Modelling the effect of soil type and grazing on nitrogen cycling in a tropical grazing system

The natural abundance ratio of soil ¹⁵N to ¹⁴N (δ¹⁵N) is a useful measure of the extent of the N cycling in an ecosystem. The aim of this study was to model the effects of soil type and grazing management on soil δ¹⁵N, using samples collected from a farm-scale cattle-grazing trial in the tropical rangelands of northern Australia. We used a statistical model, fitted with robust geostatistical methods due to a systematic sampling design and the presence of outlying values, to improve the understanding of N cycling. The model revealed a consistent interaction between soil type and grazing pressure on δ¹⁵N. For a Grey Vertosol and a complex of Brown Sodosol and Yellow Kandosol, heavy grazing was associated with relatively small values of δ¹⁵N; in contrast, for a (coarse-textured) Red Kandosol, heavy grazing was associated with relatively large values of δ¹⁵N. This suggests that, at heavy stocking rates, N cycling is particularly fast on the coarse-textured soil. Information such as this could help graziers minimize environmental impacts in tropical grazing systems. Using an existing soil map as a basis, we extrapolate our findings to the region about the study site, in a first attempt to locate at-risk areas.