Abstract：
The understanding of global carbon has rarely extended to small-scale tropical river basins. To address these uncertainties, this study aims to investigate the importance of rock weathering and organic matter turnover in the carbon cycle in a terrain dominated by crystalline silicate rocks. The geochemical composition of the dissolved and particulate carbon phases (DIC, DOC and POC) and their stable carbon isotopes were studied in the Deduru Oya River in Sri Lanka. Dissolved inorganic carbon (DIC) was the most dominant carbon phase and its contribution to the total carbon pool varied between 67 and 89 %. Furthermore, the δ13CDIC values in the river varied between -1.1 and -16.5 ‰. The lithological characteristics and molar ratios between Ca2+, Mg2+ and HCO3- indicated rock weathering mainly by CO2 and carbonic acid. The δ13CDIC values for groundwater input were -15.9 ‰, while for carbonate weathering, mainly due to fertiliser input, they reached a value of -12.7 ‰. This input was fed into an isotope mass balance to determine the relative contributions. However, the isotope mass balance was only plausible after correcting for the effects on δ13CDIC caused by degassing and photosynthesis. Our study demonstrated that carbonate weathering and organic matter turnover are essential components of the river carbon cycle even in a silicate dominated catchment. They can represent up to 60 % of the DIC pool. Combined with the higher organic matter turnover and high pCO2 in the river water, it can be suggested that the Deduru Oya River acts as a net source of CO2 in the atmosphere. Our study shows that CO2 degassing and in-stream photosynthesis in tropical river systems need to be considered along with chemical weathering to account for carbon transport and turnover in tropical rivers.
摘要：
对全球碳的理解很少扩展到小型热带流域。为了解决这些不确定性，这项研究的目的是调查的重要性，岩石风化和有机质周转碳循环在地形的结晶硅酸盐岩石。溶解和颗粒碳相的地球化学组成(DIC，在斯里兰卡的DeduruOya河中研究了DOC和POC）及其稳定的碳同位素。溶解无机碳（DIC）是最主要的碳相，其对总碳库的贡献在67％至89％之间。此外，河流中的δ13CDIC值在-1.1和-16.5‰之间变化。岩性特征和Ca2+之间的摩尔比，Mg2和HCO3-表明岩石风化主要是由CO2和碳酸引起的。地下水输入的δ13CDIC值为-15.9‰，而对于碳酸盐风化，主要是由于化肥的投入，达到-12.7‰。将该输入馈送到同位素质量平衡中以确定相对贡献。然而,只有在校正了脱气和光合作用对δ13CDIC的影响后，同位素质量平衡才是合理的。我们的研究表明，即使在硅酸盐主导的集水区中，碳酸盐风化和有机物周转也是河流碳循环的重要组成部分。它们可以代表高达60%的DIC池。再加上河水中有机物的高周转率和高pCO2，可以认为DeduruOya河是大气中CO2的净来源。我们的研究表明，热带河流系统中的CO2脱气和流中的光合作用需要与化学风化一起考虑，以解决热带河流中的碳运输和周转问题。
