Effect of arbuscular mycorrhizae on soil microbial populations and associated plant performance of the salt marsh grass Spartina patens

The effect of arbuscular mycorrhizae (AM) on soil microbial populations and on growth performance of the high salt marsh plant Spartina patens was investigated in a AM suppression study on field-collected soil cores with S. patens. The application of benomyl resulted in a significant reduction of AM colonization on roots of S. patens, but did not completely suppress AM. Non-treated cores had significantly greater colonization (26+/-6%) than either benomyl-(12+/-7%) or benomyl-phosphorus-treated (7+/-3%) cores at a depth of 2.5 cm. Colonization differences between cores declined with depth (5.0 and 7.5 cm). however, so that at 7.5 cm there was no difference between treatments. This decline was attributed to a reduction in oxygen availability with depth as evidenced by decreasing redox potential. Basic environmental conditions generally resembled those found at the field site. There were no environmental differences between treatments at the depths examined. Cell numbers and specific biomass of DAPI-stained organisms as well as members of the Domain Bacteria were significantly higher when AM colonization was suppressed. while those of the Domains Eucarya and Archaea were not significantly influenced. The increase in both microbial and bacterial population size and biomass in the presence of lower levels of AM colonization is most likely due to increases in carbon exudation to soil and rhizosphere populations that accompany AM suppression. PCR-RFLP analysis of nifH amplicons in bulk soil and rhizosphere at varying depths through the soil cores showed differences in banding patterns between rhizosphere and soil material in the presence of AM. The lack of such strong differences in the benomyl-treated cores suggests that AM colonization more strongly affects the nitrogen-fixing population than do physicochemical conditions (e.g. redox potential) alone. Plant growth performance assessed by analyzing root and leaf biomass, as well as excitation transfer efficiency of open photosynthesis system II (PS II) reaction centers (Fv/Fm) was not significantly influenced by AM. Significant differences were found between treatments for C/N ratios and nitrogen content in leaf tissue. indicating that suppression of AM increased plant nitrogen acquisition.