No-tillage with straw mulching restrained the vertical transportation of chemical fertilizer N and reduced its leaching loss in Northeast China

Intensified by the pursuit of food security, nitrogen (N) source pollution caused by excessive chemical N fertilizer application has emerged as a major environmental threat, especially through nitrate N (NO3--N) leaching. While no-tillage with straw mulching is recognized for mitigating NO3--N leaching from chemical fertilizer N, its efficacy in deep soil layers remains unclear. Thus, employing the 15N isotope labeling technique in Northeast China's black soil, we investigated the impact of no-tillage and/or straw mulching on the transformation, vertical transport, and leaching loss of fertilizer N under five treatments: conventional ridge tillage (RT), and no-tillage with varying straw coverages (NT-0, NT-33, NT-67, and NT-100). Our results revealed that over 98.3% of fertilizer-derived mineral N in the soil was NO3--N, with substantial downward movement beyond 120 cm, posing a leaching risk. Three years after N fertilizer application, no-tillage and/or straw mulching with varying coverages significantly reduced fertilizer-derived NO3--N by averaging 36.3% in 0–300 cm soil profile, compared to RT with 18.36 kg ha−1 fertilizer-derived NO3--N in soil. Notably, no-tillage and/or straw mulching of 33%, 67%, and 100% coverages effectively decelerated the vertical transport of fertilizer-derived NO3--N to 140–220 cm soil deeper layers by averaging 48.4%, where NO3--N accumulates in large quantities. The percentages of fertilizer-derived NO3--N leached to applied fertilizer N varied from 3.7% to 7.2% after three years of vertical migration. No-tillage with different straw coverages remarkably decreased this leaching percentage by 42.7% and 21.9% compared with RT and NT-0, respectively. Furthermore, we found that no-tillage with straw mulching enhanced fertilizer N utilization efficiency (NUE) and crop yields with NT-33, NT-67, and NT-100 demonstrating higher accumulative NUE of 58.4%, 56.2%, and 58.8% and average yields of 13.90, 13.48, and 13.21 Mg ha−1, respectively. Therefore, we preliminarily suggest that implementing no-tillage and straw mulching with 33% and 100% coverages in Northeast China holds promise for simultaneously enhancing crop yields and NUE, and mitigating N fertilizer leaching in the form of NO3--N. These findings offer valuable insights for optimizing field management and mitigating N pollution in agriculture, contributing to the achievement of sustainable agricultural development.