Nitrogen fixation and transfer between legumes and cereals under various cropping regimes

Intercropping legumes with cereals has historically been a common practice in agriculture for the benefits of biological nitrogen (N) fixation (BNF) of legumes in intercropping systems to enhance the biomass production of intercropping cereals. However, it still remains unclear about how the interspecific interactions of legume–cereal intercrops affect the BNF and N transfer to cereals as well as the crop yields when more plant species involved in intercropping system. To examine this issue we have established monocropping and intercropping systems with three legumes (Glycine max, Medicago sativa and Vigna radiata) and two cereals (Zea mays and Sorghum bicolor). An in situ 15N labelling experiment was conducted to quantify the amount of N fixation by legumes and its N transfer to the intercropped neighbouring cereals. The biomass of legume–cereal intercrops varied with the specific coexisting plants. The biomass of Z. mays increased and S. bicolor decreased when intercropped with legumes, and the biomass of intercropped legumes decreased relative to monocropped ones. Interestingly, the percentage of fixed N transfer to cereals from legumes was negatively correlated with biomass of legumes (y = −2.539x+179.22, R2 = 0.323, p = 0.017) and positively correlated with that of cereals (y = 3.532e0.08x, R2 = 0.598, p < 0.001). Additionally, the fixed N by legumes was positively correlated with competitive response index of legumes (y = 0.899e2.233x, R2 = 0.533, p < 0.001), this confirmed that the facilitation of plant interactions significantly promoted the BNF of legumes. All these findings approved that the inclusion of legumes into cropping systems is a sustainable and intensive method to improve the biomass production of cereals and soil sustainability. Due to the specificity of species combinations in agriculture, intercropping G. max with Z. mays appears to be a promising cropping regime because G. max fixed more atmospheric N2 than the other two legumes, and Z. mays obtained more N when interplanted with G. max.