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Deep burrowing earthworms are important ecosystem service providers but their populations are reduced by arable cultivations. We need to both better understand the impact of changes in crop management on earthworms and implement practices to enhance their in-field populations. Two current trends in arable cropping are the increased use of non-inversion tillage and over winter cover crops. Lumbricus terrestris abundances were estimated using midden counting on two field trials comparing tillage and cover cropping management practices. The long running (8-year) field trial showed that shallow non-inversion tillage had significantly (p < 0.01) greater L. terrestris abundances at 4.3 middens per m2, in comparison to deep (ca. 20 cm) non-inversion tillage (3 middens per m2) and conventional ploughing (ca. 25 cm) (1.9 middens per m2), indicating that it is inversion rather than the depth of soil disturbance that is detrimental to their abundance. All trials showed that cover cropping with oilseed radish had no significant (p > 0.05) effect on L. terrestris midden counts, either during winter cover cropping or within the arable rotation. Field observations identified that 93% of middens were associated with crop leaves (still attached to the stem) incorporated into the burrow which may be a novel route of agrochemical exposure and needs further investigation.

Oilseed rape (OSR) grown in monoculture shows a decline in yield relative to virgin OSR of up to 25%, but the mechanisms responsible are unknown. A long term field experiment of OSR grown in a range of rotations with wheat was used to determine whether shifts in fungal and bacterial populations of the rhizosphere and bulk soil were associated with the development of OSR yield decline. The communities of fungi and bacteria in the rhizosphere and bulk soil from the field experiment were profiled using terminal restriction fragment length polymorphism (TRFLP) and sequencing of cloned internal transcribed spacer regions and 16S rRNA genes, respectively. OSR cropping frequency had no effect on rhizosphere bacterial communities. However, the rhizosphere fungal communities from continuously grown OSR were significantly different to those from other rotations. This was due primarily to an increase in abundance of two fungi which showed 100% and 95% DNA identity to the plant pathogens Olpidium brassicae and Pyrenochaeta lycopersici, respectively. Real-time PCR confirmed that there was significantly more of these fungi in the continuously grown OSR than the other rotations. These two fungi were isolated from the field and used to inoculate OSR and Brassica oleracea grown under controlled conditions in a glasshouse to determine their effect on yield. At high doses, Olpidium brassicae reduced top growth and root biomass in seedlings and reduced branching and subsequent pod and seed production. Pyrenochaeta sp. formed lesions on the roots of seedlings, and at high doses delayed flowering and had a negative impact on seed quantity and quality.