SOIL ORGANIC-MATTER WITHIN EARTHWORM CASTS OF AN ANECIC-ENDOGEIC TROPICAL PASTURE COMMUNITY, COLOMBIA

Large, round-shaped surface casts derived mainly from the anecic Marti odrilus sp., family Glossoscolecidae were investigated for their struc tural stability and the composition of soil organic matter (SOM) assoc iated with different particle-size separates of the casts. Lignin (CuO oxidation) and carbohydrate (acid hydrolysis) signature of SOM was ca rried out in addition to C-13 nuclear magnetic resonance (NMR) spectro scopy, Data obtained for the casts were compared with those of the sur rounding surface soil of an Oxisol under grass/legume pasture followin g native savanna. Earthworm casts showed significantly higher contents of large (3.15-5 and 5-8 mm) water-stable aggregates than the surroun ding soil (765 g kg(-1) vs. 390 g kg(-1)). This higher structural stab ility of the casts corresponded with their higher organic C concentrat ion (56 gC kg(-1)) compared with the surrounding soil (26 gC kg(-1)). The increase was most pronounced for organic C associated with sand-si zed separates, i.e. SOM not involved in organo-mineral complexes, Sign ature of microbial lignin alteration and carbohydrate composition as w ell as C-13 NMR spectroscopy revealed that organic matter (OM) associa ted with sand was mainly composed of slightly decomposed plant residue s. Hence, higher concentrations of carbohydrates and lignin (primary r esources) and lower proportions of C-substituted aromatic C and COOH ( humic compounds) observed in whole casts compared with the unfractiona ted surrounding soil were partly due to the intense mixing of fresh li tter into the mineral soil (anecic effect). Microbial products were en riched in clay-bound SOM, and humified organic compounds dominated sil t-associated SOM, However, the structural chemical analyses showed tha t decomposition of SOM bound to silt- and clay-sized particles appeare d to be in a more advanced stage in the surrounding soil than in the c asts. The data provided evidence that carbohydrate-rich plant debris i s responsible for structural stability of earthworm casts, besides muc opolysaccharides derived from microorganisms in the intestines of eart hworms and the earthworms themselves. According to Golchin et al. (199 4) it was hypothesised that microbial metabolism of notably plant-deri ved carbohydrates results in the release of mucilage and other metabol ites which permeate the coatings of mineral particles and thus stabili se the casts, The consequence of the intimate association of slightly decomposed plant debris with the mineral phase is the build-up of a ra ther active but physically protected C pool which is released concurre ntly with the disintegration of the casts.