Coastal freshwater peat accretion – Physiologic processes and legacy impacts of dominant wetland plant species

Background/Question/Methods

Peat-accreting coastal wetlands have the potential to keep elevational pace with sea-level rise, thus providing both adaptation and mitigation for expected rises in atmospheric concentrations of greenhouse gases (GHGs).  Due to oxidation and/or sedimentation, marsh elevations are generally constrained by sea level rise, which has historically averaged ~1-2 mm yr^-1.  When subjected to greater depths and limited suspended sediment, however, organic accretion may be capable of responding rapidly to the greater accommodation space. Two lines of evidence from California’s Sacramento-San Joaquin Delta (SSJD) suggest that potential rates of organic accretion may be underestimated by historic peat records.  First, the presence of fibric remnants in historic SSJD peat cores from 0-6700 yr BP suggests that preservation of autochtonous material may be greater in the absence of mineral inputs.  Second, an experimental restoration of emergent marsh on subsided SSJD peat soil has generated new “proto-peat” at average rates of 4 cm y^-1, nearly 25-times mean sea level rise, storing an average of 1 kg C m^-2 yr^-1 since 1997, and over 2.5 kg C m^-2 yr^-1 in some locations. Using a nested approach of CO₂ flux measurements (eddy-covariance, static chamber, and leaf-scale photosynthesis) as well as quantification of soil carbon fractions at multiple scales, this poster will provide insight to environmental and botanical controls of coastal peatland carbon sequestration.  Using three distinct peat cores from a larger SSJD study with calibrated dating and geochemistry data, fibric remnants (particles >2mm) were assessed at 10 cm intervals and compared with physical and associated geochemical down-core variability.

Results/Conclusions

Organic accretion may be notably high in SSJD freshwater wetlands due to 1) high carbon use efficiency for the dominant species (tule (Schoenoplectus acutus) and cattail (Typha hybrid spp.); NPP/GPP, up to 0.7) and 2) likely use of recycled respired carbon.  Initial comparisons of plot-scale CO₂ exchange based on upscaled leaf estimates and chamber methods, indicate that microbial respiration represents less than half of the CO₂  efflux, and that night respiration is further limited by low temperatures associated with the “delta breeze”.  Fibric remnants in historic peat cores also suggest that biogeochemical conditions associated with mineral inputs promote decomposition and favor the accumulation of mineral fractions at the expense of organic fractions.  These data suggest that a) potential organic accretion may be underestimated during calibration of peat accretion models, and b) plant physiology and biochemistry may be significant factors in the future and historic development of coastal peatlands.