The Effect of Fertilization on Biomass and Metabolism in North Carolina Salt Marshes: Modulated by Location‐Specific Factors

Abstract

The resilience of salt marshes to sea level rise depends on vertical accretion through belowground biomass production and sediment deposition to maintain elevation above sea level. Increased nitrogen (N) availability from anthropogenic sources may stimulate aboveground biomass production and sediment deposition and, thus, accretion; however, increased N may also negatively impact marsh accretion by decreasing belowground biomass and increasing net CO 2 emissions. A study was conducted in Spartina alterniflora-dominated salt marshes in North Carolina, USA, to determine how responses to fertilization vary across locations with different physical and chemical characteristics. Pore water residence time, inundation time, and proximity to tidal creeks drove spatial differences in pore water sulfide, ammonium, and dissolved carbon concentrations. Although annual respiration and gross primary production were greater at the creek edge than interior marsh sites, net ecosystem CO 2 exchange (NEE) was nearly balanced at all the sites. Fertilization decreased belowground biomass in the interior sites but not on the creek edge. Aboveground biomass, respiration, gross primary production, and net CO 2 emissions increased in response to fertilization, but responses were diminished in interior marsh locations with high pore water sulfide. Hourly NEE measured by chambers were similar to hourly NEE observed by a nearby eddy covariance tower, but correcting for inundation depth relative to plant height was critical for accurate extrapolation to annual fluxes. The impact of fertilization on biomass and NEE, and thus marsh resilience, varied across marsh locations depending upon location-specific pore water sulfide concentrations. Plain Language Summary Salt marshes provide valuable services, such as protecting the coast from storms, removing excess nutrient pollution from water, and long-term burial of carbon. Because sea level is currently rising, salt marshes need to build up elevation at the same rate as sea level rise to survive. Human-produced nitrogen pollution is rising in salt marshes, often increasing the growth of grass, which may cause the marsh to trap sediment more efficiently and build elevation faster. However, increased nitrogen may also decrease root growth and increase sediment microbial activity (which decomposes sediment organic carbon to carbon dioxide), causing elevation-building to slow down. It is unclear whether the addition of nitrogen affects the marsh's elevation-building rate in a positive or negative way. We found that the effect of nitrogen on elevation-building depends on location. Factors such as tidal inundation and residence time influence sediment water chemistry and, thereby, the response to excess nitrogen. Sulfide inhibits the uptake of nitrogen by plant roots and also microbial activity; thus, marsh locations with more sulfide have diminished responses to nitrogen pollution. This knowledge may be used for management of marshes at risk due to nitrogen pollution.

Net Ecosystem Carbon Balance in a North Carolina, USA, Salt Marsh

“…NEE at control sites was in close balance at both the edge and interior; however, fertilization resulted in net CO 2 emissions at both sites. Both R and GPP were greater on the edge than the interior, and both were stimulated by fertilization (see companion paper Czapla et al, 2020). Edge control and fertilized NEE were −82.7 and 694.7 g CO 2 m −2 year −1 , respectively, but interior control and fertilized NEE were 18.5 and 216.5 g CO 2 m −2 year −1 , respectively.…”

“…Edge marsh had much lower sediment deposition rates than the interior resulting from greater elevation and lower inundation time. Fertilization increased sediment deposition in the interior due to higher AGB (see companion paper by Czapla et al, 2020). Although AGB increased on the edge with fertilization, sediment deposition did not increase substantially because of the short inundation time.…”

“…Most estimates of C accumulation do not make a distinction between edge and interior accumulation rates and, thus, may overestimate C accumulation for the whole marsh. The net loss of C from the edge resulted from low sediment C input and high metabolic rates (Czapla et al, 2020). GPP and R, although nearly balanced at both interior and edge sites, were twofold higher at the edge (Czapla et al, 2020), resulting in higher calculated lateral C export, which was estimated based on measured R.…”

“…A thorough discussion of FC vertical C fluxes and their responses to fertilization at these sites were discussed in detail in the companion paper to this work (Czapla et al, 2020).…”

“…Typical marsh morphology often consists of a high berm on the edge adjacent to the tidal creek due to a history of high sediment deposition rates (Temmerman et al, 2003) backed by a lower interior marsh with elevation gradually increasing toward the upland. Rapid flushing of sediment with oxic creek water (Gardner, 2005; Harvey et al, 1995) causes lower accumulation of metabolic products such as H 2 S and NH 4 + in pore water on the edge than in the interior (Czapla et al, 2020; Howes & Goehringer, 1994). The differences in pore water chemistry across these marsh subhabitats will give rise to distinct C and N cycling dynamics.…”