Permanent and new arable field margins support large earthworm communities but do not increase in-field populations

The maintenance of uncultivated field margins and the creation of new field margin strips are standard agri-environmental measures aimed at enhancing above-ground biodiversity, but their effects on below-ground biodiversity (in margins and also in-field) are insufficiently known. This study compared earthworm communities in permanent (grassy) margins, newly created field margin strips (unsown) and in-field in five arable fields in Ireland over a 3-year period. Permanent field margins initially supported highest earthworm abundance, biomass and species richness compared to the newly created margin strips and in-field at three conventional and two minimum (conservation) tillage sites. However, after three years, mean earthworm populations in the newly created margin strips (371 ± 181 individuals m−2 and 91.9 ± 41.7 g biomass m−2) approached those observed in the permanent margins (470 ± 47 individuals m−2 and 126.5 ± 14.7 g biomass m−2) in the three conventional tillage field sites. By contrast, overall mean population sizes inside conventionally tilled fields were 155 ± 46 individuals m−2 and 38.3 ± 12.5 g biomass m−2. Similar trends were observed at the two minimum tillage sites. Permanent field margins supported somewhat larger mean species numbers compared to in-field at conventional and minimum tillage field sites, but most species still occurred, at significantly lower abundances, inside fields. Spatially defined transect sampling (at 1, 3, 10 and 50 m in-field) at all study sites did not provide evidence for increased populations nearer to the margins or margin strips, even under reduced cultivation. It is concluded that permanent and newly created margin strips in arable systems support larger earthworm populations than are found in-field, but field margins did not appear to enhance in-field populations through colonisation or spill-over effects documented for more mobile, above-ground invertebrate taxa.