Biol Fertil Soils (1992) 14:49-53 Biology and Fertih'ty ~ Soil s 9 Springer-Verlag 1992 Nitrogen mineralization and reorganization in casts of the geophagous tropical earthworm Pontoscolex corethrurus (Glossoscolecidae) P. Lavelle 1, G. Melendez 2, B. Pashanasi 2, and R. Schaefer 1 1Laboratoire d'Ecologie des Sols Tropicaux, ORSTOM/Universit6P. et M. Curie, F-93-Bondy,France ZINIAA, Estacidn experimental San Ram6n, Yurimaguas, Loreto, Peril Received January 13, 1992 Summary. Mineral N concentrations ranged from 133.1 to 167.8 ggg-1 dry soil in fresh casts of the endogeic earthworm Pontoscolex corethrurus fed on an Amazo- nian Ultisol; this was approximately five times the con- centration in non-ingested soil. Most of this N was in the form of NH2. N also accumulated in microbial bio- mass, which increased from a control value of 10.5-11.3 to 67.5-74.1 gg g-1 in fresh casts. During a 16-day incu- bation, part of the NH~--N was nitrified and/or trans- ferred to the microbial biomass. Total labile N (i.e., min- eral+biomass N) decreased sharply at first (ca. 50% in the first 12 h), and then more slowly. The exact fate of this N (microbial metabolites, denitrification, or volatil- ization) is not known. After 16 days, the overall N con- tent of the casts was still 28% higher than that of the con- trol soil. Incubation of the soil before ingestion by the earthworms significantly increased the production of NH~ in casts. We calculate that in a humid tropical pas- ture, 50-100 kg mineral N may be produced annually in earthworm casts. Part of this N may be conserved in the compact structure of the cast where the cast is not in close contact with plant roots. Key words: Earthworm - Pontoscolex corethrurus - Nitrogen mineralization - Microbial biomass - Casts - Ultisol Recent experiments (Spain et al. 1992) have demonstrated that the growth of cultivated tropical plants may be im- proved in the presence of geophagous earthworms. In a 70-day assay, the total production of Panicum maximum significantly increased in parallel with the biomass of Millsonia anomala, a common endogeic earthworm spe- cies in humid savannas of the Ivory Coast. Maximum dry matter production of 2.8 times the control production was observed with an earthworm biomass equivalent to Correspondence to: P. Lavelle 750 kg fresh weight ha -1. The plant biomass was also significantly enriched in N (+29% to +116% and +88% to +260%, respectively, in leaves and roots) and P (+58% to + 152% in roots; no difference in leaves). At Yurimaguas, in the Peruvian Amazonia, a similar experiment was conducted for 120 days to assess the ef- fect of P. corethrurus, a geophagous earthworm common throughout the humid tropics, on the growth of tree seed- lings. The response varied, depending on the tree species; Bixa orellana grew 3 times better in the presence of earth- worms, Eugenia stipitata grew 1.3 - 1.7 times better, while no significant effect was observed with the palm-tree Bac- tris gasipaes. The different responses by the tree seedlings were probably a result of the different rooting systems, the Bixa sp. having a dense, deep rooting system that al- lowed a much better use of the nutrients released by earthworms compared with the Bactris sp., which has a low density of coarse superficial roots. Dramatic effects on N mineralization and microbial biomass were ob- served, with highly significant increases in these parame- ters in the presence of earthworms (Pashanasi et al. 1992). The present work was designed to test the hypothesis that similar improvements in plant growth would result from increased N mineralization during transit through the earthworm gut and in the casts. Previous studies have indicated that earthworm casts always have higher con- tents of mineral N than the bulk soil (Syers et al. 1979; review in Lee 1985). Nonetheless, these studies used litter- feeding species, and the increased amounts of nutrients in the casts may have simply reflected the original composi- tion of the ingested material. P corethrurus is a peregrine species with a high poten- tial for introduction in tropical cropped soils (Lavelle et al. 1987). It is therefore important to evaluate the impact on N cycling of the activities of this earthworm in order to determine the short- and long-term effects on soil fer- tility (Lavelle and Martin 1992). In the present study, mineral N (NH2-N+NO~--N) contents were measured in casts incubated for different periods (from 0 to 16.5 days) to assess the amount of min- eral N available as NH2 and NO; and the dynamics of