Changes in microbial biomass and activity during old field successions in Brittany, France

Changes in soil following grassland abandonment were studied in bocage land scapes of middle east Brittany (France). Six plant communities representing the main stages of vegetation succession were considered. Organic matter c ontents, ammonification and nitrification processes, microbial biomass and activity were followed over a period of one year. Between the initial and final stages of the succession, an overall increase in organic matter content, microbial biomass and activity occurred. import ant changes occurred also in the structure of microbial communities : the C /N ratio of biomass and the enzymic activity per mu g of biomass N decrease d. After abandonment, organic matter content, nitrogen content and activity tend to decline at least during the initial stages, whereas microbial biom ass remained stable. These changes are attributed to the termination of pas toral management, which induces large changes in the nature of organic matt er inputs. After broom and hawthorn establishment, there was an increase in organic matter content, nitrogen content and activity. However, when oaks and hornbeams established, the microbial activity slowed down and organic m atter content decreased. The spreading of oaks and hornbeams then initiated a strong increase of these parameters. According to pattern changes in soil organic matter and nitrogen contents, microbial biomass and activity, it is concluded that two steps should be co nsidered in the succession, in relation to plant community composition and soil structure. A first one from abandoned grassland to pre-forest (hawthor n) thickets is characterized by a grassland type functioning (no significan t evolution of sail structure, persistence of grassland plant species). A s econd one, when forest species established, is characterized by a forest ty pe functioning (development of B horizon, no mon grassland plant species).