Mismatch between soil nitrate and cumulative crop nitrogen uptake shape stage-specific N₂O emissions with legume cover crops under nitrogen reduction in dryland wheat systems

Abstract

Residues from leguminous cover crops can maintain crop nitrogen (N) uptake with reduced synthetic N fertilizer rates, but they can also induce N₂O emissions. However, the magnitude and dynamics of these effects on crop N uptake and N₂O emissions and the underlying soil-plant mechanisms remain insufficiently understood. We studied this knowledge gap conducting a two-year field experiment in arid northwest China with spring wheat (Triticum aestivum L.) grown with or without incorporated residues from a mixture of legume cover crops: hairy vetch (Vicia villosa Roth.) and common vetch (Vicia sativa L.), each fertilized either conventionally (full) or 15, 30, and 45 % reduction of the conventional N rate. The impacts on N₂O emissions, soil nitrate-N (NO₃^--N) accumulation (0–120 cm), and plant N uptake and their interactive effects were statistically evaluated across wheat growth stages. The N₂O fluxes peaked during seedling (2.81 and 2.19 µg N m⁻² min⁻¹ in 2023 and 2024, respectively) and jointing (3.98 and 3.71 µg N m⁻² min⁻¹ in 2023 and 2024, respectively) stages associated with elevated soil mineral N concentrations and water-filled pore space. Stage-specific N₂O emissions positively correlated with 0–120 cm soil NO₃^--N accumulation (P < 0.001), and the highest accumulation (785 and 594 N ha⁻¹ in 2023 and 2024, respectively) was observed at the jointing stage under full N fertilization without cover crops. Across treatments, cumulative plant N uptake increased from jointing to maturity stages, while stage-specific N₂O emissions declined (P < 0.001). Treatments with 15 % N reduction had about 14 % higher plant N uptake at each stage from jointing to anthesis compared to full N fertilization without cover crops in 2023. Moreover, 15 % N reductions following cover crop incorporation improved nitrogen use efficiency and reduced yield-scaled N₂O emissions relative to full N fertilization without cover crops. This study provides new soil-plant insights into the drivers of in-season N₂O emissions and underscores the potential of combining moderate N reduction with legume cover crops to simultaneously enhance nitrogen use efficiency and reduce environmental losses during the growing season in dryland wheat systems.