Plant facilitation improves carbon production efficiency while reducing nitrogen input in semiarid agroecosystem

It is critical to reduce carbon (C) emission and nitrogen (N) input in agroecosystems under a changing climate. If crop diversification is introduced, interspecific plant–plant interactions as an effective pathway may achieve this goal. However, the related process and its mechanism are poorly understood. A two-year field study was conducted to explore the effects of intercropping systems including soybean-maize, soybean- wheat and maize-wheat on the land equivalent ratio (LER), nitrogen use efficiency (NUE), seasonal carbon emission, and soil properties in a typical semiarid environment. Three N rates (N1: 150 kg ha−1, N2: 225 kg ha−1, N3: 300 kg ha−1) were applied. The result indicated that the intercropping with soybean significantly increased system productivity with LER > 1, showing a typical plant–plant facilitation. However, the LER of maize–wheat intercropping was significantly lower than 1, representing interspecific competition. With the increasing N rate, the productivity of monoculture wheat or maize was evidently promoted. Particularly, the productivity under N2 and N3 remained at a similar level due to interspecific facilitation. This trend was mechanically driven by the improved N uptake (NLER > 1) and NUE under the presence of interspecific facilitation. Critically, interspecific facilitation was observed to promote carbon emissions (CE) by 4.0%-6.3%, since root input, microbial activities and the C&N decomposition enzyme activities were significantly enhanced. To say, interspecific facilitation evidently enhanced carbon emission efficiency (Yield/CE) whereas interspecific competition turned to lower it. To sum up, plant facilitation improved crop productivity and carbon emission efficiency by reducing N input. Our findings provided a new insight into the exploration of green solution in terms of reducing emissions and increasing efficiency, as well as lowering N fertilizer application in the natural and agricultural ecosystems.