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 delta N-15 for Patagonia's rangeland soils under contrasting historical grazing intensities. The results showed that delta N-15 values declined as water availability increased. The effects of precipitation and stocking rate on soil delta N-15 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 delta N-15 of the soil. Earlier assumptions about an inflection point for N cycling and delta N-15 values related to aridity were not confirmed. We conclude that variation in water availability drives variation in delta N-15 values irrespective of grazing intensity. We also conclude that meaningful interpretation of delta N-15 in soil will require a better mechanistic understanding of the interactions between water and N in the vadose zone than we currently possess.