Quantifying Deep‐Soil and Coarse‐Soil Fractions

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 m 2 ) 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 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.