Dynamics of Soil Physical Properties in Amazonian Agroecosystems Inoculated with Earthworms

The combined use of earthworm inoculation and organic inputs is considered an efficient way to improve traditional slash‐and‐burn agriculture in the humid tropics. This study tests the hypothesis that the resistant macroaggregate structure that results from earthworm activities is likely to promote sustainability by favoring water infiltration and soil aeration. Six successive crops (maize [ Zea mays L.]‐rice [ Oryza sativa L.]‐cowpea [ Vigna unguiculata (L.) Walp.]‐rice‐rice‐rice) were grown from March 1990 to January 1993 on a fine‐sandy, siliceous, isohyperthermic 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 residues, and with crop residues plus green manure), with or without earthworm ( Pontoscolex corethrurus ) inoculation (36 g fresh weight m −2 ). Soil physical properties (bulk density, total porosity, infiltration, sorptivity, soil water tension, and aggregate‐size distribution) were measured before clearing and after harvesting each crop. The proportion of macroaggregates (>1 cm) increased from 25.1 to 32.7% in inoculated treatments, whereas the proportion of small aggregates (<2 cm) decreased 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 were 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 uptake by larger plants and changes in soil structure. Longer term experiments are necessary to confirm that the activity of the earthworm may not eventually have detrimental effects.