Data from: The importance of accounting method and sampling depth to estimate changes in soil carbon stocks

Background As interest in the voluntary soil carbon market surges, carbon
registries have been developing new soil carbon measurement, reporting,
and verification (MRV) protocols. These protocols are inconsistent in
their approaches to measuring soil organic carbon (SOC). Two areas of
concern include the type of SOC stock accounting method (fixed-depth (FD)
vs. equivalent soil mass (ESM)) and sampling depth requirement. Despite
evidence that fixed-depth measurements can result in error because of
changes in soil bulk density and that sampling to 30 cm neglects a
significant portion of the soil profile’s SOC stock, most MRV protocols do
not specify which sampling method to use and only require sampling to 30
cm. Using data from UC Davis’s Century Experiment (“Century”) and UW
Madison’s Wisconsin Integrated Cropping Systems Trial (WICST), we quantify
differences in SOC stock changes estimated by FD and ESM over 20 years,
investigate how sampling at-depth (> 30 cm) affects SOC stock
change estimates, and estimate how crediting outcomes taking an empirical
sampling-only crediting approach differ when stocks are calculated using
ESM or FD at different depths. Results We find that FD and ESM estimates
of stock change can differ by over 100 percent and that, as expected, much
of this difference is associated with changes in bulk density in surface
soils (e.g., r = 0.90 for Century maize treatments). This led to
substantial differences in crediting outcomes between ESM and FD-based
stocks, although many treatments did not receive credits due to declines
in SOC stocks over time. While increased variability of soils at depth
makes it challenging to accurately quantify stocks across the profile,
sampling to 60 cm can capture changes in bulk density, potential SOC
redistribution, and a larger proportion of the overall SOC stock.
Conclusions ESM accounting and sampling to 60 cm (using multiple depth
increments) should be considered best practice when quantifying change in
SOC stocks in annual, row crop agroecosystems. For carbon markets, the
cost of achieving an accurate estimate of SOC stocks that reflect
management impacts on soils at-depth should be reflected in the price of
carbon credits.