HR: 1340h
AN: B13C-0250    [Abstracts]
TI: Soil Microbial $^{15}$N-Natural Abundance is Enriched Relative to Other Soil N Pools and Indicates Microbial C-Limitation
AU: * Dijkstra, P
EM: Paul.Dijkstra@nau.edu
AF: Dep Biological Sciences Northern Arizona University, PO Box 5640, Flagstaff, AZ 86011 United States
AU: Doucett, R
EM: Richard.Doucett@nau.edu
AF: Dep Biological Sciences Northern Arizona University, PO Box 5640, Flagstaff, AZ 86011 United States
AU: Hart, S C
EM: Steve.Hart@nau.edu
AF: School of Forestry Northern Arizona University, PO Box 5018, Flagstaff, AZ 86011 United States
AU: Boring, L L
EM: lboring@jonesctr.org
AF: Joseph W. Jones Ecological Research Center at Ichauway, PO Box 2324, Newton, GA 39870 United States
AU: Schwartz, E
EM: Egbert.Schwartz@nau.edu
AF: Dep Biological Sciences Northern Arizona University, PO Box 5640, Flagstaff, AZ 86011 United States
AU: Hungate, B A
EM: Bruce.Hungate@nau.edu
AF: Dep Biological Sciences Northern Arizona University, PO Box 5640, Flagstaff, AZ 86011 United States
AB: Soil microbial biomass is responsible for many of the nitrogen (N) transformations that occur between different soil organic matter pools, plants and atmosphere. For this reason, it is important to learn more about the $^{15}$N natural abundance of these organisms. The microbial biomass takes up organic carbon (C) and N and inorganic N for assimilation and respiration. Under C-limited conditions, organic N compounds are mainly utilized as C-source, and excess N leaves the cell (mineralization). The processes of N assimilation, N dissimilation, and export discriminate against the heavier $^{15}$N isotope. This leaves the cells $^{15}$N enriched compared to their supposed substrates, or total soil N. Selective uptake of enriched ammonium (as a result of nitrification) may contribute to the higher \delta$^{15}$N of the micro-organisms. We measured $^{15}$N natural abundances of the microbial biomass, using the chloroform-fumigation-extraction method, in grassland soils along an elevation gradient, a fire-disturbance gradient in Florida, a ponderosa forest restoration study, and a dung-gradient near a water source in desert grassland and found 0-12\permil difference between the soil extractable N and the microbial biomass. We also found that the enrichment is quantitatively dependent on C availability. We speculate that, in addition to an indicator for C availability to the microbes, microbial transformations may explain the $^{15}$N-enrichment of soil organic matter with depth.
DE: 4806 Carbon cycling
DE: 4840 Microbiology
DE: 4845 Nutrients and nutrient cycling
DE: 4870 Stable isotopes
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting