Soil Biol. Biochem. Vol. 25, No. 6, pp. 789-792, 1993 Printed in Great Britain. All rights reserved 0038- 0717/93 W I0 + 0.00 Copyright 0 1993 Pergamon Pnss Ltd zyxwvut PHOSPHORUS TRANSFORMATIONS CONTRASTING TROPICAL SOILS IN TWO P-SORPTION DURING TRANSIT THROUGH PONTOSCOLEX CORETHRURUS (GLOSSOSCOLECIDAE : OLIGOCHAETA) D. LDPEZ-HRRNANDEZ,’ P. LAVELLE,~ J. C. FARDEAU’ and M. NIRo’ ‘Laboratorio de Estudios Ambientales, Instituto Zoologia Tropical, Facultad de Ciencias, Universidad Central de Venezuela, Caracas, Venezuela, ‘Ecole Nomiale Sup&ieure, Laboratoire de Zoologie, 46 rue d’Ulm, 75230 Paris Cedex05, France and 3Centre d’fitudes Nucleaires Cadarache, 13108 St Paul l_es Durance Cedex, France (Accepted 10 December 1992) Summary-Individuals of Pontoscolex corethrwus Muller, a geophagous tropical earthworm, were fed with two soils that have P-sorption contrasting characteristics. Water-soluble and exchangeable-P contents increased significantly in casts produced after ingestion of the low P-sorption soil, meaning that an important change occurred in the P-sorption complex. This increment was maintained for 4 days of cast aging and it could be ascribed to changes in the sorption reactive surfaces induced by competition for sorbing sites between orthophosphate and carboxyl groups of a (mucus-produced) glycoprotein. The increment in P availability in casts derived from the higher P-sorption soil decreased after 96 h perhaps due to an aging effect. The results presented, coupled with the high population of Pontoscolex corethrurus recorded in natural grasslands and cultivated agroecosystems suggest that these earthworms could be important in affecting P cycling in humid tropical soils. INTRODUCI’ION Several reports have shown the enrichment of earth- worm casts in essential plant nutrients such as P, N, K, Ca and Mg (see, e.g. Lunt and Jacobson, 1944; Parle, 1963; Russell, 1973 and a synthesis in Lee, 1985). The subject has been extensively studied by Sharpley and Syers (1976, 1977) and Sharpley et al. (1979) as regards phosphorus. These authors have shown that casts of Aporrectodea caliginosa contain inorganic P which has a high isotopic exchangeability and is rapidly released to solution. Additional inor- ganic P derived from ingested organic matter and incorporated into the casts is less tightly bound than in underlying soil materials (Sharpley and Syers, 1976) and therefore increases isotopic exchangeability of inorganic P in casts. As a consequence of the greater release of appreciable quantities of inorganic and organic P to soil solution, casts can enhance considerably the dissolved P concentration in surface run-off (Sharpley and Syers, 1976); however, under rainfall conditions where the infiltration rate is not exceeded, inorganic P may be leached from the casts into the underlying soil, and consequently surface casts may represent a significant source of available P for plant growth. A. caliginosa, the species studied by Sharpley and Syers, consumes plant materials derived mainly from the litter layer of grassland soils. Because the avail- able P content of plant material is higher than that of associated soils, the mixing of soil and plant materials and their passage through the earthworm gut, would be expected to result in a significant increase in the more labile or exchangeable P forms in casts. In the case of geophagous earthworms which feed on soils with low organic content, the labile P content of casts may be expected to be lower or comparable with the control soil, since they do not incorporate P-rich litter in their casts. We have investigated the labile P forms in recently deposited and aged casts of the geophagus Pon- toscolex corethrurus fed with two soils of contrasting P retention capacities. MATERIALS AND METHODS The earthworms came from a tropical pasture in the state of Veracruz, Mexico. The mean annual population density in this area was 121,500 ha-’ with a biomass of 350 kg fresh mass ha-’ and soil inges- tion estimated as 400 t soil (dry wt) ha-’ yr-’ (Lavelle et al., 1981). Two soils were used for the study, soil A from Lamto, Ivory Coast, with a low to moderate P sorption capacity and soil B from Mexico, with a high capacity to retain P. The main characteristics of the soils used in the experiments are presented in Table 1. 789