Seasonal variation in white clover content and nitrogen fixing (acetylene reducing) activity in a cut upland sward

Abstract

AbstractThe seasonal variation in herbage mass and nitrogen fixing (acetylene reducing) activity of white clover in an upland sward, cut weekly to 3·5 cm from mid‐May until mid‐October, was measured. Acetylene reducing activity (ARA) was measured over a 24‐h period at 3‐weekiy intervals starting on 3 March 1983. Clover leaf and Stalon biomass was measured by harvest of the assay truces, and from mid‐May quadrat euts to 3·5 cm above the soil surface provided estimates of herbage accumulation.Little A RA was detected in March, but activity increased substantially after 10 cm soil temperatures reached > 3°C, and peak activity per unit of clover leaf dry weight occurred in June and July; standing clover leaf dry matter increased during the season to a maximum of 60·5 g m−1 in June. Acetylene reducing activity was positively correlated with the number of rooted nodes, and Stalon and leaf dry weights in early spring. Thereafter, except during a period of summer drought, ARA was positively correlated with the amount of clover leaf material.Clover population density increased during the season and maximum growing point numbers (5540 m−1) occurred in September; maximum leaf number per unit area (12 984 m−1) was found in October, prior to the final cutting of the site.Results suggest that higher levels of nitrogen fixation in upland swards should be obtained if sward management regimes, which encourage a high clover leaf area, are adopted.