More frequent droughts slow down litter decomposition across European agroecosystems and increase the importance of earthworm functional diversity

Effects of increasing rainfall variability and weather extremes on litter decomposition are still uncertain, especially in agroecosystems, where the functional structure of soil communities is already affected. We conducted a mesocosm experiment to evaluate the impacts of different rain regimes and land management on litter mass loss and earthworm ecological groups (epigeic, endogeic and anecic) across European agroecosystems. We also tested if the effects of different rainfall regimes (normal, drought, flooding, intermittent) on earthworm functional diversity (FD) or community-weighted mean (CWM) of earthworm ecological groups (particularly anecic species in the case of CWM), affected litter mass loss across land-use types. We found that drought was the main factor retarding litter mass loss across European agroecosystems irrespective of management type. The effects of the rain regime on litter mass loss were coupled with the pedoclimatic conditions that were different among the studied European land-use types. Across land-use types the importance of earthworm communities for litter decomposition was higher under water depletion. These results also suggest that FD, as a proxy of niche complementarity, is crucial for the stability of the decomposition process under environmental disturbances. The FD values under drought regimes strongly indicated that climatic changes may slow down litter decomposition as a result of FD alterations that could compromise the long-term maintenance of litter mass loss. This result may be especially relevant for the European soils that are already under hydric stress, such as in most Mediterranean agroecosystems.