The influence of soil pH on plant and microbial communities in sandy grasslands

Sandy, calcareous grasslands are species rich temperate semi-natural grasslands. The species composition in 136 plots was analysed in relation to soil chemistry parameters; extractable P and N, soil pH and lime content. We found that that acidification and decalcification decrease species diversity and the number of red-listed species in these sandy grasslands. The microbial community was investigated in relation to soil pH in some of these sandy grassland soils. We found that arbuscular mycorrhiza in the roots of the plants were not affected by the soil pH differences. The extraradical hyphae in the topsoil was found to decrease with increasing soil pH, whereas the extraradical hyphae was found to decrease with increasing soil pH at the 20-30 cm depth. Our results indicate that small scale disturbance may suppress the external mycelium and we also argue that drought stress leads to decreasing AM biomass at high pH. Saprotrophic fungi do not show any relation to soil pH in our studies. The bacterial community structure of the different bacterial groups (G+, G- and actinomycetes) decrease with increasing soil pH. All together, the ratio between fungal biomass and bacterial biomass is related to soil pH.
We found that the extraradical AM mycelia and mycelia from saprotrophic fungi are reduced by increasing level of disturbance. The mycelia are destructed and therefore the AM and saprotrophic biomass in soil is reduced. Bacterial biomass is also reduced by disturbance. Soil disturbance also increase soil pH and decrease organic matter content. AM fungal biomass in the roots of the plants was found to be unaffected by soil disturbance.
We investigated the allocation of C by use of in situ 13CO2 labelling. C allocation was found to be higher to intraradical AM fungal structures than to roots in the sandy grasslands. C allocation was also found to be higher to extraradical AM structures than to saprotrophic fungi. C allocation to extraradical AM hyphae was negatively related to soil pH.
We conclude that the effects of conservation practices on microbial biomass should be taken into consideration in nature conservation programmes. Techniques which are beneficial for the plant community may be disadvantageous for the microbial community. The microbial community is also important for the C cycling in grassland soils and should be taken into account as well.