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Abstract:
Random reductions in plant diversity can affect ecosystem functioning, but it is still unclear which components of plant
diversity (species number – namely richness, presence of particular plant functional groups, or particular combinations of
these) and associated biotic and abiotic drivers explain the observed relationships, particularly for soil processes. We
assembled grassland communities including 1 to 16 plant species with a factorial separation of the effects of richness and
functional group composition to analyze how plant diversity components influence soil nitrifying and denitrifying enzyme
activities (NEA and DEA, respectively), the abundance of nitrifiers (bacterial and archaeal amoA gene number) and
denitrifiers (nirK, nirS and nosZ gene number), and key soil environmental conditions. Plant diversity effects were largely due
to differences in functional group composition between communities of identical richness (number of sown species),
though richness also had an effect per se. NEA was positively related to the percentage of legumes in terms of sown species
number, the additional effect of richness at any given legume percentage being negative. DEA was higher in plots with
legumes, decreased with increasing percentage of grasses, and increased with richness. No correlation was observed
between DEA and denitrifier abundance. NEA increased with the abundance of ammonia oxidizing bacteria. The effect of
richness on NEA was entirely due to the build-up of nitrifying organisms, while legume effect was partly linked to modified
ammonium availability and nitrifier abundance. Richness effect on DEA was entirely due to changes in soil moisture, while
the effects of legumes and grasses were partly due to modified nitrate availability, which influenced the specific activity of
denitrifiers. These results suggest that plant diversity-induced changes in microbial specific activity are important for
facultative activities such as denitrification, whereas changes in microbial abundance play a major role for non-facultative
activities such as nitrification.
