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 N-15 natural abundance of the microbial biomass relative to other soil N pools. We analysed the N-15 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 delta N-15 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 N-15-enriched relative to these N-pools at lesser C availability farther away. Possible mechanisms for this variable N-15-enrichment include isotope fractionation during N assimilation and dissimilation, and changes in substrate use from a less to a more N-15-enriched substrate with decreasing C availability.