Precipitation is the key determinant of topsoil δ¹⁵N values in southern Patagonia's semiarid rangelands

Nitrogen (N) cycling in rangeland soils could potentially be controlled by water supply, stocking rates, or a range of other variables, such as ecosystem N stocks. To gauge the relative importance and elucidate possible interactions among these factors, we measured many abiotic variables to identify first-order controls of δ¹⁵N for Patagonia's rangeland soils under contrasting historical grazing intensities. The results showed that δ¹⁵N values declined as water availability increased. The effects of precipitation and stocking rate on soil δ¹⁵N values were additive, and the effect of precipitation far outweighed the effects of grazing pressure. The soil N stock was a weak predictive variable for modeling variation in δ¹⁵N of the soil. Earlier assumptions about an inflection point for N cycling and δ¹⁵N values related to aridity were not confirmed. We conclude that variation in water availability drives variation in δ¹⁵N values irrespective of grazing intensity. We also conclude that meaningful interpretation of δ¹⁵N in soil will require a better mechanistic understanding of the interactions between water and N in the vadose zone than we currently possess.