Lumbricid earthworm effects on incorporation of root and leaf litter into aggregates in a forest soil, New York State

Plant litter, microorganisms, and soil minerals assemble into aggregates in soils. Soil aggregates protect organic matter and are a primary site for storage of stabilized soil carbon, and thus many studies have evaluated how agriculture and soil disturbance affects aggregates. However, relatively little is known about aggregation in forest soils and the role of non-native earthworms. In a temperate forest where earthworm invasion has greatly reduced soil C stocks, we measured the abundance of aggregate fractions in plots with and without earthworms. We also quantified the flow of C-13 from leaf litter and root litter into soil aggregates and evaluated how lumbricid earthworms affect this process. Macroaggregates comprised the majority of the bulk soil mass in these fine-textured soils both in the presence and absence of earthworms. The principal effect of earthworms on the composition of macroaggregates was to greatly reduce the proportion of both coarse POM and mineral-sorbed C in them. Earthworms also reduced the proportion of free microaggregates in soil. Two years after addition of isotope-labeled leaf litter most of the label was recovered in macroaggregates in the form of microaggregates held within them. Earthworms also greatly increased the proportion of root-derived C that was incorporated into macroaggegates, thereby apparently accelerating the process of root C incorporation into soil aggregates. However, much of the C incorporated into aggregates over a three-year time scale remained labile and was eventually mineralized. These observations indicate that earthworm effects on C mineralization may exceed effects on stabilization over longer time scales than for previous laboratory experiments.