Effects of biochar incorporation and fertilizations on nitrogen and phosphorus losses through surface and subsurface flows in a sloping farmland of Entisol

Nitrogen (N) and phosphorus (P) loss in runoffs has been a serious problem in China due to the amount of chemical fertilizer currently applied to farmland. Biochar has been used as a soil amendment for increasing nutrient retention, water holding capacity, and crop yield. However, little is known about the influence of biochar on the runoff erosion from both surface flow and subsurface flow, especially in sloping regions. Experimental runoff plots were, therefore, set up on Entisols in a sloping farmland, involving four treatments: control (CT), conventional NPK fertilization (CF, based on local practices), optimum NPK fertilization (OF, based on the recommendation from local agriculture committee), and biochar combined with 85% of optimum NPK fertilizer (BF). The effects of the four treatments on the runoff fluxes, sediment yields and the runoff coefficients, as well as the N (ammonium (NH₄⁺), nitrate (NO₃⁻) and total N (TN)) and P (phosphate (PO₄³⁻), total P (TP), dissolved TP (DTP), particulate P (PP)) losses via both surface runoff (i.e., surface flow, 0–20 cm, ploughed layer) and subsurface runoff (i.e., subsurface flow, 20–60 cm, non-ploughed layer) were all monitored and evaluated from April 2018 to September 2019. Results show that, in this sloping farmland, subsurface flow was the main process of runoff and dominated N transport, but surface flow dominated P transport. The three fertilization treatments reduced the runoff erosion by surface flow, while increasing it from subsurface flow (p < 0.05). Compared with two NPK fertilizations, BF treatment significantly increased the total sediment yield, subsurface runoff flux and subsurface runoff coefficient. Among all three fertilization treatments, the differences of grain yields are found not significant (except for the 4th crop). In surface flow, biochar addition increased the loss flux of NH₄⁺ and P fractions, but reduced the loss flux of NO₃⁻ with the comparison of the CF and OF treatments. In subsurface flow, biochar addition didn’t significantly changed the runoff of N fractions, but increased P runoffs, especially for TP and PP runoffs (p < 0.05). Overall, the risk of N and P loss, and runoff erosion through subsurface flow in biochar incorporated Entisols sloping farmland needs be highlighted.