HR: 12:05h
AN: B22C-08    [Abstracts]
TI: What Determines the N Isotope Composition of Soil Organic Matter Fractions?
AU: * Dijkstra, P
EM: paul.dijkstra@nau.edu
AF: Department of Biological Sciences, Northern Arizona University PO Box 5640, Flagstaff, AZ 86011, United States
AU: Coyle, J S
AF: Department of Biological Sciences, Northern Arizona University PO Box 5640, Flagstaff, AZ 86011, United States
AU: LaViolette, C M
AF: Department of Biological Sciences, Northern Arizona University PO Box 5640, Flagstaff, AZ 86011, United States
AU: Selmants, P C
AF: School of Forestry, Northern Arizona University PO Box 15018, Flagstaff, AZ 86011, United States
AU: Schwartz, E
AF: Department of Biological Sciences, Northern Arizona University PO Box 5640, Flagstaff, AZ 86011, United States
AU: Hart, S C
AF: School of Forestry, Northern Arizona University PO Box 15018, Flagstaff, AZ 86011, United States
AU: Hungate, B A
AF: Department of Biological Sciences, Northern Arizona University PO Box 5640, Flagstaff, AZ 86011, United States
AB: Soil organic matter, tightly bound to the soil mineral phase, exhibits characteristically high ¹⁵N/¹⁴N ratios. It has been proposed that this high ratio is the result of repeated microbial processing of organic materials. It is well known that animals are ¹⁵N enriched relative to their diet. This enrichment is related to N assimilation, dissimilation, and preferential export of the lighter ¹⁴N isotope. We investigated whether similar ¹⁵N enrichments exist for the soil microbial biomass using a broad range of ecosystems spanning semiarid, temperate and tropical climates, grassland and forests, and over four million years of soil development. We show that 1- the soil microorganisms are ¹⁵N enriched relative to other organic and inorganic soil N pools, 2- this enrichment is variable and related to C:N ratio, 3- ¹⁵N enrichment correlates positively with net N mineralization, and 4- ¹⁵N enrichment increases during soil incubation. We find, using published data, that similar negative relationships between C:N ratios and δ¹⁵N can be found for soil organic matter fractions and animal organisms. We propose that the higher ¹⁵N/¹⁴N ratios of older soil organic matter fractions reflects repeated microbial processing analogous to C and N transformations occurring in animal food webs.
DE: 0428 Carbon cycling (4806)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0469 Nitrogen cycling
DE: 0486 Soils/pedology (1865)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting