Regulation of nitrogen fixation in a grazed pasture

Self‐contained, rotationally grazed ryegrass‐white clover pasture was established at Palmerston North on a sandy silt loam, to investigate factors regulating N‐fixation in a high fertility soil. N‐fixation, as estimated by the acetylene‐reduction assay, gave rates of 180 and 230 kg N/ha/annum for years with a dry warm summer and a moist cool summer respectively. Seasonal and regrowth stage variation in N‐fixation could be explained in terms of widely with season, from 19 to 110 kg N fixed/t legume herbage DM. Reduction of N‐legume growth and soil‐N uptake by legumes. N‐fixation per unit of legume growth varied fixation as a result of grazing was associated with an increase in soil mineral nitrogen. The proportion of N‐fixation occurring in the top 75 mm soil horizon was 90% in winter, and 10% under dry summer conditions.