The effects of soil moisture and pH on soil organic carbon content in tidal freshwater marsh wetlands

The carbon stock in tidal freshwater wetlands in the upper estuaries has often been overlooked and not typically included in either the teal or blue carbon accounting. It was in this context that we measured the carbon stock at three selected representative sites (one is distal and the other two are proximal to a water tributary) in a wetland sanctuary of the Chesapeake Bay, United States. The measured average organic carbon stock of 243 ± 31 MgC ha⁻¹ (<i>n</i> = 5) in the top 68 cm soil is consistent with the recently reported high carbon stocks in the tidal freshwater wetlands. More intriguing and potentially more important, however, are that the latitude comparison between the data garnered from the sites distal and proximal, respectively, to a water source and the longitude comparison between the data of this and a 1993 study reveal the remarkable importance of soil moisture in keeping organic carbon. Particularly, the distal site that suffered from soil drying (i.e. had less soil moisture content) revealed a loss of ca. 57% carbon stock over a 28-year span. The measured lower soil pH at the distal site is possibly caused by the lower-moisture-enabled higher activity of oxidizing organic matters, particularly in warm seasons. Accordingly, we proposed that a 59% soil moisture content could be used as the minimum soil-moisture-level criterion for ensuring an effective preservation of carbon stocks and a soil pH decrease in warm seasons as an indicator of potential degradation in its carbon stocking capacity. In addition to confirming the remarkable organic carbon content in soil of tidal freshwater wetlands which has been overlooked until recently, newly uncovered evidence suggests that soil drying over a 28-year span led to ca. 57% decrease of soil organic carbon and soil pH lowering could be indicative of the degrading processes. Unlike the blue and teal carbon stocks in respective coastal and inland wetlands, the organic carbon stock in tidal freshwater wetlands has been overlooked.In addition to confirm a remarkable stock of 243 ± 31 MgC ha⁻¹ in the latter, we found that: (1) soil drying over a 28-year span led to ca. 57% decrease in its soil organic carbon at the site distal to the water resources; (2) soil pH lowering in the warm season could indicate its loss.As the tidal freshwater wetlands occupy non-negligible terrestrial area (14.4% of coastal watershed in US), these findings have profound implications in the studies of various tidal freshwater wetlands ecosystems. Unlike the blue and teal carbon stocks in respective coastal and inland wetlands, the organic carbon stock in tidal freshwater wetlands has been overlooked. In addition to confirm a remarkable stock of 243 ± 31 MgC ha⁻¹ in the latter, we found that: (1) soil drying over a 28-year span led to ca. 57% decrease in its soil organic carbon at the site distal to the water resources; (2) soil pH lowering in the warm season could indicate its loss. As the tidal freshwater wetlands occupy non-negligible terrestrial area (14.4% of coastal watershed in US), these findings have profound implications in the studies of various tidal freshwater wetlands ecosystems.