Objective  To explore changes of nitrogen and nutrient element contents of root bleeding saps of soybeans Glycine max (L.) Merr. with different yield levels at late growth stages, and provide a physiological basis for soybean breeding and high-yielding cultivation.

Method  Soybean cultivars with three different yield levels (low-, middle- and high-yielding) were planted under the same cultivation condition. Root bleeding intensity, the contents of nitrogen compounds (total nitrogen, nitrate nitrogen, ammonium nitrogen and amino acids) and other nutrient elements (P, K, Ca and Na) in root bleeding saps of soybeans were measured at flowering (R2), podding (R4) and seed-filling (R6) stages respectively.

Result  Root bleeding intensities, total nitrogen and amino acid contents of high-yielding cultivars were 13.4% and 24.0%, 31.9% and 74.2%, 46.3% and 81.0% higher than those of middle- and low-yielding cultivars at late growth stages. Nitrate nitrogen content of high-yielding cultivar was 19.9% higher than that of low-yielding cultivar, while ammonium nitrogen content of high-yielding cultivar was 16.5% lower than that of low-yielding cultivar. The P, K and Ca contents of high-yielding cultivar were higher than those of low- and middle-yielding cultivars. The bleeding intensities of soybeans with different yield levels reached the maxinum values at R4 stage, and the grain yield was significantly positively correlated with bleeding intensity at R4 stage (r = 0.765^**). The contents of total nitrogen and amino acid in root bleeding saps reached the maximum values at R2 stage while the contents of P and K were the highest at R4 stage.

Conclusion  Root metabolism of high-yielding cultivar was vigorous and had stronger absorption capacity and vitality, which might be the key factors to increase soybean yield.