Poor recovery of fungal denitrification limits nitrogen removal capacity in a constructed Gulf Coast marsh

Abstract

Widespread degradation and destruction of coastal wetlands over the last century have spurred on the practice of creating salt marshes to mitigate losses of wetland area and ecosystem function. Constructed marshes can quickly recover plant biomass, but biogeochemical functions, such as nitrogen removal capacity through denitrification, can take decades to centuries to recover. One potential mechanism for this uncoupling of structural and functional recovery is that an impaired microbial community subsequently impedes the nitrogen removal capacity of constructed marshes. While both bacteria and fungi can contribute to nitrogen removal via denitrification, little is known about fungal denitrification in wetlands, even though it has the potential to contribute to climate change via the production of the greenhouse gas nitrous oxide. Here, we measured fungal and bacterial denitrification potential rates and fungal biomass in sediments collected from a 33-year-old constructed marsh and a reference natural marsh to assess both a) the relative contribution of each group to total nitrogen removal and b) whether fungal biomass accrual is a driver of functional recovery in salt marshes. To assess the relative contributions of fungi and bacteria to denitrification, we added selective inhibitors (antifungal, antibacterial, or both) to sediments from each marsh and measured potential fungal and bacterial denitrification rates. We also measured sediment ergosterol concentrations seasonally as a proxy for fungal biomass. In the absence of inhibitors, denitrification potential rates in the constructed marsh were three times lower than in natural marsh sediment. Further, denitrification rates in the constructed marsh remained similarly low regardless of increasing inhibitor concentration, whereas denitrification rates in the natural marsh declined with increasing inhibitor concentration. When either inhibitor was added alone, denitrification was stimulated in the natural marsh, but was suppressed in the constructed marsh, suggesting an apparent competitive release, where the reduction of either bacterial or fungal competition for substrate thereby allowed the remaining competitor greater access to the substrate, in the natural marsh. We observed that compared to the constructed marsh, fungi in the natural marsh contributed ∼30% more (50% vs. 21%) to denitrification, and that fungal biomass was generally higher. Collectively, these data suggest that 3 decades post construction, denitrification rates and the contribution of fungal denitrification to total denitrification are lower in the constructed marsh than the natural marsh. These data also suggest that fungal biomass may limit the recovery of nitrogen removal in constructed marshes and that fungal denitrification can be an important pathway of N removal in salt marsh ecosystems.