Katarina Hedlund

Agricultural overproduction has led the European Union to encourage long-term abandonment of agricultural land. To enhance the transition of agricultural land to natural grasslands or forests different management practices in relation to vegetation can be used. The aim of this study was to understand the interactions between plant and associated soil microbial communities during the succession of agricultural land. A field experiment was established on newly abandoned agricultural land where the development of the soil microbial community was studied in plots after sowing a mixture of 15 plant species and in plots that were left to be naturally colonized by plants. The plants sown contained five species each of grasses, legumes and forbs representing three functional groups. A subset of these plots were inoculated with soil cores from a later successional stage. Adjacent soils with ongoing agricultural practices and deciduous beech forest were also studied. The microbial community composition and biomass were studied by determining fatty acid signatures (PLFA, NFLA) of soil microorganisms. The microbial community had changed within 2 years from that of the agricultural field diverging into one type present in sown plots and another in plots that were naturally colonized by plants. Sowing plant seed mixtures promoted growth of the bacterial community and sarophytic fungi. Respiration measurements showed higher microbial activity and biomass in sown plots. Bait plants for arbuscular mycorrhizal (AM) fungi and NLFA 16:1omega5 showed that the biomass of AM-fungi was reduced when seed mixtures were sown, which also decreased the formation of mycorrhizal associations on plant roots. (C) 2002 Elsevier Science Ltd. All rights reserved.