HR: 11:35h
AN: B52A-04    [Abstracts]
TI: Distribution of Organic Matter in Nano- and Micropores of Soil Microaggregates
AU: * McCarthy, J F
EM: jmccart1@utk.edu
AF: University of Tennessee, Department of Earth and Planetary Sciences, Knoxville, TN 37996-1410 United States
AU: Ilavsky, J
EM: ilavsky@aps.anl.gov
AF: Argonne National Laboratory, 9500 S. Cass St., Argonne, IL 60439 United States
AU: Mayer, L M
EM: lmayer@gwi.net
AF: University of Maine, Darling Marine Center, Walpole, ME 04573 United States
AU: Jastrow, J D
EM: jdjastrow@anl.gov
AF: Argonne National Laboratory, 9500 S. Cass St., Argonne, IL 60439 United States
AU: Perfect, E
EM: eperfect@utk.edu
AF: University of Tennessee, Department of Earth and Planetary Sciences, Knoxville, TN 37996-1410 United States
AU: Zhuang, J
EM: jzhuang@utk.edu
AF: University of Tennessee, Department of Earth and Planetary Sciences, Knoxville, TN 37996-1410 United States
AB: The processes underlying the sequestration of organic matter (OM) in soil microaggregates are being studied using ultra-small small angle x-ray scattering (USAXS) and nitrogen adsorption to evaluate the pore size distribution of the total- and OM-filled porosity within microaggregates. Soil microaggregates (50 to 250 um) are particularly crucial to long-term sequestration because they protect C against decomposition, resulting in much longer residence times. Systematic changes in the pore-size distribution of OM were evaluated at two long-term field manipulations: a chronosequence of tallgrass prairie restoration (Fermilab, Batavia, IL, USA; Mollisol) and a 30-year comparison of till/no-till cultivation at two levels of N inputs (University of Kentucky, USA; Alfasol). The soil OM levels increased over time after restoration of cultivated soils to a prairie, and with decreased tillage and increased fertilizer inputs. The distribution of pores in microaggregates was measured in microaggregates before and after the OM was removed by combustion at 350oC. The total porosity was determined using data from the combusted, OM-free microaggregates. The distribution of the OM within the pores was determined by USAXS from differences in the porosity estimates for the combusted and intact samples. USAXS advantages include a very wide range of length scales measured (1 nm to 5 um) and not requiring connectivity of the pores to a probe molecule. The USAXS data reflect differences in the composition and density of pores, OM and minerals. There is a strong change in x-ray contrast when an entirely OM-filled void is cleared by combustion, compared to the case when a coating of OM in an air-filled pore is removed. This latter property was exploited to evaluate the protection of OM residing in pores with reduced access of microbes or microbial exoenzymes. The conversion of a cultivated soil to a prairie results in restructuring of the OM distribution, with new OM migrating to increasingly fill pores. Additionally, although the volume fraction of the total porosity increases with the pore diameter, there is a sharp decline in the abundance of OM-filled pores at length scales greater than 1 um. This pattern suggests that OM accumulation and preservation occurs via limiting physical access of microorganisms to OM within filled pores. Similar, but less strongly developed patterns were seen for the Kentucky site, but it is likely that strong adsorption to the Fe-oxide rich Alfasols could also contribute to OM accumulation at that site.
DE: 1699 General or miscellaneous
DE: 1615 Biogeochemical processes (4805)
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting