Root-derived carbon and the formation and stabilization of aggregates

It is hypothesized that particulate organic matter (POM) contributes to agg regate stability. However, little is known about the dynamics of the POM fr action or its role in aggregate formation. A simulated no-till study was co nducted 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 l oess-derived silt loam. Two water pretreatments (capillary-wetted and slake d) were applied to soil samples collected during the incubation, and the sa mples were then wet sieved to obtain five aggregate size fractions, Densiom etric separations were used to isolate free and released POM (frPOM) and in traaggregate POM (iPOM). Small macroaggregates (250-2000 mu m) were enriche d in iPOM-C-14 on Day 0 which suggested that many of these aggregates forme d 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 ma croaggregates (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 we re released when unstable macroaggregates (>250 pm) slaked. These data supp ort the hypothesis that new microaggregates are formed within existing macr oaggregates and provide strong evidence that, in no-till, aggregate formati on and stabilization processes are directly related to the decomposition of root-residue and the dynamics of POM C in the soil.