peat-draining rivers (