Evaluating the delayed, causal effects of summer irrigation on winter earthworm communities in a Mediterranean agroforestry trial

Mediterranean agroecosystems increasingly face prolonged summer drought, extended bare-soil periods and heat extremes, all of which constrain soil biota and threaten long-term soil functioning. Management practices maintaining plant cover during summer, such as cover crops, may alleviate these constraints but generally require irrigation in dry regions. However, the ecological consequences of combining summer cover crops with irrigation remain poorly quantified for soil fauna, particularly in heterogeneous agroforestry systems. We monitored earthworm communities for six years (2019–2024) at the DIAMS experimental platform, a replicated Mediterranean alley-cropping agroforestry trial where an irrigated summer cover crop was implemented factorially. Earthworms were surveyed each winter across contrasted habitat types to assess delayed (legacy) effects of summer management. To isolate irrigation effects from spatial heterogeneity and interannual climatic variability, we applied a Difference-in-Differences framework complemented by event-study analyses. Summer crop irrigation did not affect earthworm biomass or species richness but induced habitat-specific changes in abundance. Abundance increased in irrigated agroforestry crop alleys (×3. 5 in AFC and ×2. 5 in AFC1m), while no detectable effects occurred in monoculture or tree-dominated habitats. Communities were largely dominated by Microscolex dubius, suggesting that irrigation primarily enhanced the survival or recruitment of this disturbance-tolerant species. Irrigation effects were stronger following dry autumns, indicating climate-dependent legacy responses. Overall, our results show that irrigation benefits for soil fauna are strongly context-dependent, emerging primarily in exposed crop habitats and shaped by habitat heterogeneity, seasonal legacy effects and interannual climatic variability. • Summer irrigation increased earthworm abundance in agroforestry crop alleys but not in monoculture or tree habitats. • A Difference-in-Differences design revealed delayed irrigation effects emerging in winter communities. • Responses were driven by numerical increases of the disturbance-tolerant species Microscolex dubius.