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This dissertation explores changes over time in soil organic matter (SOM) stabilization of two irrigated production fields: a continuous maize sequence that was converted from no-till to conservation deep tillage (Site 1), and a no-till maize-soybean rotation (Site 2). An integrated approach using humic acid extractions and density-based physical fractionation of SOM within aggregate size classes was developed to evaluate the changes in SOM stabilization (physical protection, organo-mineral associations and humification). At Site 1, loss of SOM in the surface layer was compensated for by increased SOM in deeper soil layers with no net change in C stocks. Whole field C-stocks in the 0-400 kg soil m-2 layer of Site 1 were 7.09 ± 0.29, 7.47 ± 0.36, 7.15 ± 0.37 and 7.18 ± 0.41 kg C m-2 in fall of 2005, 2006, 2007 and 2008, respectively. This outcome is encouraging, given the challenge of stabilizing aboveground residues into the mineral matrix without increasing tillage-induced C losses from native SOM and residues due to enhanced decomposition. At Site 2, there was an increase in C and N stocks in surface soil after the second maize year (2007). Whole field C stocks in the 0-400 kg soil m-2 layer of Site 2 were 6.42 ± 0.30, 6.18 ± 0.32, 6.81 ± 0.26 and 6.72 ± 0.29 kg C m-2 in fall of 2005, 2006, 2007 and 2008, respectively. The changes in soil 13C signature and monosaccharide composition, together with changes in SOM fraction amount support a large input of soybean residues as key to promote stabilization of standing maize litter through physical protection and humification. Preferential SOM accumulation in the largest, most C-enriched aggregates was associated with less humified SOM fractions while similar accumulation rates of more humified fractions were found among aggregate classes. These findings support stabilization of crop residues as SOM under high yield maize and soybean management at field scale.
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