Preliminary studies of the impact of excreted N on cycling and uptake of N in pasture systems using natural abundance stable isotopic discrimination

The possibility of using natural abundance techniques to determine N transformations and flows after deposition of cattle dung has been examined. These preliminary results showed that δ¹⁵N in dung was greater than in plants growing in association with particular pats. This, and other observational information, indicated that dung pats of different ages were being examined. There were significant variations in plant δ¹⁵N signatures within and between species grown in association and away from the dung. It was probable that variation in plant δ¹⁵N was brought about by changes in soil mineral N pools after transfer of N derived from the dung. This resulted in different δ¹⁵N signatures in Trifolium repens (because of changes in N utilization from soil or atmospheric pools), in Lolium perenne (because of changes in δ¹⁵N in soil mineral N), but not in Ranunculus repens (because the majority of active roots were outside the range of immediate influence of the deposited dung). The differences in δ¹⁵N allowed the development of hypotheses for changes in soil N pools and the acquisition of N by plants from soil, dung or atmospheric sources.