Soil organic matter within earthworm casts of an anecic-endogeic tropical pasture community, Colombia

Large, round-shaped surface casts derived mainly from the anecic Martiodrilus sp., family Glossoscolecidae were investigated for their structural stability and the composition of soil organic matter (SOM) associated with different particle-size separates of the casts. Lignin (CuO oxidation) and carbohydrate (acid hydrolysis) signature of SOM was carried out in addition to ¹³C nuclear magnetic resonance (NMR) spectroscopy. Data obtained for the casts were compared with those of the surrounding surface soil of an Oxisol under grass/legume pasture following native savanna. Earthworm casts showed significantly higher contents of large (3.15-5 and 5-8 mm) water-stable aggregates than the surrounding soil (765 gkg^-1 vs. 390 gkg^-1). This higher structural stability of the casts corresponded with their higher organic C concentration (56 gCkg^-1) compared with the surrounding soil (26 gCkg^-1). The increase was most pronounced for organic C associated with sand-sized separates, i.e. SOM not involved in organo-mineral complexes. Signature of microbial lignin alteration and carbohydrate composition as well as ¹³C NMR spectroscopy revealed that organic matter (OM) associated with sand was mainly composed of slightly decomposed plant residues. Hence, higher concentrations of carbohydrates and lignin (primary resources) and lower proportions of C-substituted aromatic C and COOH (humic compounds) observed in whole casts compared with the unfractionated surrounding soil were partly due to the intense mixing of fresh litter into the mineral soil (anecic effect). Microbial products were enriched in clay-bound SOM, and humified organic compounds dominated silt-associated SOM. However, the structural chemical analyses showed that decomposition of SOM bound to silt- and clay-sized particles appeared to be in a more advanced stage in the surrounding soil than in the casts. The data provided evidence that carbohydrate-rich plant debris is responsible for structural stability of earthworm casts, besides mucopolysaccharides derived from microorganisms in the intestines of earthworms and the earthworms themselves. According to Golchin et al. (1994) it was hypothesised that microbial metabolism of notably plant-derived carbohydrates results in the release of mucilage and other metabolites which permeate the coatings of mineral particles and thus stabilise the casts. The consequence of the intimate association of slightly decomposed plant debris with the mineral phase is the build-up of a rather active but physically protected C pool which is released concurrently with the disintegration of the casts.