COS 142-1 - Influence of organic matter and nutrient loading on nitrogen removal and retention across different salt marsh vegetation types.

Abstract: Anthropogenic activities have doubled the amount of bioavailable nitrogen (N) in the biosphere over the last 100 years leading to damaging impacts such as coastal biodiversity loss and harmful algal blooms. Coastal marshes help mitigate N pollution through denitrification and anammox, two microbially mediated pathways of permanent N removal. Though salt marshes can permanently remove N, there is a competing microbial process, dissimilatory nitrate reduction to ammonium (DNRA), which retains bioavailable N in the ecosystem. The partitioning of N-removal versus N-retention is driven largely by sediment organic matter (OM) accumulation and nutrient availability. Marsh vegetation can influence both OM and N availability via OM decay and root exudates. Our objective was to test how alterations in OM and N alter the balance between N-removal and N-retention in coastal marshes in sediments from patches dominated by different vegetation types.

We utilized a lab-based “thin disc” incubation approach to mechanistically determine the balance of coastal N-removal and N-retention pathways by amending sediments from different vegetation patches with OM and N in a full-factorial design (control, OM only addition, N only addition, OM+N addition). We incubated sediment discs from Juncus spp., Cladium spp., and Distichlis spp. — all common northern Gulf of Mexico marsh plants — in the four different treatment tanks for one month. After the incubation, we found that N-removal was the dominant pathway in control sediments across all vegetation types. OM only treatments shifted the dominant pathway to N-retention in all vegetation types. In fact, OM additions, regardless of N treatment, increased N-retention across all vegetation types by over 300%, although, N-removal still remained the dominant pathway in Cladium and Distichlis sediments in tanks amended with OM+N. Sediments that received N only addition did not differ significantly in N-removal or N-retention from the control sediments in any vegetation type.

These findings suggest that both N-retention and N-removal may be limited by OM, however, N-retention is favored in sediments with higher OM availability relative to N availability. Additionally, we determined that biogeochemical processing of N may differ among sediments from different vegetation patches, whereas sediments from Cladium and Distichlis patches are more likely to respond to N or OM enrichment, while sediments from Juncus patches are more resilient to fluctuations in resource availability. Consequently, climate driven sea level rise which impacts marsh vegetation distribution may affect N removal capacity of marshes.