Abstract. Tropical peat-draining rivers are known as potentially large sources of carbon dioxide (CO2) to the atmosphere due to the high loads of carbon they receive from surrounding soils. However, not many seasonally resolved data are available, limiting our understanding of these systems. We report the first measurements of carbon dioxide partial pressure (pCO2) in the Rajang River and Estuary, the longest river in Malaysia. The Rajang River catchment is characterized by extensive peat deposits found in the delta region, and by human impact such as logging, land use and river damming. pCO2 averaged 2540±189 µatm during the wet season and 2350±301 µatm during the dry season. Using three different parameterizations for the gas transfer velocity, calculated CO2 fluxes to the atmosphere were 1.5 (0.5–2.0) g C m−2 d−1 (mean, minimum – maximum) during the wet season and 1.7 (0.6–2.6) g C m−2 d−1 during the dry season. This is at the low end of reported values for Southeast Asian peat-draining rivers, but similar to values reported for Southeast Asian rivers that do not flow through peat deposits. In the Rajang River, peatlands probably do not contribute much to the CO2 flux due to the proximity of the peatlands to the coast, which limits the opportunity for degradation of organic C during transport. Thus, we suggest that peat coverage is, by itself, insufficient as the sole predictor of CO2 emissions from peat-draining rivers, and that other factors, like the spatial distribution of peat in the catchment and pH, also need to be considered.

<p><strong>Abstract.</strong> Tropical peat-draining rivers are known as potentially large sources of carbon dioxide (CO<sub>2</sub>) to the atmosphere due to high loads of carbon they receive from surrounding soils. However, not many seasonally resolved data are available, limiting our understanding of these systems. We report the first measurements of carbon dioxide partial pressure (<i>p</i>CO<sub>2</sub>) in the Rajang River and Estuary, the longest river in Malaysia. The Rajang River catchment is characterized by extensive peat deposits found in the delta region, and by human impact such as logging, land use and river damming. <i>p</i>CO<sub>2</sub> averaged 2919 ± 573 µatm during the wet season and 2732 ± 443 µatm during the dry season. This is at the low end of reported values for Southeast Asian peat-draining rivers, but higher than values reported for Southeast Asian rivers that do not flow through peat deposits. However, dissolved inorganic carbon (DIC) and δ<sup>13</sup>C-DIC data did not suggest that peatlands were an important source of inorganic carbon to the river, with an average DIC concentration of 203.9 ± 59.6 µmol L<sup>−1</sup> and an average δ<sup>13</sup>C-DIC of −8.06 ± 1.90 ‰. Also, compared to rivers with similar peat coverage, the <i>p</i>CO<sub>2</sub> in the Rajang was rather low. Thus, we suggest that peat coverage is, by itself, insufficient as sole predictor of CO<sub>2</sub> emissions from peat-draining rivers, and that other factors, like the spatial distribution of peat in the catchment and pH, need to be considered as well. In the Rajang River, peatlands probably do not contribute much to the CO<sub>2</sub> flux due to the proximity of the peatlands to the coast. CO<sub>2</sub> fluxes to the atmosphere were 2.28 ± 0.52 gC m<sup>−2</sup> d<sup>−1</sup> (wet season) and 2.45 ± 0.45 gC m<sup>−2</sup> d<sup>−1</sup> (dry season), making the Rajang River a moderate source of carbon to the atmosphere.</p>

Abstract. Southeast Asian peatlands represent a globally significant carbon store that is destabilized by land-use changes like deforestation and the conversion into plantations, causing high carbon dioxide (CO2) emissions from peat soils and increased leaching of peat carbon into rivers. While this high carbon leaching and consequentially high DOC concentrations suggest that CO2 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 by showing that carbon decomposition is hampered by the low pH in peat-draining rivers. This limits CO2 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 increases CO2 emissions from peat-draining rivers by counteracting the pH limitation. As such inputs of carbonate minerals can occur due to human activities like deforestation of river catchments, liming in plantations, and enhanced weathering application, our study points out an important feedback mechanism of those practices.

Abstract. Southeast Asian peatlands represent a globally significant carbon store that is destabilized by deforestation and the transformation into plantations, causing high carbon dioxide (CO2) emissions from peat soils and increased leaching rates of peat carbon into rivers. While global model studies assumed that CO2 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 CO2 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 CO2 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.

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