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Abstract:
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. SCI. 57(1): 11-18.
Keywords: deep carbon, carbon pools, soil carbon concentrations, missing carbon
SOIL CONTAINS APPROXIMATELY 2,344 Pg of global C and is the second largest actively cycled pool of C on the earth (Hedges and Oades 1997, Jobbagy and Jackson 2000). Of terrestrial pools, soil is the largest active C pool (Post et al. 1990). Historically, studies estimating soil C have been based almost exclusively on the fine (<2 mm) soil fraction and have not included the coarse (>2 mm) fraction. Whitney and Zabowski (2004) found that up to 37% of total soil N was in the coarse soil fraction with the highest concentration often found in forest soils. In addition to excluding the coarse soil fraction, another common practice in soil sampling is to limit sampling depth. These practices may result in a large component of soil carbon being missed, causing soil carbon stocks to be underestimated and potential treatment changes to remain undetected.
Ugolini et al. (1996) suggested that agricultural soils, which are often free of rock fragments, have received a large amount of attention, and, as a...