Nitrification inhibitor enhances nitrogen use efficiency and crop yield more than biochar in calcareous soils

Calcareous soils are characterized by high pH and rapid nitrification, which often lead to excessive nitrate accumulation, nitrogen (N) loss, and increased nitrous oxide (N2O) emissions. Effective inhibition of nitrification or enhancement of inorganic N retention is therefore crucial for enhancing N use efficiency (NUE) and mitigating N2O emissions in such systems. Biochar and 3,4-dimethylpyrazole phosphate (DMPP) are widely used N management strategies; however, their relative effectiveness in improving NUE and mitigating N2O emissions, as well as the underlying mechanisms regulating soil N transformations, remain poorly understood, particularly in calcareous soils. In this study, two consecutive seasons of pot experiments were conducted with eight treatments: control, phosphate and potash fertilizer (PK), N, phosphate and potash fertilizer (NPK), NPK + DMPP, NPK + low biochar (10 t ha‒1), NPK + high biochar (30 t ha‒1), NPK + low biochar + DMPP, and NPK + high biochar + DMPP. The effects of biochar and DMPP, applied alone, or in combination with, on crop N uptake, NUE, yield, soil gross N transformation rates, and N2O emissions, were systematically evaluated. Results showed that DMPP significantly enhanced crop N uptake and yield by 30.8%‒49.1% and 19.0%‒48.9%, increased NUE by 14.4%–17.9%, and reduced cumulative N2O emissions by 77.0%–85.1% relative to NPK across both seasons. Mechanistically, compared to NPK, DMPP effectively suppressed ammonia-oxidizing bacteria activity by 27.4%‒42.4%, and nitrification rates by 50.0%‒55.5%, but increased the microbial ammonium immobilization-to-nitrification ratio by 11.9%‒20.8%, and prolonged the residence time of inorganic N, thereby enhancing N retention and utilization. In contrast, both low and high biochar additions promoted microbial N immobilization but accelerated nitrification, decreased NUE, and stimulated N2O emissions; even co-application with DMPP did not counteract these effects. Overall, DMPP proved more effective than biochar in stabilizing inorganic N, improving NUE and crop yield, and mitigating N2O emissions in calcareous soils, representing a key strategy for optimizing N management in such agroecosystems.