Low P availability is a vital constraint for nodulation and efficient N2 fixation of legume, including soybean. To elucidate the mechanisms involved in nodule adaption to low P availability under legume/cereal intercropping systems, two experiments consisting of three cropping patterns (monocropped soybean, monocropped maize, soybean/maize intercropping) were studied under both sufficient- and deficient-P levels. Our results demonstrated that intercropped soybean with maize showed a higher nodulation and N2 fixation efficiency under low P availability than monocropped soybean as evidenced by improvement in the number, dry weight and nitrogenase activity of nodules. These differences might be attributed to increase in P level in intercropping-induced nodules under low P supply, which was caused by the elevated activities of phytase and acid phosphatases in intercropping-induced nodules. Additionally, the enhanced expression of phytase gene in nodules supplied with deficient P level coincided with an increase in phytase and acid phosphatase activities. Our results revealed a mechanism for how intercropped maize stimulated nodulation and N2 fixation of soybean under P deficient environments, where enhanced synthesis of phytase and acid phosphatases in intercropping-induced nodules, and stimulated nodulation and N2 fixation.
To investigate the effects of maize and soybean intercropping on nitrogen and phosphorus uptake, nodule growth, and nitrogen fixation in soybean, a pot experiment was conducted with two phosphorus (P) rates (low P -P50 and sufficient P -P100). The results showed that, compared with monocropped soybean, intercropping of soybean and maize significantly increased the nodule number, nodule weight, leghemoglobin content, and nitrogenase activity of nodule, and promoted the growth and nitrogen (N) and phosphorus uptake of soybean under P50 and P100 rates. The concentrations of N, P, and the activities of acid phosphatase, phytase in nodules in intercropped soybean were significantly higher than those of monocropped soybean under P50 and P100 rates, and the activities of acid phosphatase and phytase showed the highest values under IS-P50 treatment. In addition, the P concentration in the nodules of intercropped soybean under P50 rate was significantly higher than that of monocropped soybean under P90 rate. so In summary, to maintain the larger phosphorus content for nitrogen fixation of soybean under phosphorus deficiency, the soybean and maize intercropping system increased the phosphorus concentration in the nodules mainly by enhancing the activities of acid phosphatase and phytase in the nodules, and thus promoted the growth and nitrogen uptake of soybean.
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