Interactive effects of climate change and grazing history on soil microbial diversity in Mediterranean agroforestry systems

The impacts of climate change drivers on the diversity of soil microorganisms can be strongly influenced by the soil management history. We conducted a field experiment in Mediterranean agroforestry systems (dehesas) of SW Spain with contrasting grazing histories to assess how climate change drivers and grazing legacy interact to influence soil microbial activity and diversity. Four climatic treatments were applied: warming (W, +2–3 °C), drought (D, −30% rainfall), combined warming + drought (WD), and control (C), across plots representing high, moderate, and ungrazed conditions. Plots were distributed between open grasslands and tree-covered microsites to test the potential buffering effect of canopy cover. Over three years, we measured microbial biomass, soil enzyme activities, N mineralization rates, and bacterial and fungal diversity. Warming decreased bacterial richness by 40% and reduced the abundance of ectomycorrhizal fungi (EMF) by 51% across all land-use legacies. Drought had very little effect on the microbial diversity or abundance indices. Land-use legacy influenced the impacts of climate manipulation; a history of high grazing intensity exacerbated the impact of climatic stressors, with the decline in arbuscular mycorrhizal fungi (AMF) under warmer conditions being significant only in areas with an intensive grazing legacy. The presence of scattered trees was associated to a greater soil fertility and a buffering effect on some microbial groups (Acidobacteria and AMF). These findings suggest that intensively-managed dehesas are particularly vulnerable to climate change, highlighting the critical importance of reducing livestock stocking rates and maintaining tree cover to mitigate negative consequences on soil biodiversity and ecosystem functioning. • Warming reduced soil bacterial richness and ectomycorrhizal abundance in agroforestry systems. • Warming had a greater impact than drought on soil microbial diversity. • Intensive grazing history exacerbated the negative effect of warming. • Tree canopy provided a buffering effect for some microbial groups.