DYNAMICS OF SOIL PHYSICAL-PROPERTIES IN AMAZONIAN AGROECOSYSTEMS INOCULATED WITH EARTHWORMS

The combined use of earthworm inoculation and organic inputs is consid ered an efficient way to improve traditional slash-and-burn agricultur e in the humid tropics. This study tests the hypothesis that the resis tant macroaggregate structure that results from earthworm activities i s likely to promote sustainability by favoring water infiltration and soil aeration. Six successive crops (maize [Zea mays L.]-rice [Oryza s ativa L.]-cowpea [Vigna unguiculata (L.) Walp.]-rice-rice-rice) were g rown from March 1990 to January 1993 on a fine-sandy, siliceous, isohy perthermic Typic Paleudult previously covered by forest at Yurimaguas (Peruvian Amazonia). The experimental design included a combination of three organic residue treatments (without residues, with crop residue s, and with crop residues plus green manure), with or without earthwor m (Pontoscolex corethrurus) inoculation (36 g fresh weight m(-2)). Soi l physical properties (bulk density, total porosity, infiltration, sor ptivity, soil water tension, and aggregate-size distribution) were mea sured before clearing and after harvesting each crop. The proportion o f macroaggregates (>1 cm) increased from 25.1 to 32.7% in inoculated t reatments, whereas the proportion of small aggregates (<2 cm) decrease d from 33.2 to 26.1%, and no change was observed in the intermediate ( 2-10 mm) category. In the control treatment, no significant changes we re observed. Earthworm activities significantly increased bulk density (from 1.12 to 1.23 Mg m(-3)), and decreased porosity (from 58 to 53%) and sorptivity (from 0.45 to 0.15 cm s(-1/2)). Soil water tension was also affected by the presence of earthworms through increased water u ptake by larger plants and changes in soil structure. Longer term expe riments are necessary to confirm that the activity of the earthworm ma y not eventually have detrimental effects.