Symbiotic nitrogen fixation in a sequence of pastures of increasing age measured by a15N dilution technique

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Abstract
A 15N dilution technique was used to measure the uptake of soil N by pasture herbage, and the rate and efficiency of symbiotic N fixation in 4 white clover (Trifolium repens)-based pastures ranging in age from 2 to > 20 yr at Darfield, Canterbury [New Zealand]. Pasture dry matter (DM) yields ranged from 9800 kg/ha per year in the youngest pasture to 6350 kg/ha per year in the oldest pasture. The proportion of clover DM decreased with increasing pasture age. The uptake of soil N by clover was 7-19% of the total N uptake and reflected the clover composition of the pastures. Rates of symbiotic N fixation were 45, 101, 78 and 142 kg N/ha per year for the pastures in order of decreasing pasture age. The efficiency of symbiotic N fixation, expressed in terms of the ratio N-fixed/total N in the legume, was greater in the 2 older pastures (87.6 and 87.7%) than in the younger pastures (81.6 and 82.3%).