Interactions Between Microorganisms and Intertidal Plant Communities

Current knowledge on the link between plant communities and the microbial community in intertidal salt marshes is still rather limited. In this chapter we discuss (i) how root-growth strategies of intertidal salt-marsh species may affect microbial activity and community composition and (ii) a case study on nitrogen mineralisation. Intertidal salt-marsh communities are generally characterised by a clear elevational gradient, which causes a gradient in inundation regimes that strongly affects the soil chemistry. The resulting below-ground stress gradient has led to contrasting root growth strategies at different elevations. The contrasting root traits related to these root growth strategies (root architecture, root longevity, root porosity, etc.) are likely to strongly affect the microbial communities in the sediment. This influence can be via effects on oxygen, carbon, nutri- ent, and water availability in the sediment and may influence both the community structure and microbial activity. At a larger scale, the substrate composition of a marsh (mineral vs. peat) is also likely to have a major impact on the microbial community and activity. Microbial communities are, however, also likely to have a major impact on the plant community. A clear illustration is the effect of microbial nitrogen mineralisation on overall nitrogen availability, which is an important factor in competition among plant species. A detailed case study on N-mineralisation at Schiermonnikoog demonstrates both the spatial and temporal variability of microbial activity, and the importance of good quantitative knowledge of microbial processes for understanding whole ecosystem functioning.