Soil properties inside earthworm patches and gaps in a tropical grassland (la Mancha, Veracruz, Mexico)

Earthworms often form spatially structured populations characterised by alternate clusters of either low or high density. This study compared various soil properties and herbaceous plant biomass between areas of high and low earthworm density (i.e. patches and gaps) in a tropical grassland (la Mancha, Veracruz, Mexico). We aimed at identifying those variables that might explain the observed spatial distribution or, conversely be explained by the community pattern. We examined the spatial aggregation by means of the Spatial Analysis by Distance IndicEs (SADIE) system and showed the presence of significant patches and gaps. Only a few variables significantly differed between patches and gaps. Areas corresponding to earthworm patches and gaps had more silt and clay, respectively, in the 10-30 cm soil layer. There was no difference in the C content in the different particle-size classes expressed in absolute values (mg C g−1 soil) but the proportion of C associated to > 200 μm particles was larger in the patches (0-10 cm layer) while there was more C associated to the fine particle (< 50 μm) in the gaps (10-30 cm soil layer). Patches and gaps did not differ significantly in terms of cation concentrations (K, Mg and Ca), pH, soil bulk density or herbaceous plant dry mass. The lack of clear relationships between earthworm distribution and soil parameters in this study suggest that earthworm populations and soil properties may occur at different spatial scales.