Tuesday, November 14, 2006
178-2

15N Natural Abundance of the Soil Microbial Biomass is Related to C and N Availability.

Paul Dijkstra1, Jeffrey S. Coyle1, Paul C. Selmants2, Egbert Schwartz1, Stephen Hart3, and Bruce A. Hungate1. (1) Dept Biol Sciences, Northern Arizona Univ, PO Box 5640, Flagstaff, AZ 86011, (2) Northern Arizona Univ, School of Forestry, PO Box 15018, Flagstaff, AZ 86011-5018, (3) PO Box 15018, Northern Arizona Univ., Northern Arizona University, School of Forestry, Flagstaff, AZ 86011-5018

Stable isotope measurements are a powerful tool in soil and ecological research. Natural abundance measurements of soil N pools have the potential to provide information regarding N-transformations. Recently we have developed a protocol to measure the 15N natural abundance of the soil microbial biomass using the chloroform fumigation-extraction procedure. We measured the 15N isotope signatures of the microbial biomass, extractable and total N pools in soils from an elevation gradient, a semi-arid and tropical soil age gradient, and a dung gradient. Results show that 1) the δ15N of the microbial biomass differs from that of the soil extractable and total N, and 2) the difference between the natural abundance 15N composition of the microbial biomass relative to that of the extractable N pool varies with C and N availability in the soil. We postulate that the 15N-enrichment of the microbial biomass relative to other soil pools is associated with fractionation during N assimilation and dissimilation processes. This interpretation suggests that a high 15N enrichment of the microbial biomass is associated with low C availability, high dissimilation and thus high N-mineralization rates.