C and N availability affects the ¹⁵N natural abundance of the soil microbial biomass across a cattle manure gradient

The availability of C and N to the soil microbial biomass is an important determinant of the rates of soil N transformations. Here, we present evidence that changes in C and N availability affect the ¹⁵N natural abundance of the microbial biomass relative to other soil N pools. We analysed the ¹⁵N natural abundance signature of the chloroform-labile, extractable, NO₃^-, NH₄⁺ and soil total N pools across a cattle manure gradient associated with a water reservoir in semiarid, high-desert grassland. High levels of C and N in soil total, extractable, NO₃^-, NH₄⁺ and chloroform-labile fractions were found close to the reservoir. The δ¹⁵N value of chloroform-labile N was similar to that of extractable (organic + inorganic) N and NO₃^- at greater C availability close to the reservoir, but was ¹⁵N-enriched relative to these N-pools at lesser C availability farther away. Possible mechanisms for this variable ¹⁵N-enrichment include isotope fractionation during N assimilation and dissimilation, and changes in substrate use from a less to a more ¹⁵N-enriched substrate with decreasing C availability.