Earthworms (Lumbricus rubellus) and the fate of 15N in surface-applied sorghum residues

Earthworms are known to enhance litter decomposition and mineralization processes, but their temporal effects on nutrient release from crop residues and uptake by plants are not well known. To study this phenomenon, two experiment were performed. In the first, the transfer of 15N from surface-applied 15N-labeled grain sorghum (Sorghum bicolor L.) residues of different ages into different soil pools was assessed when earthworms (Lumbricus rubellus Hoff.) were introduced into microcosms. Different aged litter was obtained by microbially conditioning for 0, 30 and 60 d sorghum litter placed on the surface of two Ultisols of different texture (Pacolet sandy clay loam and Cecil sandy loam). After 60 d microcosms were sampled for: earthworm biomass, surface litter, soil organic, mineral and microbial flush N and 15N. In the second, sorghum was sown in the soil from this experiment and grown for 35 d. N and 15N of the roots and shoots were measured. In the first experiment, earthworms buried most of the surface litter transferring 15N from the litter to the soil. Much of the litter 15N in these treatments probably remained in the organic pool, although significantly more mineral 15N and flush 15N was found in worm treatments when all data (from both soils) were combined, indicating mineralization was occurring. In treatments without earthworms, the litter remained on the surface and less 15N entered soil pools. In the second experiment no significant differences were detected in shoot growth and only one earthworm treatment had significantly higher root growth. Nevertheless, root and shoot 15N uptake and recovery of 15N from the original amount of 15N added were higher in all worm-worked soils and significant at two dates for Pacolet and one for Cecil. This accelerated transfer of 15N from surface litter to soil in the presence of earthworms and increased uptake of 15N by plants grown in worm-worked soils, highlights the importance of litter incorporation by earthworms to improve the transfer of litter-derived N into the soil and N availability to plants in systems where residues are deposited on the soil surface (e.g. no-tillage agroecosystems).