Characteristics of dissolved organic carbon transport via overland flow and interflow on sloping cropland of purple soil

Runoff is a major driver for Dissolved Organic Carbon (DOC) diffusing into aquatic ecosystems. The transportation of DOC during storm events could have measurable impacts on the global carbon cycle, but the magnitude and pathway of the impacts remain uncertain due to a lack of data. A better understanding of DOC transport via overland flow or interflow on sloping cropland is favorable for the carbon sequestration of cultivated upland. This study aimed to gain insight into the process and flux of transport of DOC in runoff as affected by rainfall characteristics. An experiment was conducted to monitor DOC transport via overland flow and interflow on sloping cropland of purple soil with free-drain lysimeter at Yanting Agro-ecological Experimental Station, Chinese Academy of Science. The experimental plots were conducted in traditional mineral fertilizer treatment (NPK) with fertilization rates at 130 kg/hm2 of N, 90 kg/hm2 of P2O5, 36 kg/hm2 of K2O for wheat and 150 kg/hm2 of N, 90 kg/hm2 of P2O5, 36 kg/hm2 of K2O for maize growing seasons, respectively. DOC transport via overland flow and interflow was continuously monitored from 2010 to 2011 in the rainy seasons by determining DOC concentrations via means of flow injection analysis (FIA) technology by Auto Analyzer3 (AA3, Bran+Lubbe, Norderstedt, Germany). Results indicated that DOC concentration was greatly affected by overland flow and interflow process. DOC concentrations in the drainage water of overland flow exhibited sharp declines at the initial period, whereas those in interflow increased at the initial period of a rainfall event and declined steadily. Transport processes of DOC were greatly affected by rain intensity. DOC concentration in the initial stage of overland flow was high while the peak DOC concentration in interflow was higher and occurred earlier in storm rain compared with heavy rain event. Average annual cumulative discharge of overland and interflow from 2010 to 2011 was 61.2、300.3 mm, accounting for 7.7% and 37.6% in the whole rainy season respectively. The average DOC concentrations in overland flow and interflow were 3.9 and 3.4 mg/L, respectively, during the experiment period. Average DOC transport fluxes and annual cumulative loads of interflow were 105.2 and 1007.6 mg/m2, respectively. The average transport load of DOC in interflow was 4.8 times more than that in overland flow. It indicated that interflow was the main transport pathway of dissolved organic carbon in the rainy season. Furthermore, soil temperature measured at soil depths (5, 20, 40, and 60 cm) had a significant negative correlation on DOC concentration in interflow while there was no effect on DOC concentration in overland flow, causing greatly different seasonal and annual patterns of dissolved organic carbon concentration in interflow.