Soil organic carbon flux in riparian ecosystems

Background/Question/Methods

Riparian zones are important components of the landscape providing numerous environmental services. Questions abound regarding to what degree wetland riparian soils function as a sink for organic carbon on the landscape. These questions remain because of difficulty of quantifying soil organic C (SOC) sequestration in hydric riparian settings. We development and tested a series of stratigraphic markers to identify timeframes for calculating SOC sequestration in twenty-four 1^st and 2^nd order riparian zone in southern New England. We compared these SOC sequestration rates to rates derived from process-based metrics (mass balance approach): inputs included roots, litterfall, and deadfall; outputs included CO₂ and dissolved organic carbon (DOC).  

Results/Conclusions

Supported by C-14 and Pb-210 dates, we found that human artifacts (1950), total Pb concentrations (1900), and distribution of non-arboreal weed pollen (beginning and end of peak agriculture) were the most effective markers for identifying time-frames for SOC-sequestration. On average, 84% of the net SOC in the upper m of riparian soils accumulated after European settlement at an average rate of 0.81 Mg C ha^-1 yr^-1. SOC-sequestration rates calculated from process-based data were on the order of 3 times higher than these rates suggesting our measures of C losses from the soil are lower than expected.  DOC has minimal affects on total SOC flux. Total CO₂ respiration was equivalent between upland and hydric riparian soils, however, root respiration was significantly higher in upland soils supporting the need to separate root and microbial respiration when calculating SOC fluxes. Riparian zone SOC pools, to a depth of 1 m, average 246 Mg C ha^-1. Although the riparian zone only covers 8% of the watershed as much as 20% of the SOC is stored in the upper m of these soils with more SOC below that depth. Riparain zones are very important SOC sinks on the landscape.