Interactive effects of plants and earthworms on the physical stabilization of soil organic matter in aggregates

Plants and earthworms are key ecosystem engineers and important regulators of soil aggregation and C dynamics, yet research to date has mainly considered their impacts in isolation thereby ignoring potential interactions between these organisms. We conducted a microcosm experiment under greenhouse conditions to assess the impacts of plants (Brachiaria decumbens) and earthworms (Pontoscolex corethrurus) on soil structure and C stabilization. Aggregate stability was assessed by wet-sieving. Large macroaggregates (> 2 mm) were also visually separated according to origin (e.g., earthworms, roots) and then further fractionated into particle size fractions to assess aggregate composition and C distribution. Earthworms increased aboveground biomass of B. decumbens by nearly 30 %. The presence of plant roots increased aggregate stability (mean weight diameter) by 2.6 %. While earthworms alone had no simple impacts on aggregation, a significant interaction revealed that earthworms increased aggregate stability in the presence of roots by 6 % when compared to microcosms without plants. Additionally, the presence of roots increased the C concentration of coarse particulate organic matter in earthworm casts, while earthworms increased C storage in microaggregates and the silt and clay fraction within root-derived aggregates. These findings suggest that plants and earthworms are intimately linked in soil aggregate formation and that both organisms need be considered simultaneously for proper management of soils.