Abstract
The role of soil fauna in the incorporation of carbon into soil aggregates of tropical soils has been understudied. We conducted a field experiment within a secondary forest and a microcosm experiment at the University of Puerto Rico in Mayagüez to test soil aggregate formation and carbon incorporation by earthworm activity. Using 13C natural abundance in vegetation and the difference in δ13C between C3 and C4 plants, carbon sources in the soil were tracked. Maize leaves (C4 carbon isotopic signal) were used to track the carbon incorporation into soil aggregates (C3 carbon isotopic signal). Earthworms and soil samples (Typic Haplohumults) were collected at 0 to 10 cm soil depth. The treatments for microcosms were: (I) soil, (II) soil + C4 leaves, (III) soil + C4 leaves + two earthworms (low density), and (IV) soil + C4 leaves + three earthworms (high density). Aggregate size classes were separated by the wet sieving method. At the study site, we found two earthworm species belonging to epigeic and endogeic ecological categories. Over six months, our field data suggested that endogeic P. corethrurus can reorganize small macroaggregates to form large macroaggregates. The microcosm experiment corroborated that P. corethrurus consumes soil and transfers soil-derived carbon from microaggregates to macroaggregates. The treatment with the highest earthworm density did not show higher carbon incorporation (0.23 g C/kg sand-free aggregates) into the soil compared to low earthworm density (0.25 g C/kg sand-free aggregates). Our results suggest that P. corethrurus prefers consuming soil-derived carbon and can translocate it from microaggregates to macroaggregates by restructuring soil aggregates.