Two aspects of earthworm bioturbation: Crop residue burial by foraging and surface casting in no-till management

In no-till agroecosystems, presence of the earthworm Lumbricus terrestris L. can be a key driver in the replenishment of soil organic matter stocks post-harvest, through surface residue foraging and incorporation. The impact on such systems under different climatic conditions is, however, still unquantified. A field experiment was designed to determine incorporation of cereal harvest residues at three no-till agricultural sites in boreal conditions (SW Finland) focusing on L. terrestris impacts over the period from autumn harvest to spring sowing. Either spring barley, wheat or oats were cultivated at the sites. Following the harvests, representative masses of residues were applied on small experimental plots (0.25 m2) with cleaned soil surfaces in areas of inherently high (LT+: 10.6 ± 2.1 ind. 0.25 m-2) and low (LT-: 1.8 ± 0.7 ind. 0.25 m-2) L. terrestris density within the fields. Residues were covered with metal mesh caging and left until spring sowing, when changes in residue mass were recorded and plots sampled for earthworms. The reduction of straw residue mass varied between sites, from no difference between the LT+ and LT- treatments to 19 % and 59 % higher mass reduction in LT+. Amount of earthworm castings on the residues was also quantified and findings indicated a positive correlation with earthworm (also endogeic) density. For L. terrestris, earthworm species abundance relationships indicated competitive interaction with epigeics and positive interaction with endogeics. Results showed that L. terrestris foraging can enhance the incorporation of cereal surface residues outside of the boreal growing season and that earthworm surface casting also has a role in residue burial. However, the increase of incorporation by L. terrestris activity, interrupted by winter conditions, was moderate and did not lead to marked exposition of the soil surface by experimental end, even at those plots with the highest L. terrestris foraging. The incorporation rate estimates may have been unrepresentative due to the exceptionally late harvest during the study period. Investigations covering the whole year from multiple growing seasons are needed for a comprehensive assessment of earthworm impacts on crop surface residue dynamics.