Fungal Contribution to Carbon and Nutrient Cycling in a Subtropical Freshwater Marsh

Despite the well-known occurrence of emergent standing litter in freshwater marshes, very little quantitative data is available concerning the functional role and contribution of fungal decomposers to litter decomposition or their impact on ecosystem scale carbon and nutrient cycling. In the present study, microbial respiration and seasonal biomass and production rates of fungal decomposers associated with standing plant litter were examined to assess the quantitative contribution of fungi to both fine scale litter decay processes and large-scale ecosystem carbon flow pathways in a subtropical Typha domingensis and mixed Cladium jamaicense/Juncus roemarianus freshwater marsh, respectively. In addition, the growth and biomass dynamics of fungal decomposers in standing litter were also examined in relation to changes in detrital nitrogen and phosphorus concentrations in order to assess the importance of fungi to nutrient cycling. When integrated over the annual study period, estimated cumulative fungal production in T. domingensis standing litter totaled 39 mgC/g initial detrital C, indicating that 10.5% of the observed Typha leaf carbon lost was assimilated into fungal biomass. Corresponding estimates of microbial respiration (CO2 flux) totaled 133 mgC/g initial detrital C, providing evidence that a considerable amount of Typha leaf carbon is also mineralized by microbial communities, most likely fungi. When examined at the ecosystem scale, estimated annual fungal production within the mixed C. jamaicense/J. roemarianus marsh totaled 90±4 gC/m2, which was equivalent to ~14% of mean annual standing-dead litter mass (643±104 gC/m2). Estimated annual carbon flux from microbial respiration totaled 124±5 gC/m2, representing ~19% of mean annual standing-dead litter mass. In addition to assimilating carbon, fungi also were a significant contributor to the transformation and assimilation of detrital nutrients. Using a fungal stoichiometric ratio of C106N16P1, fungi inhabiting standing litter immobilized \u3e35% the total detrital N and P during decomposition. Results obtained in this study provide compelling evidence that fungal decomposers can play a key role in the cycling of carbon and nutrients in freshwater marshes. Hopefully, these findings will extend our current conceptual understanding of biogeochemical pathways in freshwater marshes