Biochar, grass, and cross-ridge reshaped the surface runoff nitrogen under consecutive rainstorms in loessial sloping lands

Many options for reducing nitrogen (N) losses from agriculture rely on changing the surface pathways of N runoff in one way or another, but the runoff N mechanism by agricultural interventions such as biochar, grass, and cross-ridge is not well understood. Herein, 12 artificial simulated rainstorms by three biochars (0%, 3%, and 6%), two planting modes (conventional flat planting, cross-ridge), and two underlying surfaces (bare, grass) were designed to investigate the surface pathways of N loss and possible tradeoffs in loessial sloping lands under 60 mm·h−1 rainfall intensity. Results indicate that: (i) 3% and 6% biochar application can aggravate soil erosion on bare slope, but planting grass on 0% or 3% biochar bare slope has good reduction effects on sediment and N loss, while 6% belongs to excessive biochar on grassed slopes. (ii) Grass planting may decrease the loss fractions of particulate nitrogen (PN) more obviously than bare slope because it decreases the kinetic energy of raindrops, slows the rate of water flow, and improves the shear strength of soils with deep penetrating roots. Configuring cross-ridge with 0% or 3% biochar on grassed slope can reduce PN fraction obviously but the interception of cross-ridge on bare slope is limited and easy to fail. (iii) The hydrological pathways transporting various N fractions vary greatly owing to the interaction of biochar, grass, and cross-ridge. Runoff N from 10°, 15°, and 20° bare slope materializes mainly in particulate fractions (65.58%, 50.15%, and 60.46%), while from grassed slope it occurs mainly in dissolved fractions (54.01%, 59.50%, and 66.48%). These findings may help provide new insights needed to enhance soil health and maintain agroecosystem sustainability through decreasing N losses via sloping farmland management in loess hilly and gully regions.