Detritivores and exogenous nitrogen influence litter microbial communities in coastal salt marshes

Saprotrophic fungal communities and animal detritivores are known to regulate litter decomposition, yet their interaction has seldom been examined in coastal salt marshes. Such studies are important because salt marshes have some of the highest carbon storage rates of any ecosystem, and litter plays an important role in carbon cycling. Furthermore, it is currently unknown how species loss and nitrogen (N) pollution will impact the detritivore-fungi or detritivore-algae interactions that occur on litter. To test for changes in the microbial community of decaying Spartina alterniflora Loisel litter, we factorially altered the N concentrations of sediment surrounding the litter as well as the densities of Melampus bidentatus Say, 1822, an abundant but declining detritivorous snail that increases fungal abundance and litter decomposition rates. In each of 36 experimental plots, we analyzed the fungal and algal community composition of litter bundles (collected after 50 and 100 d, respectively) along with detritivore densities, plant traits, sediment and litter N, sediment chemistry, and microclimate. Notably, the densities of snails and isopods were associated with changes in the evenness and relative abundances of fungi in the litter, while amphipods influenced both fungal and algal communities. N additions further altered fungal community structure and increased the dominant saprotroph Natantispora retorquens at both timepoints. In sum, our novel field study revealed that N pollution and the loss of key detritivores, both of which are expected to increase in the future, will likely impact salt marsh fungi. Therefore, global change may impact the animal-microbial dynamics that influence above-ground carbon cycling.