A Method Intercomparison Dataset for Quantifying Tidal Wetland Soil Organic Matter and Carbon Content in California, U.S.A.

Coastal vegetated habitats, although small in areal extent, store carbon in their soils at rates higher than many terrestrial environments. This coastal carbon sink is frequently referred to as blue carbon. Most reported values of blue carbon storage and sequestration are based on estimates of soil organic carbon (SOC) derived from the more widely measured loss-on-ignition (LOI), a proxy for organic matter content. Carbon content is often estimated from regression equations fit on subsets of paired samples that have both LOI and less frequently measured carbon content. Carbon content is typically measured by elemental analysis (EA). Protocols for LOI and EA involve methodological choices that can affect measurements that can bias blue carbon assessments.  This dataset presents sets of comparative measurements of organic matter content using LOI and carbon content using EA for 50 soil samples collected from cores taken from six salt marsh sites along the California coast. Loss-on-ignition was calculated for four different combinations of ignition temperatures, as well as with and without samples being homogenized before the procedure. Elemental analysis was performed both with and without acid treatment for removing carbonates. Particle size distribution, dry bulk density, and carbonate content of samples were also calculated, as they can be informative in interpreting the reasons for lab errors. Although standardization of LOI protocol is difficult because of inherent differences in soil properties, data generators and users can take simple, cost-effective steps to estimate common sources of error such as random laboratory error (less than 2%), measurement of inorganic carbon (generally less than 10%), or measurement of non-carbon elements (generally less than 30%).