Spatio-temporal structure of earthworm community and soil heterogeneity in a tropical pasture

Abstract

The spatio-temporal structure of an earthworm community was assessed in a tropical Colombian pasture and compared with the patterns of soil heterogeneity. Earthworms were sampled at six dates during a period of one year, while soil properties were assessed at a single date at the end of this period. Sample design was spatially explicit and consisted in a regular grid of 5 m mesh. At each date, 120 samples were taken for earthworm evaluations, while the single soil sampling consisted in 60 points. Multivariate and geostatistical analyses were performed to describe the spatial structure of collected data. Mantel and partial Mantel's tests were used to assess the links between the spatio-temporal patterns of earthworm distribution and soil heterogeneity. The results clearly demonstrated the existence of structured spatial patterns in the earthworm community which were stable in time, at least at the temporal scale concerned by the study period. It mostly corresponded to an alternation of patches where particular species assemblages dominated. The ecological significance of these assemblages is discussed. Soil properties also displayed a significant spatial organisation. Only a small part of the spatio-temporal structures of earthworm distribution was correlated with soil properties. Small species were more abundant in patches where soil was more compact, while large species were associated with high root biomass and total C levels. The question whether earthworm spatio-temporal distribution induces similar patterns in soil properties or the contrary occurs is discussed. Our results suggest that earthworm community organisation mostly corresponds to a non-equilibrium system where mobile earthworm populations influence soil properties. The overall effect of earthworm community on soil is difficult to assess due to the predominance of factors that preferentially express themselves at large scales in the ecological hierarchy.