| Legume/cereal intercropping is one of the most common practices due to effective utilization of environment resources and high productivity. The yield advantage of intercropping systems has been well documented during the past decades. However, studies on the effect of interspecific root interactions on N nutrient advantage in intercropping systems still need to be further enhanced. The aim of our present work was to discuss biological nitrogen fixation of fababean by inoculating rhizobium in maize/fababean intercropping systems, and the emphasis was focused on the mechanism of nitrogen advantage in maize/fababean and maize/peanut intercropping system with different co-existence periods. Meanwhile, effects of different nitrogen levels and interspecific interaction on biological nitrogen fixation of bean and iron nutrition of peanut were investigated. The main results were as the following:(1) The biomass of fababean was significantly enhanced by N fertilization, inoculating rhizobium and intercropping, while the grain yield was increased only by intercropping. The biomass and grain yield of maize were significantly enhanced by N fertilization, but the biomass of maize not by inoculation or intercropping under N225 treatment. The biomass and grain yield of maize intercropping with fababean inoculated rhizobium under No treatment were significantly increased by 13.8% and 36.3% compared with monocropping maize, respectively. Land equivalent ratio (LER) was raised by inoculating rhizobium, with LER values of 1.43~1.57 based on grain yields in maize/fababean system. In addition, nodulation of fababean was markedly restrained by N fertilization, at the later growth stage of fababean, but facilitated remarkably by inoculation, and the facilitation of intercropping on nodulation was erratic.(2) Intercropping enhanced N, P uptake by fababean significantly, and inoculating rhizobium was effective even under N225 treatment. N, P uptake by crops was facilitated by N fertilization, and N, P uptake by maize intercropping with inoculating rhizobium fababean under N0 treatment were significantly increased. Intercropping had no effect on N, P uptake by maize at its muturation stage.(3) Before fababean was harvested, there was no significant difference in NO3--N concentration of the rhizosphere and bulk soil between intercropped and monocropped fababean, because nutrient competitive ratios of fababean relative to maize were greater than 1.0 during maize/fababean co-growth stage. After fababean was harvested, NO3--N concentration in 0~20cm soil under intercropped fababean was significantly reduced compared with that of monocropped fababean. NC3--N concentration in the rhizosphere and bulk soil under monocropped and intercropped maize was relatively invariable during its growth period. Thus, N nutrient advantage in the maize/fababean intercropping system was mostly attributed to both fababean competing nutrient from maize root zone soil before fababean harvested and maize competing nitrogen from fababean root area soil after fababean harvested.(4) The visible nodule number and nodule nitrogenase activities per plant peanut were increased significantly in maize/peanut mixed cropping system, not only due to decreasing of NO3--N concentration in the rhizosphere soil of mixed peanut because of competition of maize for soil nitrogen,but also due to the obvious improvement of mixed cropping peanut under different nitrogen levels. Moreover, iron nutrition of mixed cropping peanut was related to maize nitrogen nutrition and growth status of crops. Nodule nitrogenase activities were enhanced by both mixed cropping in needling stage, and were inhibited by nitrogen in all growth stage. Thus, it was important for biological nitrogen fixation of peanut to decrease nitrogen concentration in rhizosphere soil and to improve iron nutrition of peanut on calcareous soil, and it was a main reason for nitrogen nutrition advantage to improve biological nitrogen fixation of peanut in maize/peanut mixed cropping system.
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