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Title Declines in plant productivity drive loss of soil elevation in a tidal freshwater marsh exposed to saltwater intrusion
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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 ± 308 vs. 2964 ± 204 g/m2 in control plots and an overall 2.8 ± 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 ± 0.4 and 1.7 ± 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 1.4 ± 0.08 kg C/m2 in press plots. In contrast, the control and the fresh treatment plots gained 0.7 ± 0.05 and 0.8 ± 0.05 kg C/m2, respectively, which represents a net change in C storage of more than 2 kg C/m2. 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.

Contributors Elena Solohin, Sarah Widney and Christopher B. Craft

Solohin, E., Widney, S. and Craft, C.B. 2020. Declines in plant productivity drive loss of soil elevation in a tidal freshwater marsh exposed to saltwater intrusion. Ecology. 101(12):13. (DOI: 10.1002/ecy.3148)

Key Words decomposition, root production, saltwater intrusion, sea level rise, Signature Publication, soil elevation, tidal wetlands
File Date 2020
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This material is based upon work supported by the National Science Foundation under grants OCE-9982133, OCE-0620959, OCE-1237140 and OCE-1832178. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.