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 (n = 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.