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

Summary 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 15 N natural abundance of the microbial biomass relative to other soil N pools. We analysed the 15 N natural abundance signature of the chloroform‐labile, extractable, NO 3 – , NH 4 + 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, NO 3 – , NH 4 + and chloroform‐labile fractions were found close to the reservoir. The δ 15 N value of chloroform‐labile N was similar to that of extractable (organic + inorganic) N and NO 3 – at greater C availability close to the reservoir, but was 15 N‐enriched relative to these N‐pools at lesser C availability farther away. Possible mechanisms for this variable 15 N‐enrichment include isotope fractionation during N assimilation and dissimilation, and changes in substrate use from a less to a more 15 N‐enriched substrate with decreasing C availability.