Hydrologic separation and their contributions to N loss in an agricultural catchment in hilly red soil region

Abstract

The impacts of hydrological processes on N loss is of great value to understand the N transport at catchment scale, which is far from clear. Rainfall, soil water, groundwater and stream water and their N concentrations were monitored from March 2017 to February 2018 in Sunjia agricultural catchment of the red soil critical zone. Objectives of this study were: (1) to determine the dynamics of N concentration of different waters and their N loads; (2) to assess their contributions to N load of streamflow in the paddy and upland mixed agricultural catchment. Our results showed that the N concentrations of soil water (4.8 mg L−1) and groundwater (6.0 mg L−1) were the highest, approximately 2 to 5 times higher than those of stream water (2.7 mg L−1), rain water (1.7 mg L−1) and irrigation water (1.2 mg L−1). The N net loss of the catchment (38.2 kg ha−1 yr−1) accounted for 15% of the total fertilizer N input. Rainy season (April–June) was a high-risk period of N loss, contributing to more than one third of the total annual loss amount. Using end-member mixing analysis model (EMMA), we found groundwater (whose discharge accounted for 25% of the catchment streamflow) was an important source for the N loss in the agricultural catchment. Even in this catchment with coexisting upland and paddy field ecosystems, identified end-members could be used to predict the N load well (R2>0.87, p<0.001). These results can deepen our understanding of the relationship between hydrological process and N transport in the red soil critical zone and are also helpful to improve the water and fertilizer management in subtropical agricultural catchment.