Citation:

Herbert, E., Joseph P. Schubauer-Berigan, AND C. Craft. Effects of 10 yr of nitrogen and phosphorus fertilization on carbon and nutrient cycling in a tidal freshwater marsh. LIMNOLOGY AND OCEANOGRAPHY. American Society of Limnology and Oceanography, Lawrence, KS, 65(8):1669-1687, (2020). https://doi.org/10.1002/lno.11411

Impact/Purpose:

Tidal freshwater marshes (TFMs) can protect downstream ecosystems from eutrophication by intercepting excess nutrient loads; however, recent studies in saltmarshes suggest that long-term nutrient loading compromises the structural and functional integrity of tidal wetlands. Here we present data on changes in macrophyte productivity and biogeochemical cycling of carbon (C), nitrogen (N), and phosphorus (P) from experimental plots in a TFM on the Altamaha River (GA) fertilized for 10 years with N, P, or N+P. Nitrogen fertilization doubled aboveground biomass and N storage and enhanced N removal via denitrification, but did not enhance N storage in soils. Phosphorus fertilization enhanced microbial productivity and P storage in soils, but reduced belowground biomass. In combination, NP in a 15:1 (mole:mole) ratio increased aboveground biomass, microbial biomass and activity, and enhanced both the N and P removal capacity of the TFM more than either nutrient alone without decreasing belowground biomass or soil C. While NP together enhance the overall productivity and nutrient removal capacity of the wetland, only 6% of N and 35% P added over the 10-year period was retained, suggesting our TFM may have a limited capacity to assimilate excess nutrients. Rates of denitrification measured in N and NP plots in laboratory incubations were high (784-1356 nmol N dry g-1 d-1) indicating there may be high potential for excess N removal. This work highlights the importance of long-term nutrient enrichment studies that examine both macrophyte and microbial responses and suggests that TFMs are sensitive to change in the stoichiometry of nutrient loading.

Description:

Tidal freshwater marshes (TFMs) can protect downstream ecosystems from eutrophication by intercepting excess nutrient loads; however, recent studies in saltmarshes suggest that long-term nutrient loading compromises the structural and functional integrity of tidal wetlands. Here we present data on changes in macrophyte productivity and biogeochemical cycling of carbon (C), nitrogen (N), and phosphorus (P) from experimental plots in a TFM on the Altamaha River (GA) fertilized for 10 years with N, P, or N+P. Nitrogen fertilization doubled aboveground biomass and N storage and enhanced N removal via denitrification, but did not enhance N storage in soils. Phosphorus fertilization enhanced microbial productivity and P storage in soils, but reduced belowground biomass. In combination, NP in a 15:1 (mole:mole) ratio increased aboveground biomass, microbial biomass and activity, and enhanced both the N and P removal capacity of the TFM more than either nutrient alone without decreasing belowground biomass or soil C. While NP together enhance the overall productivity and nutrient removal capacity of the wetland, only 6% of N and 35% P added over the 10-year period was retained, suggesting our TFM may have a limited capacity to assimilate excess nutrients. Rates of denitrification measured in N and NP plots in laboratory incubations were high (784-1356 nmol N dry g-1 d-1) indicating there may be high potential for excess N removal. This work highlights the importance of long-term nutrient enrichment studies that examine both macrophyte and microbial responses and suggests that TFMs are sensitive to change in the stoichiometry of nutrient loading.

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