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Intercropping, the simultaneous production of two or more crops on the same field, offers opportunities for sustainable agricultural intensification by providing higher yield per unit of land than monoculture. However, intercropping is largely ignored when evaluating regional carbon (C) cycle and ecosystem service function. We hypothesized that interspecific interactions in intercropping systems might induce changes in rhizosphere soil properties and accordingly affect carbon emission. To test the hypothesis, three intercropping systems including maize-soybean (2 M:4 S, 2-row maize: 4-row soybean), wheat-soybean (4 W:4 S), and maize-wheat (2 M:4 W) were established to explore the effects of interspecific interactions on land equivalent ratio (LER), C emission and soil properties in a typical semiarid rainfed site from 2019 to 2020. The results showed that interspecific facilitation (LER>1 and RII>0) was observed when soybean was intercropped with maize and wheat. On average, soil C emission was substantially increased by 10.6%, compared with their respective monocultures. Particularly, the intercropping with soybean not only alleviated N deficiency in the rhizosphere of maize or wheat, but also improved water use efficiency (WUE) of maize or wheat via complementary effect. Moreover, facilitating process evidently promoted the input of dissolved organic carbon (DOC) and easily oxidized organic carbon (EOOC) in rhizosphere soils (P < 0.05). This resulted in greater microbial activities, N mineralization rate, and inorganic N supply in the maize and wheat strips. However, the belowground competition from the intercropped wheat significantly lowered soil moisture and inorganic N content in maize strips. Also, soil labile C input (DOC and EOOC) and microbial activities were declined, while carbon emission was evidently lowered by 7.4%. Interestingly, the trade-off between grain yield production and carbon emission showed that interspecific facilitation promoted the yield per unit of carbon emission, indicating higher carbon emission efficiency. Yet, interspecific competition led to an opposite trend. The findings enhanced the understandings on carbon emission efficiency and its potential management strategy regarding crop diversity and green production in semiarid agroecosystems. [Display omitted] • Soybean intercropped with cereal promoted crop yield, showing plant facilitation. • Plant facilitation was resulted from water complementary use and N enrichment. • Increased labile carbon input and microbial activities enhanced carbon emission. • Yet, wheat-maize mixture showed an opposite trend due to interspecific competition. • Soybean-led rhizosphere interaction promoted carbon production efficiency & yield. [ABSTRACT FROM AUTHOR]

The sole and integrated effects of N rates and cropping patterns on grain yield advantage (a-c), and carbon emission efficiency advantage (d-f). Notes: CEE, the carbon emission efficiency; N1-150, N2-225, and N3-300 represented three N levels of 150, 225 and 300 kg ha−1, respectively. [Display omitted] • Soybean intercropped with wheat or maize promoted LER, showing plant facilitation. • Plant facilitation increased LER and reduced nitrogen application by increasing NUE. • Facilitation promoted carbon emission (CE) with C&N degradation enzyme activity. • Wheat-maize intercropping showed opposite trend due to interspecific competition. • Plant facilitation increased carbon emission efficiency (yield/CE) but lowered N application. 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. [ABSTRACT FROM AUTHOR]