Soil substrate utilization pattern and relation of functional evenness of plant groups and soil microbial community in five low mountain NATURA 2000

Species rich, semi-natural grassland systems provide several ecosystem functions. The goal was to assess how aboveground composition and evenness affects soil substrate utilization pattern and soil microbial functional evenness. At five German NATURA 2000 grassland sites, the interactions of plant functional groups (graminoids, forbs and legumes) and belowground microbial functional evenness were investigated in relation to soil properties and sampling date. Functional evenness of soil microorganisms was measured with high spatial resolution by community level physiological profiling (CLPP) using multi-SIR (substrate-induced respiration) at three sampling dates during the vegetation period. Evenness indices were used to compare plant functional group diversity and soil microbial functional diversity. All sites differed in the consistently high soil microbial functional evenness, which was strongly predicted by soil pH, but not by plant functional groups or aboveground plant dry matter production. However, soil microbial functional evenness was particularly decreased by an increasing legume proportion and showed seasonal changes, probably driven by shifts in resource availability and soil water content. Our results suggest that changes in soil chemical properties or in a single key plant functional group may have stronger effects on soil microbial functional evenness than changes in plant functional group evenness.