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SYMPOSIUM

Quantifying Deep-Soil and Coarse-Soil Fractions

Avoiding Sampling Bias

^a College of Forest Resources, Box 352100, University of Washington, Seattle, WA 98195-2100
^b National Council for Air and Stream Improvement, P.O. Box 13318, Research Triangle Park, NC 27709-3318

^* Corresponding author (robh{at}u.washington.edu).

Forest soils are often deep and/or coarse-textured, which does not always lend itself to easy unbiased sampling. Two important Pacific Northwest (PNW) forest soil series that are deep and coarse-textured were studied to evaluate methods of estimating soil C: (i) a loamy sand glacial outwash soil (Indianola series, mixed, mesic Dystric Xeropsamments) and (ii) a very gravelly sandy loam glacial outwash soil (Everett series, sandy-skeletal, isotic, mesic Vitrandic Dystroxerepts). Four methods were compared for estimating soil C, including: (i) large pit (0.5 m²) excavation, (ii) dug pit with 54-mm hammer-core bulk-density sampling, (iii) 31-mm soil push sampler, and (iv) clod method. Coarse (>2 mm) fragments were also collected, processed, and analyzed for soil C. Extending soil sampling deeper than 15 cm increased soil C estimates by as much as 120%. The pit excavation method with sand-displacement volume measurements, which is by far the most labor-intensive and time-consuming, was considered the "standard" by which other methods were compared, as it didn't contain any obvious biases. Soil core methods overestimated the <2-mm soil fraction (samples taken between large rocks). Biased methods are often accepted as the "best available" due to the high time requirement of pit excavation. The 31- or 54-mm soil core methods often didn't work due to the high rock content (>50%) of the Everett soil. Including C analysis of the >2-mm soil fraction increased soil C estimates by 170% for the Everett series soil (due to organic C contained in the rocks; there were no carbonates) but did not substantially increase the estimate in the Indianola series soil.

Abbreviations: PNW, Pacific Northwest

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