Declines in plant productivity drive loss of soil elevation in a tidal freshwater marsh exposed to saltwater intrusion

Abstract

We experimentally increased salinities in a tidal freshwater marsh on the Altamaha River (Georgia, USA) by exposing the organic rich soils to 3.5 yr of continuous (press) and episodic (pulse) treatments with dilute seawater to simulate the effects of climate change such as sea level rise (press) and drought (pulse). We quantified changes in root production and decomposition, soil elevation, and soil C stocks in replicated (n = 6) 2.5 × 2.5 m field plots. Elevated salinity had no effect on root decomposition, but it caused a significant reduction in root production and belowground biomass that is needed to build and maintain soil elevation capital. The lack of carbon inputs from root production resulted in reduced belowground biomass of 1631 AE 308 vs. 2964 AE 204 g/m 2 in control plots and an overall 2.8 AE 0.9 cm decline in soil surface elevation in the press plots in the first 3.5 yr, whereas the control (no brackish water additions) and the fresh (river water only) treatments gained 1.2 AE 0.4 and 1.7 AE 0.3 cm, respectively, in a 3.5-yr period. There was no change in elevation of pulse plots after 3.5 yr. Based on measurements of bulk density and soil C, the decline of 2.8 cm of surface elevation resulted in a loss of 0.77 AE 0.5 kg C/m 2 in press plots. In contrast, the control and the fresh treatment plots gained 0.25 AE 0.04 and 0.36 AE 0.03 kg C/m 2 , respectively, which represents a net change in C storage of more than 1 kg C/m 2. We conclude that, when continuously exposed to saltwater intrusion, the tidal freshwater marsh's net primary productivity, especially root production, and not decomposition, are the main drivers of soil organic matter (SOM) accumulation. Reduced productivity leads to loss of soil elevation and soil C, which has important implications for tidal freshwater marsh persistence in the face of rising sea level.

Differential effects of press vs. pulse seawater intrusion on microbial communities of a tidal freshwater marsh

“…Press plots had considerably less above- (Fig. S1) and below-ground biomass, root production (Solohin et al 2020), pore-water DOC, net ecosystem exchange, and CH 4 emissions than other treatments (Herbert et al 2018). Ecosystem respiration and extracellular enzyme activity (EEA) also was lower in the press treatment ( Table 1), suggesting that the microbial community was carbon limited.…”

Saltwater and phosphorus drive unique soil biogeochemical processes in freshwater and brackish wetland mesocosms

“…2019, Solohin et al. 2020). The brackish soils collected for our experiment were taken from a brackish marsh that has document soil subsidence or peat collapse (Wilson et al.…”

Salinity reduces coastal marsh respiration more than photosynthesis

“…Ecosystems that become salinized or oxygen-depleted from prolonged inundation can experience plant dieback, leading to loss of soil aggregates, increased erosion, and loss of production 14,15 . Wetland plant roots exposed to elevated salinity can lose biomass, especially of ne roots, which also results in soil elevation loss and reduced dissolved and particulate carbon export 16,17 . Further acceleration of soil carbon loss can occur from the production of labile detrital carbon from dying plant material, which can be exacerbated by concomitant changes in porewater and surface water nutrients and redox conditions 8,14 .…”

“…Further acceleration of soil carbon loss can occur from the production of labile detrital carbon from dying plant material, which can be exacerbated by concomitant changes in porewater and surface water nutrients and redox conditions 8,14 . Saltwater intrusion may alter dissolved organic carbon concentrations through physical interactions between cations and dissolved organic matter 18 or through loss of ne root biomass, soil compression, and release of dissolved organic carbon from soil 16,17 . Changes in carbon and nutrient concentrations may result in saltwater-induced changes in land-ocean biogeochemical uxes, as well as shifts in the landatmosphere exchange of greenhouse gases 4,19,20 .…”

“…Saltwater intrusion is increasing worldwide with rapidly rising sea levels, requiring a broader understanding of the consequences for coastal communities and ecosystems. Site-speci c studies have recorded varying productivity responses to elevated salinity despite decreases in soil carbon storage in coastal wetlands 4,[12][13][14][15][16][17] . Reduced plant biomass and growth, and increased soil organic matter processing and export will contribute to carbon losses.…”