Runoff harvesting engineering and its effects on soil nitrogen and phosphorus conservation in the Sichuan Hilly Basin of China

Abstract

The complex terrain and concentrated rainfall results in the loss of rainwater in the form of runoff, which aggravates soil erosion and nutrient loss in hilly areas around the world. Thus, low rainwater utilization rates and severe agricultural non-point source pollution (AGNSP) from hillslope land are the main global issues of concern in agriculture. The aims of this study were to improve in situ rainfall use, retain soil nutrients from loss and reduce AGNSP. This study characterized runoff and planned its harvesting engineering in Juxing village, Guandu town, which is located in the northeastern portion of Hechuan District, Chongqing municipality, the Sichuan Hilly Basin of China, by applying a geographic information system (GIS) based on the soil conservation service (SCS) and overland flow models, revealing the effects on soil nitrogen (N) and phosphorus (P) conservation. The results showed that more than 97% of the total annual runoff occurred from April to October, and the total annual runoff in each catchment ranged from 373 mm to 529 mm. Slope land leveling increased the soil thickness of slope land to within approximately 20–30 cm and decreased the grades to within approximately 5–15°, thereby effectively improving the residence time and effective soil water storage by approximately 20–140% and 30–131%, respectively. Although the “soil reservoir” improved after leveling, the maximum water deficit (84.2 mm) in unirrigated land was recorded in August. Thus, considering the spatial variation in soil, land use patterns, topography, and runoff in the study area, 24 additional ponds were built. The irrigation water of these ponds could supply approximately 89% of the required water storage. In addition, compared with the values before runoff harvesting engineering (RHE) construction, the total N (TN), total P (TP), available N (AN) and available P (AP) capacities of the unirrigated land soils in the study area after RHE construction increased by approximately 25.5, 27.4, 78.4 and 139.4%, respectively, and the TN, NH4+-N, TP and total dissolved P (TDP) contents in the runoff decreased by approximately 32.2, 59.7, 36.2 and 59.6%, respectively. Therefore, RHE, including slope land leveling and pond construction, improved the efficiency of water resources, increased the capacity for soil N and P conservation, and reduced AGNSP in the Sichuan Hilly Basin of China.