Effect of Contrasting Patterns of Nitrate Application on the Nitrate Uptake, N2-Fixation, Nodulation and Growth of White Clover

Abstract

White clover (Trifolium repens L.) plants were grown from seed in perlite, inoculated with effective rhizobia and exposed to the same ‘concentration x days’ of 15N-labelled nitrate in four contrasting patterns of doses. Acetylene reduction was measured at intervals using an open, continuous-flow sytem. Mean dry weight per nodule and rates of acetylene reduction fell rapidly (2–3 d) during periods of exposure to high nitrate concentrations (> 7 mM N) and rose again, equally rapidly, when nitrate was withdrawn or substantially reduced. The fall in mean dry weight per nodule (50–66 per cent) was almost certainly too large to be accounted for by loss of soluble or storage carbohydrate only. No new nodules were formed during periods of high nitrate availability. When nitrate was supplied continuously at a moderate concentration (5.7 mM N) nodule numbers stabilised although existing nodules increased in dry weight by almost four-fold over the 30 d measurement period. Treatment had no effect on the percentage nitrogen in plant tissues although there were large differences in the proportions derived from nitrate and N2-fixation. Plants exposed continuously or frequently to small doses of nitrate took up more nitrate, and hence relied less heavily on N2-fixation, than those exposed to larger doses less often. Increased reliance on nitrate brought with it increased total dry weight and shoot: root ratios. Possible mechanisms involved in bringing about these differences in nitrogen nutrition and growth are discussed.