Water salinity and inundation control soil carbon decomposition during salt marsh restoration: An incubation experiment

Abstract

Coastal wetlands are a significant carbon (C) sink since they store carbon in anoxic soils. This ecosystem service is impacted by hydrologic alteration and management of these coastal habitats. Efforts to restore tidal flow to former salt marshes have increased in recent decades and are generally associated with alteration of water inundation levels and salinity. This study examined the effect of water level and salinity changes on soil organic matter decomposition during a 60‐day incubation period. Intact soil cores from impounded fresh water marsh and salt marsh were incubated after addition of either sea water or fresh water under flooded and drained water levels. Elevating fresh water marsh salinity to 6 to 9 ppt enhanced CO 2 emission by 50%−80% and most typically decreased CH 4 emissions, whereas, decreasing the salinity from 26 ppt to 19 ppt in salt marsh soils had no effect on CO 2 or CH 4 fluxes. The effect from altering water levels was more pronounced with drained soil cores emitting ~10‐fold more CO 2 than the flooded treatment in both marsh sediments. Draining soil cores also increased dissolved organic carbon (DOC) concentrations. Stable carbon isotope analysis of CO 2 generated during the incubations of fresh water marsh cores in drained soils demonstrates that relict peat OC that accumulated when the marsh was saline was preferentially oxidized when sea water was introduced. This study suggests that restoration of tidal flow that raises the water level from drained conditions would decrease aerobic decomposition and enhance C sequestration. It is also possible that the restoration would increase soil C decomposition of deeper deposits by anaerobic oxidation, however this impact would be minimal compared to lower emissions expected due to the return of flooding conditions.

Tidal effects on ecosystem CO2 exchange at multiple timescales in a salt marsh in the Yellow River Delta

“…Second, part of the CO 2 could dissolve in the tidal water. Also, standing water and water in pores reduces the availability of O 2 in the soil, which means an inhibition effect of heterotrophic respiration (Jimenez et al, 2012;Han et al, 2015;Wang et al, 2019a). Furthermore, we also found that tidal flooding reduced the temperature sensitivity of the nighttime ecosystem respiration (Table 2).…”

A new in situ method showed greater persistence of added soil organic matter in natural than restored wetlands

“…Plant roots can also cause higher SOM decomposition rates as a consequence of increased aeration due to radial oxygen leakage (Bernal et al 2017). Prolonged soil saturation, on the other hand, slows SOM decomposition (Wang et al 2019), and increased inundation reduces soil carbon dioxide (CO 2 ) emissions (Neubauer 2013). Moreover, invertebrate detritivores may increase SOM mineralization via colonization and processing of soil and leaf litter (Scatolini & Zedler 1996; Craft 2000).…”

Plant litter composition and stable isotope signatures vary during decomposition in blue carbon ecosystems

“…The varied inundation conditions for seagrass (subtidal settings for leaf wrack, and within the rhizosphere for root/rhizome) compared to A. marina and C. glauca (exposed to sub-daily wetting and drying cycles on the wetland surface in the mid intertidal zone) were likely important. Less oxic conditions in the seagrass deployment would explain slower decomposition (Wang et al 2019). A lack of significant differences in composition of t 0 versus all collections other than t 729 for Z. muelleri leaves, might also be explained by our use of leaf wrack from t 0 which would have undergone some early leaching and/ or diagenesis prior to collection (Trevathan-Tackett et al 2020), thereby missing potentially significant early changes in macromolecular content and/or isotopic signatures.…”