3.1 Field Methods
We followed the field methods described in Hinshaw and Wohl (2021) and collected 11 soil samples per moisture class (wet or dry), where possible, in each category (degraded, treatment, reference) of floodplain using a 3-cm-diameter 30-cm-length spoon sampling soil corer. The sample size of 11 per category is drawn from supplemental information in Sutfin and Wohl (2017), where bias and variance are shown to stabilize after 11 samples per geomorphic unit. Moisture categories were determined based on vegetation (riparian vs upland species), microtopography, and soil moisture conditions at the time of sampling. Moist soil with wetland vegetation (e.g., sedges, rushes) was categorized as wet; all other sites were categorized as dry. All sampling was conducted during relatively dry summer conditions. Dividing samples into separate moisture categories is intended to account for differences in carbon content of saturated vs dry soil found in previous literature (e.g., Moyano et al., 2012; Manning et al., 2015) and we used simple t-tests to test for differences between wet and dry samples. At each sampling location, we noted vegetation present and sample depth.
Samples were obtained at 30-cm vertical intervals at multiple depths to 90 cm from the same sampling hole where the floodplain sediment was sufficiently deep. Soil texture by hydrometer and total and organic carbon analyses were done by a commercial laboratory. Bulk density estimates were assigned based on soil type using the median estimate from a collection of approximations using pedotransfer functions from Leonaviciute (2000) and Ruehlmann and Korschens (2009); regression analyses of data from Chaudhari et al. (2013); and a table of common bulk density values from StructX (Structx 2023). In total, 653 samples collected over the summers of 2020-2022 were used in the analyses for this study. The bulk density values were used in the following equation to convert from organic carbon content to organic carbon stock (Eq. 1):
\(\text{Organic\ Carbon\ Stock\ }\left(\frac{\text{Mg}}{\text{ha}}\right)=Organic\ Carbon\ \%\ *1\ m\ depth*\text{bulk\ density\ }\left(\frac{\text{Mg}}{m^{3}}\right)*10,\ 000\frac{m^{2}}{\text{ha}}\)[1]