Interactive effects of mulching practice and nitrogen rate on grain yield, water productivity, fertilizer use efficiency and greenhouse gas emissions of rainfed summer maize in northwest China

Soil mulching and nitrogen application have long been applied in rainfed agriculture, but their interactive effects on crop productivity and especially environments remain poorly understood. Field experiments were carried out in 2018 and 2019 to investigate the responses of grain yield, water productivity, nitrogen use efficiency, economic benefit and greenhouse gas (N2O, CO2 and CH4) emissions of rainfed summer maize in northwest China to various mulching practices (NM: no mulching, SM: straw mulching, and RF: ridge-furrow cultivation with film mulching on the ridge), nitrogen rates (N1: 100 kg N ha−1, N2: 200 kg N ha−1 and N3: 300 kg N ha−1) and their interactions. The results showed that there were no significant differences in grain yield, water productivity and NUE between RF and SM irrespective of nitrogen rates, but RF and SM significantly increased grain yield by 12.4% and 7.9%, water productivity by 13.7% and 10.2%, and partial factor productivity by 12.1% and 7.2% compared with NM (7620.8 kg ha−1, 18.86 kg ha−1 mm−1 and 45.3 kg kg−1), respectively. SM obtained comparable net income to RF under N2 (1567.0 and 1632.4 USD ha−1) and N3 (1593.9 and 1709.9 USD ha−1). Grain yield and net income increased by 12.0% and 14.8% from N1 (7408.6 kg ha−1 and 1356.6 USD ha−1) to N2, and by only 5.5% and 3.6% from N2 (8297.3 kg ha−1 and 1558.0 USD ha−1) to N3, respectively. Water productivity was increased by 14.3% from N1 (18.6 kg ha−1 mm−1) to N2, but it decreased by 3.3% in 2018 and increased by 3.0% in 2019 from N2 (23.2 and 19.4 kg ha−1 mm−1) to N3. Both SM and RF increased CO2 emission compared to NM, while N2O emission decreased under SM and increased under RF. RF increased soil CH4 absorption, while CH4 patterns were not consistent under SM. SM significantly decreased global warming potential by 23.1% and greenhouse gas intensity by 29.0% relative to NM (405.4 kg CO2-eq and 52.0 kg CO2-eq t−1 yield), but they were significantly increased by 62.1% and 22.4% compared with NM under RF, respectively. Compared to N1 (74.09 kg kg−1), N2 and N3 decreased partial factor productivity by 44.0% and 60.9%, respectively. N rate had little influence on CO2 and CH4 emissions, but it significantly promoted N2O emission. There were significant interacting effects of mulching practice and nitrogen rate on N2O emission and global warming potential. In conclusion, straw mulching with 200 kg N ha−1 achieved a better balance between agronomic, economic and environmental benefits of rainfed summer maize in northwest China.