Relationships between C and N availability, substrate age, and natural abundance 13C and 15N signatures of soil microbial biomass in a semiarid climate

Soil microbial organisms are central to carbon (C) and nitrogen (N) transformations in soils, yet not much is known about the stable isotope composition of these essential regulators of element cycles. We investigated the relationship between C and N availability and stable C and N isotope composition of soil microbial biomass across a three million year old semiarid substrate age gradient in northern Arizona. The delta N-15 of soil microbial biomass was on average 7.2 parts per thousand higher than that of soil total N for all substrate ages and 1.6 parts per thousand higher than that of extractable N, but not significantly different for the youngest and oldest sites. Microbial N-15 enrichment relative to soil extractable and total N was low at the youngest site, increased to a maximum after 55,000 years, and then decreased slightly with age. The degree of N-15 enrichment of microbial biomass correlated negatively with the C:N mass ratio of the soil extractable pool. The delta C-13 signature of soil microbial biomass was 1.4 parts per thousand and 4.6 parts per thousand enriched relative to that of soil total and extractable pools respectively and showed significant differences between sites. However, microbial C-13 enrichment was unrelated to measures of C and N availability. Our results confirm that N-15, but not C-13 enrichment of soil microbial biomass reflects changes in C and N availability and N processing during long-term ecosystem development. (C) 2009 Elsevier Ltd. All rights reserved.