Nitrogen stable isotopes in streams: effects of agricultural sources and transformations

The nitrogen stable isotope ratio of biological tissue 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 streams in agricultural settings in Wisconsin, USA, to evaluate the relative influence of animal manure, inorganic fertilizer, and denitrification on biotic delta(15)N. Variance in biotic delta(15)N was explained by inorganic fertilizer inputs and the percentage of wetland land cover in the watershed, but not by animal manure inputs. These results suggest that denitrification of inorganic fertilizer is the primary driver of delta(15)N variability among the study sites. Comparison with previously collected stream water NO(3)-N concentrations at the same sites supports the role of denitrification; for a given N application rate, streams with high biotic delta(15)N had low NO(3)-N concentrations. The lack of a manure signal in biotic delta(15)N may be due 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 United States, inorganic fertilizer is more likely than manure to drive variability in biotic delta(15)N and to cause excessive nitrogen concentrations in streams.