Land use determines the composition and stability of organic carbon in earthworm casts under tropical conditions.

Environmental conditions play an important role in controlling the fate of organic carbon (OC) in earthworm casts. However, the mechanisms that lead to the destabilization of OC in casts deposited in different land use systems are poorly understood. In this study, we investigated the impact of land use on the fate and composition of particulate organic matter (POM) and mineral-associated organic matter (MAOM) in earthworm casts under tropical conditions. We conducted a 400-day field exposure experiment in a woodland and a meadow in northern Vietnam with earthworm casts and soil aggregates without trace of earthworm activity (reference soil) under natural rainfall conditions. We analyzed the element content, stable carbon isotope composition and mid-infrared signatures of the two types of materials after 9 and 400 days of field exposure. The results showed that the casts were initially enriched in OC as compared to the reference soils, with the MAOM fraction accounting for over 90% of the earthworm-induced OC accumulation. The POM fraction occluded in casts consisted of fresh plant material and disappeared during the 400 days of field exposure. Enrichment and composition of OC differed between woodland and meadow casts. POM and MAOM showed contrasting persistence in the two land use systems. While woodland casts showed the highest potential to stabilize OC, the actual amount of cast OC stabilized for more than 400 days was more important under meadow. The processes affecting cast OC dynamics were contrasting in both systems. Under woodland, cast OC destabilization was most probably related to microbial degradation, while under meadow, OC accumulation was most probably related to root activity. Our study highlights that the impact of earthworms on the origin, composition, and fate of cast OC in tropical environments is strongly influenced by land use. • Earthworms induce SOC storage as MAOM rather than occluded POM. • Landuse influences composition of earthworm induced SOC in fractions. • Occluded POM in earthworm casts is susceptible to loss on an annuel timescale. • Landuse influences persistance of earthworm induced SOC in MAOM fractions. [ABSTRACT FROM AUTHOR]