Interactions between nitrate uptake and N2 fixation in white clover

Abstract

White clover (Trifolium repens L.) plants grown in pots and supplied with the same ‘concentration x days’ of15N labelled nitrate, but in contrasting patterns and doses had similar N concentrations but differed in the proportions devived from N2 fixation and nitrate. N2-fixation and nodule dry weight responded rapidly (2–3 days) to changes in nitrate availability. Plants exposed frequently to small doses of nitrate took up more nitrate (and hence relied less on N2-fixation) and had greater dry weights and shoot: root ratios than those exposed to larger doses less often. In mixed ryegrass (Lolium perenne L.)/clover communities clover's ability to either successfully compete for nitrate or fix N2 gave it consistently higher N concentrations than grass whether they were given high or low nitrate nutrient. This higher N concentration was accompanied by greater dry weights than grass in the low nitrate swards but not where high levels of nitrate were applied.