Total Soil Carbon in the Coarse Fraction and at Depth

Historically, most studies estimating soil carbon have been based almost exclusively on 2-mm soil particles (fine soil fraction) and generally excluded soil 2-mm (coarse soil fraction) or soil at 1-m depth. In fact, many studies have not examined soil carbon below 20 cm. This study was conducted to determine how much total soil C is underestimated by following traditional, or standard, soil sampling and analysis for both particle size fractions and depth. Seventeen soil series and one soil classified to the level of Great Group, were sampled in Alaska, Oregon, Puerto Rico, and Washington. Total soil C was quantified for the soil fine and coarse fractions, as well as for soil 1-m deep. Results of this study showed substantial soil C contained in the coarse soil fraction (1-25%) and in soil greater than 1-m depth (3-48%). The combined exclusion of the coarse fraction and soil below 1 m could miss as much as one-half of the soil total C of a profile. These results indicate that to obtain a true value of soil total carbon, the entire soil must be sampled, including the coarse soil fraction and soil below 1 m. FOR .S CI. 57(1):11-18.