Agricultural practices and the spatial distribution of earthworms in maize fields. Relationships between earthworm abundance, maize plants and soil compaction

The relationships between the spatial heterogeneity of maize fields, due to row-cropping and farm machinery traffic, and earthworm abundance were studied in three plots receiving different organic matter treatments: no organic fertilizer, pig slurry and farmyard-manure. In all plots, there was no significant effect of farm machinery traffic although there was a tendancy for earthworms to be less abundant under inter-rows (wheel tracks) than in traffic-free inter-rows. In both the maize field without organic fertilizer and the maize treated with pig slurry, earthworms were primarily located along the maize row. Earthworm abundances were greater within than between rows (16 vs 6 nos 0.1 m−2 in control, 30 vs 15 nos 0.1 m−2 in slurry). In the farmyard-manure treatment, no row effect on the spatial pattern in earthworm numbers was found. However, worm biomass was approximately twice as high under the maize row as under the inter-row. This suggested a greater migration of adults to the maize roots or that juvenile worms grew faster there. Earthworm populations showed spatial variance in life cycle stage with populations under the maize row having proportionally more adults than populations between rows. Soil bulk-density was lower in than between maize rows and lower in maize fields amended with organic matter. Soil bulk-density and earthworm biomass were shown to be negatively correlated (r = −0.92, n = 6). Image analysis of resin-impregnated soil blocks within and between rows showed that the soil under the row was characterized by a higher macroporosity (5.7 vs 0.7%) and also a greater diversity in size and shape of the macropores than occurred between the rows.