Field observations of soil water content and nitrogen distribution on two hillslopes of different shape

Abstract

Summary The features of water regimes and nitrogen distribution in hillslope soil could provide information on the design suitability of hillslope shapes near water bodies and knowledge about the interception of nutrients before entering water bodies. Hillslope shape effects on overland flow and subsurface flow have been demonstrated by many previous studies, particularly in arid and semiarid regions. We present results from a 1-year monitoring study of two naturally formed hillslopes (convex and concave) with average gradients of 41% in a humid area (eastern China) to better understand the soil water content (SWC) variations and nitrogen concentration distributions. The SWC at the upper position of the convex hillslope (CVS) varied widely with relatively high coefficients of variance, while those at the lower position of the concave hillslope (CCS) maintained continuous, relatively high values during the study period. Comparatively, the low segment soil profile (top 80 cm) of the CCS maintained the highest soil water storage, followed by the up segment (top 80 cm) of the CCS and the low segment of the CVS, and finally the up segment of the CVS. Lateral subsurface flow discharged out from the 30–80 cm soil layer at the mid positions of the two hillslopes and the lower position of CVS; while the lateral subsurface flow from the upper hillslope recharged into the 30–80 cm soil layer at the upper and lower positions of the CCS and the upper position of CVS. Most of the soil nitrogen infiltrated the hillslope with water. Nitrogen concentrations at 30 cm depth on both the CVS and CCS varied temporally, responding strongly with rainfall events, while nitrogen concentrations at 80 cm depth changed slightly throughout the study period and maintained relatively low values. Nitrogen concentrations remained relatively low at the upper position on the CCS compared with those at the mid and lower positions. Inversely, on the CVS, the upper position had relatively higher or proximate nitrogen concentrations compared to its mid and lower positions. Nitrogen storage in the low segment soil profile of the CCS was higher than its up segment, but they were similar on the CVS. The nitrogen redistributions on the hillslopes were dominantly from water regimes, in particular, from the lateral subsurface flow. Due to the diversity of water regimes in different shaped hillslopes, the interception of lateral subsurface flow and its nitrogen pollution should receive more attention in a humid region.