Intercropped soybean boosts nitrogen benefits and amends nitrogen use pattern under plastic film mulching in the semiarid maize field

Legume-based intercropping has the potentials for improving nitrogen (N) behavior in semiarid cereal production. Yet, it is unclear how the intercropped legume mediates the fate and benefits of fertilizer N in plastic film mulched cereal field. A two-year field experiment was conducted with cropping patterns, including maize monoculture, soybean monoculture and maize-soybean intercropping, with N levels (0, 180 and 225 kg N ha−1 for maize; 0 and 40 kg N ha−1 for soybean), and with and without plastic film mulching in maize. Both data qualification from the methods of N-difference and 15N isotope tracer confirmed that maize had a great enhancement of NUE when intercropped with soybean. The use of 180 kg N ha−1 with film mulching for intercropped maize resulted in the highest NUE with 61% 15N recovery, 40% N agronomic efficiency and 91 kg grain kg−1 N partial productivity. Under no mulching, intercropped maize with 180 kg N ha−1 had a 28% higher 15N recovery and 43% less 15N loss with 12% more fertilizer 15N allocation into maize grain than monoculture maize with 225 kg N ha−1. To some extent, this effect was amplified by film mulching. Interestingly, film mulching caused an overuse of soil-derived N in maize, and in this regard, an influence of 67.3% was eliminated by soybean-based intercropping. This deficiency of N was compensated by enhancing the use of fertilizer-derived N and soybean-transferred N (30 kg N ha−1) under low-N rate. The intercropping pattern FM180S40, i.e. film mulched maize with 180 kg N ha−1 and soybean with 40 kg N ha−1, had the highest N fertilization efficiency (FNER=1.29) due to the high land use efficiency (LER=1.19) and maize yield (14 Mg ha−1) with less N fertilizer use. Soybean-based intercropping and film mulching sustained a 20% reduction of N fertilizer based on the optimized N level (225 kg N ha−1) for dryland maize. This further induced a great intercropping advantage of N fertilization efficiency via high NUE of maize and positive processes of NUE formation in maize strips. This study highlights the key role of intercropped soybean in boosting N fertilization benefits and reducing N loss in semiarid film-mulched agriculture systems.