No consistent effects of plant diversity on root biomass, soil biota and soil abiotic conditions in temperate grassland communities

We examined the effects of decreasing plant diversity and functional group identity on root biomass, soil bulk density, soil nitrate and ammonium concentrations, microbial basal respiration, density of predaceous and non-predaceous nematodes, earthworm biomass and density and Shannon-Wiener indices of earthworm diversity in a temperate grassland. Plant species and functional group diversity did not have significant effects on any of these measured variables. However, functional group identity of the plants did significantly affect root biomass and soil abiotic factors. In addition, root biomass, Shannon-Wiener indices of earthworm diversity and density of epigeic earthworms were significantly higher in the presence of legumes, while we found no correlation between functional group identity and other groups of soil biota. We also found several significant relationships among root biomass density, soil microbial basal respiration, nematodes and earthworms. Thus, how can the lack of correlation between soil variables and plant diversity manipulation be explained? Firstly, plant litter input and historical land use have long-term effects on soil properties, which suggests effects of plant diversity manipulation might only be manifested over the long-term. However, we found significant relationships between root biomass density and soil biota suggesting that changes in root biomass density with declining plant diversity could lead to changes in soil biota that might be detectable despite large temporal lags in the soil system. Secondly, we did not find significant effects of plant diversity on root biomass but we found that it was significantly related to the identity of functional groups. Thirdly, we observed complex interactions among trophic levels of soil organisms, which may counteract and buffer effects of changes in plant diversity on soil biota. Fourth, our results suggest that belowground properties might be much more influenced by the identity (and litter quality) of plant functional groups than plant species diversity per se. (C) 2002 Elsevier B.V. All rights reserved.