Formation of aggregates and storage of carbon mediated by earthworms in two soil series of Puerto Rico

Abstract

Earthworms play an important role in soil aggregate formation and carbon incorporation. However, there are no currently published reports of soil aggregate formation mediated by earthworms in the soils of Puerto Rico. This study investigated the carbon incorporation from maize leaves litter (C4 plant) into aggregates in field conditions and the effect of endogeic earthworms on water stable aggregates and carbon incorporation into the aggregates in laboratory conditions. The 6-month field experiment was conducted after application of maize leaves residue on micro-plots of Dagüey and Consumo soils within a secondary forest at Alzamora farm of the University of Puerto Rico at Mayagüez (UPRM). Measurements of aggregate size distribution, total carbon and nitrogen concentratios, 13C concentration, earthworm abundance and biomass were performed. Relation of the earthworm abundance and biomass with aggregate size proportion was analyzed. Only Consumo soil presented change in δ 13C signal between treatments. There was a negative relation between earthworm abundance and large macroaggregates+C4 (P=0.02; R2=0.49) and small macroaggregates+C4 (P=0.01; R2=0.51) for Dagüey soil. In Consumo soil with large macroaggregates+C4 (P=0.001; R2=0.95) and small macroaggregates+C4 (P=0.01; R2=0.80). The microcosm experiment was conducted by 20-day laboratory incubation using 13C naturally labeled maize leaves (-13.76 ±0.08 ‰) and soil collected from Dagüey (-25.48 ± 0.31 ‰) and Consumo (-27.02 ±0.15 ‰) soils. Soil crushed and sieved (< 250 µm) was subjected to four treatments: (I) soil; (II) soil + C4-leaf litter; (III) soil + C4-leaf litter + earthworms (low abundance); and (IV) soil + C4-leaf litter + earthworms (high abundance). At the end of the experiment, aggregate size distribution, total carbon and nitrogen concentrations, 13C concentration, and carbon concentration from soil and C4-leaf litter were measured. The results showed that treatment with earthworms (III and IV) had approximately 3 to 5 times higher small macroaggregates than treatments I and II for Dagüey soil. Also, Consumo soil showed higher mass proportion of small macroaggregates in treatment IV than treatments II and III, however it was similar to treatment I. This study showed that endogeic earthworm species increase the small macroaggregate mass proportion at the expense of other fractions.