Water and Organic Carbon Cycles in Monsoon-driven Humid Tropics of the Western Ghats Mountain Belt, India: Insights from Stable Isotope Approach

The Western Ghats form a major mountain belt, next to the Himalayas, in controlling the flux of water and carbon to the northern Indian Ocean. This study attempts to understand the water and carbon cycles in two humid tropical river basins with its streams originating at higher altitudes of the Western Ghats, India. Water and suspended particulate matter (SPM) were collected on a monthly scale during summer monsoon season (June-September) from Swarna and Nethravati rivers draining into the Arabian Sea. For the source apportionment, samples have been measured for stable isotopes of oxygen (O-18) and hydrogen (H-2) in water and stable isotopes of carbon (C-13(POC)) in particulate organic matter (POM) at spatial scale from tributaries and main channel of rivers, and runoff water from agricultural land (dominant paddy field) and forest in the downstream region. The association between O-18 and deuterium-excess in river water and rain water shows that water in these tropical basins depicts rainout effect of marine source moisture during the onset of summer monsoon. As the monsoon intensifies, the fresher rain water replenishes older water stored previously in sub surface soil layer leading to its flushing into the river during summer monsoon season. Stable carbon isotope ratio and elemental ratio of POM (C-13(POC) = -27.1 +/- 0.4 parts per thousand and C/N = 8.1 +/- 1.7) in two humid tropical river water during summer monsoon season is an admixture of suspended particulates from runoff water of forest (C-13(POC) = - 27.82 +/- 0.4 parts per thousand) and agricultural land (C-13(POC) = -26.29 +/- 0.4 parts per thousand). It is found that C-13(POC) shows minimal variability with SPM content and C/N ratio within the same organic carbon pool. The study emphasizes the need to consider the agricultural runoff contribution to the rivers while establishing the global elemental budget and observing the global climate change.