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

摘要

Changes in soil following grassland abandonment were studied in bocage landscapes of central eastern Brittany (France). Six plant communities representing the main stages of vegetation succession were considered. Organic matter contents, 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. Important changes occurred also in the structure of microbial communities: the C:N ratio of biomass and the enzymic activity perμg of biomass N decreased. After abandonment, organic matter content, nitrogen content and activity tend to decline at least during the initial stages, whereas microbial biomass remained stable. These changes are attributed to the termination of pastoral management, which induces large changes in the nature of organic matter inputs. After broom (Cytisus) and hawthorn (Crataegus) establishment, there was an increase in organic matter content, nitrogen content and activity. However, when oaks (Quercus) and hornbeams (Carpinus) established, the microbial activity slowed down and organic matter content decreased. The spreading of oaksand 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 considered in the succession, in relation to plant community composition and soil structure. The first one from abandoned grassland to pre-forest (hawthorn) thickets is characterized by a grassland type functioning (no significant evolution of soil structure, persistence of grassland plantspecies). The second one, when forest species established, is characterized by a forest type functioning (development of B horizon, no more grassland plant species).