HR: 0830h
AN: H51C-04    [Abstracts]
TI: Stable Isotopic Signatures of Soil Nitrogen and Carbon Across Forty Years of Forest Development
AU: * Billings, S A
EM: sharonb@ku.edu
AF: Dept. of Ecology and Evolutionary Biology, Kansas Biological Survey, University of Kansas, 2101 Constant Ave., Lawrence, KS 66047
AU: Richter, D D
EM: drichter@duke.edu
AF: Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC 27708
AB: The δ¹⁵N and δ¹³C signatures of soil organic matter provide integrative measures of ecosystem-level N and C processes. Current understanding of the extent to which various mechanisms govern soil δ¹⁵N and δ¹³C is limited by the complexity of processes occurring simultaneously in soils, and the notable absence of δ¹⁵N and δ¹³C data from individual sites throughout the ecosystem's development. We examined δ¹⁵N and δ¹³C and associated data of archived soil samples from four depths in an aggrading loblolly pine forest in the southeastern US. We examine these data in conjunction with O horizon, root, litterfall, and foliage isotopic data, as well as with previously published information about soils at this site, and identify mechanisms driving these parameters' observed shifts with time. Soil δ¹⁵N increased with depth, and across time in the three deepest layers, by a maximum of 5.5‰, to 9.1‰. We isolate two key mechanisms that have governed changes in N isotopes in these soils. First, though discrimination against ¹⁵N during SOM mineralization is relatively small compared to other processes in the N cycle, the mineralization and transfer of >800 kg ha^-1 soil organic N into aggrading vegetation and forest floor suggest that isotopic fractionation associated with SON mineralization has been a governing feature of these soils' δ¹⁵N values. Second, accretion of microbial residues enriched in ¹⁵N likely has influenced soil δ¹⁵N via microbial excretion of ¹⁵N-deplete compounds. We suggest that the greater N isotope fractionation associated with microbial dissimilation of N compounds compared to the fractionation associated with N assimilation results in a significant enrichment of microbial biomass and subsequent residues. Though most chemical reactions in the N cycle result in isotopic fractionation, several of these processes are likely insignificant in these soils, permitting us to isolate these two likely influences on soil δ¹⁵N. Soil δ¹³C increased with depth, and declined by 1.5‰ during forest development in the surface layer. Deeper layers exhibited no significant trend in δ¹³C with time. Because these soils periodically supported C4 vegetation prior to forest planting, the initial δ¹³C values were more enriched than pure C3 vegetation (-24.5‰). We expect that as C3 plant residues become more fully incorporated into the soil profile, an increasing proportion of SOM comprised of microbial residues will promote an increase in δ¹³C_SOM, particularly deeper in the soil profile.
DE: 1615 Biogeochemical processes (4805)
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Hydrology [H]
MN: 2005 Joint Assembly