The nitrogen stable isotope ratio of biological tissue (biotic δ¹⁵N) has been proposed as an indicator of anthropogenic N inputs to aquatic ecosystems, but overlap in the isotopic signatures of various N sources and transformations make definitive attribution of processes difficult. We collected primary consumer invertebrates from Wisconsin streams in agricultural settings to evaluate the relative influence of animal manure, inorganic fertilizer, and denitrification on biotic δ¹⁵N. Sample sites were selected to minimize correlations among potential explanatory variables, which were calculated using land cover and agricultural census data.

Results/Conclusions

Biotic δ¹⁵N was significantly correlated with inorganic fertilizer inputs and the percent wetland land cover in the watershed, but not with animal manure inputs. These results suggest that denitrification of N derived from inorganic fertilizer is the primary driver of δ¹⁵N variability among the study sites. Comparison with previously collected stream water NO₃-N concentrations at the same sites supports the role of denitrification; for a given N application rate, streams with high biotic δ¹⁵N had low NO₃-N concentrations. The lack of a manure signal in biotic δ¹⁵N may be due to its high ammonia content, which can be dispersed outside the range of its application by volatilization. Based on our findings and on agricultural census data for the entire U.S., inorganic fertilizer is more likely than manure to drive variability in biotic δ¹⁵N and to cause excessive N concentrations in streams.