The use of N-15 labelling to study the turnover and utilization of ruminant manure N

An improved understanding of the cycling of animal manure N is a prerequisite for malting better use of this N source. A sheep was fed N-15-Iabelled grass in order to study the fate of N-15-Iabelled ruminant manure N in the plant-soil system. The uniformity of labelling was found to be satisfactory when an appropriate feeding strategy was used. The mineralization of labelled faecal N was compared to the mineralization of labelled feed N and indigestible feed N by measuring residual labelled organic N in unplanted topsoil in the field. After18 months, 61% of both faecal N and feed N was recovered in organic form in the topsoil, while 94% of the indigestible feed N was still present in the soil. The influence of slurry distribution in soil on the crop uptake of labelled faecal N in slurry was studied in a sandy and a sandy loam soil. The crop uptake of labelled faecal N was compared with the uptake of N-15- labelled mineral fertilizer in a reference treatment. The uptake was 28-32% of that of the reference treatment with simulated slurry injection, 13-25% with incorporated slurry and 18-19% with slurry on the soil surface. The mineralization of faecal N in the autumn after application in spring was low irrespective of the slurry distribution in soil. The results demonstrate that the contact between animal manure and the soil matrix significantly influences the short-term turnover and availability of faecal and ammonium N in slurry, especially in fine-textured soils.