Salt marshes are crucial to mitigating the environmental effects of nutrient runoff, yet marshes are being rapidly degraded. Subsequently, marsh construction and restoration has increased, but are these efforts enough to mitigate lost function? Constructed marshes quickly recover aboveground biomass, but biogeochemical function lags behind and can take centuries to regain fully recover. A potential mechanism for this uncoupling of structure and function is that differing microbial community structure subsequently impairs the nitrogen (N) removal capacity of constructed marshes. We conducted an experimental study on sediment from a 33-year-old constructed salt marsh and an adjacent natural marsh from the northern Gulf of Mexico in Mobile county, Al. We evaluated bacterial and fungal denitrification and fungal biomass at both sites to assess both a) the relative contribution of both groups to total N removal in salt marsh ecosystems and b) whether fungal community differences corresponded to functional loss in constructed salt marshes. Sediments from each habitat were amended with glucose and potassium nitrate (KNO3) to stimulate denitrification and exposed to one of three treatments (control, streptomycin, cycloheximide, or both inhibitors) before measuring denitrification potential (DNP). In situ sediment ergosterol was measured seasonally for one year.
Results/Conclusions
DNP in control sediment was significantly different between marshes (p < 0.0005), with rates three times lower in the constructed marsh than the natural marsh. Additionally, natural marsh sediment increased DNP rates in response to increasing N additions, while constructed marsh sediment failed to respond past the lowest concentration of KNO3 amendment (p < 0.001). When exposed to either inhibitor alone, DNP was stimulated relative to control in the natural marsh (p > 0.05) but suppressed relative to control in the constructed marsh (p < 0.00001). Surprisingly, fungi may contribute as much as 73% of total N removal in natural marshes. Additionally, fungal biomass was higher in the natural marsh than the constructed marsh in almost every season (p < 0.05). Our data suggest that fungi and bacteria compete for N in the rhizosphere of natural salt marshes and each group experiences a release from competition when the other is suppressed. Additionally, while DNP is limited by N in natural systems, our data suggest that microbial biomass rather than N availability may limit DNP in constructed marshes. This research highlights the importance of including microbial communities in assessing restoration success and the role of fungi in salt marsh N removal.