Microbial transformation of soil organic matter plays a critical role in carbon (C) cycling making it essential to understand how land use and management practices influence microbial physiology and its connection to C dynamics. One factor that is likely to impact soil microbial physiology is crop diversification via its influence on belowground diversity (e.g., chemical heterogeneity of C inputs, microbial community composition). However, the effect of crop diversification measures on microbial physiology and potential effects on C cycling in agricultural soils is still unclear. To address this knowledge gap, we sampled topsoil from eight experimental sites covering different crop diversification measures across Europe (i.e., cover crops, ley farming, vegetation stripes). We used the 18O-labelling method to analyse microbial C use efficiency (CUE), growth, respiration and biomass C. Additionally, a second sampling at five selected sites examined whether the growing season influenced the impact of crop diversification. Meta-analysis revealed no overall effect of crop diversification on CUE, microbial activity, biomass or soil organic C (SOC). However, the effects varied with the type of diversification measure: cover crops did not affect carbon processing, vegetation stripes increased microbial activity, and ley farming enhanced CUE. The largest variation in CUE was observed between samplings at the same sites, indicating seasonal dynamics. Temperature, precipitation and photosynthetically active radiation predicted seasonal variation in CUE (R-2 = 0.36). While cover crops did not significantly impact C storage in our study, both ley farming and vegetation stripes increased SOC. The overall effect of crop diversification on SOC seems to be decoupled from highly temporally variable CUE in the bulk soil and rather relate to C-inputs.
