Sources and features of particulate organic matter in tropical small mountainous rivers (SW China) under the effects of anthropogenic activities

Abstract Small mountainous rivers (SMRs), particularly those in the tropics, play a disproportionate role in global carbon cycling and have been heavily disturbed by anthropogenic activities during the Anthropocene. Identifying the sources of particulate organic matter (POM) in SMRs and their influencing factors have crucial scientific significance in terms of filling a cognitional gap concerning the fate of POM in tropical land–ocean ecosystems and practical values regarding environmental management and ecological evaluation. In this study, total suspended solids concentration, particulate organic carbon and particulate nitrogen contents, stable carbon isotope (δ13C), and C/N ratio of suspended particulates were investigated from five SMRs (Fangcheng, Maoling, Qin, Dafeng, and Nanliu) in Guangxi Province, southwest China. The soil (OMsoil), C3 plants (OMC3), and autochthonous production (OMauto) were identified as potential POM sources, and their relative contributions were quantified via the use of end-member mixing models that were based on the classic indicators of δ13C and C/N. The results showed that soil was the biggest POM contributor in Nanliu, Qin, and Maoling rivers, whereas OMC3 was more important in Fangcheng River. Compared to other watersheds, the OMauto accounted for a very minor proportion of the POM sources in Fangcheng River. Nanliu and Fangcheng, as typical representatives of agricultural and forest watersheds, respectively, exhibited remarkable differences in terms of POM sources. Land use patterns that were dominated by agricultural expansion, the soil erosion that resulted from the destruction of vegetation and engineering construction, and hydrological characteristics such as flow velocity and nutrient concentration were all found to vastly impact the sources of POM reaching the different types of SMR. These influencing factors are mainly derived from anthropogenic activities, whose effects on the tropical SMRs ecosystems and even the global biogeochemical cycle, may exceed our expectation.