The effect of nitrogen in cattle slurry and mineral fertilizers on nitrogen fixation by white clover

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AbstractThe influence of N in slurry or in mineral fertilizers on herbage yield and nitrogen fixation by white clover grown in mixed swards was investigated. Two levels of N in cattle slurry were compared with a range of mineral N fertilization. The percentage of N derived from symbiosis (% Nsym) was measured by the 15N‐isotope dilution method. The measurements were made in spring 1987 in two 4‐year‐old field trials, and included two out of five harvests.The % Nsym was approximately 90% without N fertilization. It decreased in response to both mineral N and N in slurry but did not fall below 63%. The sum of two harvests revealed that 51·2 kg N ha−1 were fixed in the absence of N fertilization. With 75 kg mineral N ha−1 or 50 m3 cattle slurry ha−1, the yield of fixed N decreased to 17·2 and 24·9 kg ha−1, respectively. When compared on the basis of the fertilizer effect on dry matter yield and N concentration of perennial ryegrass, the decrease in yield of N fixed due to treatment with cattle slurry was less than that due to mineral N fertilizer. This was owing to the smaller extent of the depression in the proportion of white clover in the sward when the same amount of N was applied in cattle slurry, as compared with mineral fertilizer, although % Nsym responded similarly to both types of N fertilization.