Population dynamics of earthworms (Lumbricidae) and their role in nitrogenturnover in wheat and wheat-clover cropping systems

The temporal dynamics of earthworm populations were studied for three years in two farm-scale winter wheat production systems, conventionally monocrop ped wheat and low-input, direct-drilled wheat intercropped with white clove r. Populations were estimated by soil sorting monthly in the first two crop ping cycles and bimonthly in the third cycle. Earthworm population dynamics were strongly related to changes in soil moisture, with low population lev els coinciding with low moisture content. Populations in conventional wheat were generally quite stable but showed a sharp drop after autumn ploughing followed by a marked recovery in the first two cropping cycles. In wheat-c lover, drought conditions during the first summer probably disrupted popula tion build-up. Population levels in the two systems diverged in the second cropping cycle and were widely separated in the third cycle when 319 indivi duals m(-2) (55 g biomass m(-2)) were found in conventional wheat and 1160 individuals m(-2) (175 g biomass m(-2)) in wheat-clover. Nine earthworm spe cies were present in both cropping systems and shifts in species compositio n were relatively small. Murchieona minuscula, a little known endogeic spec ies previously unreported from Ireland, was abundant in both systems and ap peared to benefit from annual ploughing. Earthworm tissue production was estimated to be 81-209 g m(-2) in conventio nal wheat and 122-552 g m(-2) in wheat-clover, while N output resulting fro m mortality was calculated at 0.75-4.24 g m(-2) in wheat and 1.16-10.61 g m (-2) in wheat-clover N output via excretion and mucus was estimated to acco unt for a further 2.93-3.65 g m(-2) in the wheat and 4.98-10.69 g m(-2) in the wheat-clover crop.