|Haile-Mariam, Shawel - WASHINGTON STATE UNIV|
|Paul, E - COLORADO ST UNIVERSITY|
Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 12, 2007
Publication Date: March 1, 2008
Citation: Haile-Mariam, S., H.P. Collins, S.E. Wright, and E.A. Paul. 2008. Fractionation of soil organic matter following long-term laboratory incubation. Soil. Sci. Soc. Am. J. 72:370-378. Interpretive Summary: Research on well managed, long-term plots can provide the most useful information on SOC dynamics and sequestration. The analysis of soil from plots before and after laboratory incubation in which the microbial enzymes allowed to decompose the labile soil components provided a great deal of useful information on physically separated fractions to complement our previously published reasech on SOM dynamics. The results of 800 day laboratory incubations identified the active, slow and labile pools of SOM. The changes in corn derived C during incubation were especially informative about the controls on SOC dynamics that must be considered in interpreting data on C sequestration and ecosystem stability. The plant residues remaining in soil as represented by the light fraction were increased by no till. Their stability during incubation was not different bewteen the conventional and no-till systems nor was it site (soil) specific showing that its inherent chemical characteristics controlled turnover rates.
Technical Abstract: Soil organic matter (SOM) in agricultural soils comprises a significant part of the global terrestrial C pool. It has often been characterized by utilizing a combination of chemical dispersion of the soil followed by physical separation. These fractions include a non aggregate protected, light fraction (LF) consisting of material with a density of less than 1.7 g cm-3, a particulate organic matter fraction (POM) that is associated with sand size particles after dispersion and the SOM associated with silt and clay size particles. We analyzed soil samples from conventional (CT) and no-till (NT), continuous-corn rotations at four long-term sites in the Corn Belt to determine the concentration of C and N associated with soil fractions (LF, POM, silt and clay), determine the mean residence time (MRT) of the LF and POM, and identify the change in C concentration and '13C signal of each fraction using extended laboratory incubations. Carbon concentrations increased with decreasing size fractions. Light fractions, prior to incubation ranged from 0.5 to 0.8 g C kg-1 and comprised 3 to 5 % of the SOC with no significant difference between CT and NT treatments and decreased 70% after 800 d of incubation. The POM fraction accounted for 5 to11% of the SOC for the heavy textured (>31 % clay) soils at Lamberton, MN and Hoytville, OH compared to a range of 17 to 23 % for the KBS, MI and Wooster, OH soils that contained 19 % clay. The highest proportion of the SOC content was found in the clay size fraction ranging from 36 % (3.8 g C kg-1) of the SOC in the CT treatment of the Wooster soil to 68.4% (13.9 g C kg-1) in the fertilized Lamberton soil. Measurement of the 13C change during long-term incubation showed that the average MRT of corn derived C in the LF across sites was 3.5 y and 9.4 y for the POM fraction. The POM fraction of the Wooster and Hoytville soils had average MRT’s of 6.1 to 8.2 y; the KBS and Lamberton soils averaged ~12 y. Measurement of these fractions and knowledge of their turnover rate when combined with long-term field studies can provide a basis for C models to test long-term impacts of land use and different management scenarios on carbon sequestration, soil properties and agricultural sustainability.