Carbon dynamics of surface residue- and root-derived particulate organic matter in the whole soil and aggregate size fractions

The importance of particulate organic matter (POM) in the soil ecosystem has been emphasized by many researchers, but the dynamics of POM and its role in aggregate formation are not well understood. I concluded a simulated no-till experiment which utilized {dollar}sp{lcub}14{rcub}{dollar}C labeled plant material, periodic sampling, and a combination of physical and chemical fractionation methods to measure progressive changes in aggregation and the distribution of new POM C as surface residue and in situ roots decomposed. The main objectives of this dissertation were: (i) to characterize the relative contribution of surface residue- and root-derived C to soil organic matter (SOM) under no-till; (ii) to compare the concentration of new POM C in aggregates of different size and stability; (iii) to quantify changes with time in the amount, concentration, and distribution of surface residue- and root-derived C in free and intraaggregate POM.; After one year, 66% of the {dollar}sp{lcub}14{rcub}{dollar}C contained in surface residue on d 0 had been respired as CO{dollar}sb2,{dollar} 12% of the {dollar}sp{lcub}14{rcub}{dollar}C remained in residue on the soil surface, and 16% of the {dollar}sp{lcub}14{rcub}{dollar}C was in the soil. In contrast, 56% of the root-derived {dollar}sp{lcub}14{rcub}{dollar}C was respired as CO{dollar}sb2{dollar} and 44% of the {dollar}sp{lcub}14{rcub}{dollar}C remained in the soil after one year. The concentration of root-derived POM {dollar}sp{lcub}14{rcub}{dollar}C was significantly higher in stable (slaked) compared to less stable (capillary wetted) macroaggregates, but there was no difference in the concentration of surface residue-derived POM {dollar}sp{lcub}14{rcub}{dollar}C in stable compared to unstable macroaggregates. The disruption of macroaggregates during the first 90 d of the incubation resulted in the release of root-derived intraaggregate POM into the free POM pool. After d 90, most of the root-derived C lost from macroaggregates with slaking was contained within microaggregates. The amount of new, root-derived C associated with microaggregates released from macroaggregates with slaking increased linearly with time which supports the hypothesis that microaggregates form within macroaggregates. The data present compelling evidence that in no-till, aggregate formation and stabilization processes are directly related to the decomposition of root-residue and the dynamics of POM C in the soil.