Root-derived carbon and the formation and stabilization of aggregates

It is hypothesized that particulate organic matter (POM) contributes to aggregate stability. However, little is known about the dynamics of the POM fraction or its role in aggregate formation. A simulated no-till study was conducted to examine changes in free and aggregate-associated POM during the decomposition of in situ C-14-labeled roots during a 1-yr incubation in a loess-derived silt loam. Two water pretreatments (capillary-wetted and slaked) were applied to soil samples collected during the incubation, and the samples were then wet sieved to obtain five aggregate size fractions, Densiometric separations were used to isolate free and released POM (frPOM) and intraaggregate POM (iPOM). Small macroaggregates (250-2000 mu m) were enriched in iPOM-C-14 on Day 0 which suggested that many of these aggregates formed around cores of new, root-derived POM during the growth and senescence of the oat plants. Slaking resulted in the disruption of many of the small macroaggregates (250-2000 mu m) and a large increase in frPOM-C-14 on Day 0. The amount of C-14 released into the frPOM pool with slaking declined with time. In contrast, there was a significant linear increase in the amount of new, root-derived iPOM-C-14 in large microaggregates (53-250 mu m) that were released when unstable macroaggregates (>250 pm) slaked. These data support the hypothesis that new microaggregates are formed within existing macroaggregates and provide strong 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.