Do wetland soils emit or sequester carbon? A novel method to assess carbon sequestration rate in natural and restored tidal freshwater wetlands

Background/Question/Methods

Coastal wetlands provide a wide range of ecosystem services that include improving water quality and offsetting greenhouse (GHG) emissions. Wetlands store more Carbon (C) than upland soils, meaning that through wetland conservation and restoration we can offset the accumulation of GHGs. The majority of methods for evaluating carbon sequestration rate (CSR) are based on radionuclide ¹³⁷Cs or ²¹⁰Pb dating, which are expensive and not affordable by many lab facilities or researchers. Our goal was to develop a novel method and test an in-situ field bioassay (IFB) using a soil carbon ingrowth cores (SCIC) to assess CSR in restored and natural wetlands. Our bioassay is inexpensive and effective, based on our 2-year field experiment. We first created three types of bags (3.5 cm x 50 cm) by filing them with native wetland soil materials that were well homogenized and sieved. The pore size for the first bag was 6-8 mm and <2mm for the second and the third bag, while the third bag was inside a slotted (0.01 mm) PVC pipe to maintain a proper hydrology. Five habitats (mudflat, low marsh, high marsh, swamp and upland) were selected at Patuxent Wetland Park, a natural wetland, and four habitats (mudflat, low marsh, high marsh and upland) were selected at Wootons Landing Wetland Park, a restored wetland. Both study sites are tidal freshwater wetlands (salinity < 0.3 ppt) located at the Patuxent River, a Chesapeake Bay tributary in Maryland, USA. One hundred-sixty SCIC were deployed in the different habitats for natural and restored wetlands in December 2014, with 81 retrieved after one year and processed to evaluate CSR and the second set retrieved after two years (February 2017). SCIC were sectioned into 5 cm increment then soil bulk density and percent of soil carbon were estimated.

Results/Conclusions

Preliminary results, based on our IFB, showed that the natural wetland was able to sequester 22.3±15.2 kg C ha^-1 year^-1, while restored wetlands lost 51.0±19.2 kg C ha^-1 year^-1 (P_site< 0.05). The results indicate that there is a need to improve wetland restoration techniques to increase restored wetland’s ability to sequester C. The results also show the importance of conserving natural wetlands that play an important role in offsetting GHGs emissions. In conclusion, the novel IFB method is inexpensive and readily available for research scientists to use and apply for estimating CSR without the overwhelming cost of radionuclide dating.