Interactions between the salt marsh grass Spartina patens, arbuscular mycorrhizal fungi and sediment bacteria during the growing season

The interaction between the salt marsh plant Spartina patens, arbuscular mycorrhizal fungi (AMF) and bacteria in salt marsh sediment was examined in a long-term arbuscular mycorrhizas (AM) suppression study by applying the systemic fungicide benomyl to field-collected sediment cores with and without S. patens plants. Microbial populations were sampled four times corresponding to major plant phenological stages (dormancy, vegetative growth, reproduction, and senescence) previously linked to changes in microbial populations under field conditions. Benomyl-treatment of soil cores significantly suppressed AM colonization on S. patens, keeping values relatively consistent throughout the growing season (11.5%) whereas plants in non-treated cores experienced seasonal increases and declines in AM colonization (26.6% during vegetative growth to 11.5% during dormancy). Soil physicochemical parameters were not affected by benomyl application. In unvegetated cores, no benomyl- or seasonal effects were displayed by cell numbers and specific biomass of DAPI-stained organisms, members of the domain bacteria and here especially members of the α-, β-, γ- and δ-subdivisions of proteobacteria that were the most abundant bacterial groups. In vegetated cores, the microbial community as well as specific bacterial populations were at least twice as large in terms of number and biomass than in samples from unvegetated cores with significant seasonal changes for DAPI-stained cells, for members of the domain bacteria and for members of the α- and γ-subdivisions of proteobacteria. In benomyl-treated cores, the population of γ-subdivision of proteobacteria was significantly smaller than in non-treated cores, and a positive association was found between this bacterial group and root length colonized by AM suggesting that AM-suppression can affect populations of specific soil bacterial populations in salt marsh sediment. Benomyl-treatment had no effect on the diversity of N-fixing bacteria as evidenced by PCR-RFLP analysis, but seasonal changes were noted in vegetated cores with populations during active plant growth substantially different from populations during dormancy and senescence.