Compensation mechanism of green manure on grain yield and nitrogen uptake of wheat with reduced nitrogen supply

Long-term continuous cropping, excessive nitrogen input, and low nitrogen use efficiency are prevalent issues in wheat production in the Northwest Oasis irrigation area. This study examined the compensatory mechanism on grain yield and nitrogen utilization by reducing nitrogen input in wheat and multi-cropping of green manure after wheat harvest. The goal was to provide a theoretical basis for developing efficient wheat production technologies with reduced nitrogen input. This long-term field experiment, initiated in 2018, collected data from 2020 to 2022. The main plot included four green manures: multi-cropped common vetch mixed with hairy vetch (HCV), common vetch (CV), rapeseed (R), and fallow (F) after the previous wheat harvest. Subplots applied three nitrogen rates: the local conventional rate (N3, 180 kg hm-2), reduced by 20% (N2, 144 kg hm-2), and reduced by 40% (N1, 108 kg hm-2). Our results indicated that reducing conventional nitrogen by 20% and 40% significantly decreased both grain yield and nitrogen uptake. However, multi-cropped hairy vetch mixed with common vetch after wheat harvest could compensate for the yield and nitrogen uptake losses caused by a 40% nitrogen reduction. When combined with a 20% nitrogen reduction, grain yield and nitrogen uptake increased by 21.4% and 6.9%, respectively (P < 0.05). Additionally, this cropping pattern compensated for the decreased nitrogen use efficiency resulting from a 40% nitrogen reduction and, when combined with a 20% nitrogen reduction, enhanced nitrogen use efficiency by 13.4% (P < 0.05). The compensation mechanism was attributed to: (1) Under a 40% nitrogen reduction, multi-cropped hairy vetch mixed with common vetch compensated for nitrogen uptake rate, increased net nitrogen assimilation rate by 34.3% (P < 0.05), maintained nitrogen distribution in the ear, and enhanced the nitrogen transportation rate from the stem by 6.6% (P < 0.05). (2) Compared with fallow after wheat harvest and the conventional nitrogen application rate, multi-cropped hairy vetch mixed with common vetch under a 20% nitrogen reduction increased mean nitrogen uptake efficiency and net nitrogen assimilation rate by 7.2% and 34.1%, respectively (P < 0.05). It also improved nitrogen distribution in the ear from early filling to maturity stages by 6.7% (P < 0.05) and enhanced the contribution rate of nitrogen transportation from stem and leaf to ear by 17.8% and 8.9%, respectively (P < 0.05). Therefore, multi-cropped hairy vetch mixed with common vetch after wheat harvest is a viable measure to reduce nitrogen fertilizer input. When combined with a 20% nitrogen reduction, it can increase grain yield and nitrogen use efficiency in wheat by improving nitrogen uptake rate, net nitrogen assimilation rate, and the contribution rate of nitrogen transportation from leaf and stem to ear, thereby promoting nitrogen distribution in the ear in arid oasis irrigated areas.