Effect of pasture age on the efficiency of nitrogen fixation by 10 accessions of Stylosanthes spp

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Abstract
The proportion of legume nitrogen that had been symbiotically fixed in 10 accessions of Stylosanthes spp. was determined by an isotope dilution method in microplots within grazed pastures 1, 4 and 6 years after the legumes were sown in association with buffel grass at Lansdown, near Townsville, Queensland. The proportion, averaged over all accessions, varied between years from 0.79 to 0.83, but was not related to pasture age, differences in legume yield, or total uptake of soil nitrogen. Within years, the proportion in individual plots was weakly and negatively correlated with legume yield and soil nitrogen uptake 4 years after sowing, but not at other times. No significant differences in proportions between the 10 accessions of Stylosanthes were demonstrated. It is concluded that, in these pastures, the efficiency of nitrogen fixation by the legume is not greatly affected by changes in the availability of soil nitrogen as the pasture ages. Consequently, the rate of symbiotic nitrogen fixation will depend mainly on the growth of the legume.