Quantifying Deep‐Soil and Coarse‐Soil Fractions

Abstract

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.

Carbon Storage in Coarse and Fine Fractions of Pacific Northwest Old‐Growth Forest Soils

“…Furthermore, small pieces (<2 mm) of charcoal may have contributed C in our samples, although large identifiable pieces of charcoal had been removed. Finally, we did not make C measurements of the material we classified as >2-mm rocks, although organic C may be present in sedimentary rocks (Des Marais et al, 1992), and rock has been mentioned as a possible pool of organic C in forest soils (Bauhus et al, 2002;Harrison et al, 2003).…”

“…Recent investigations of forests indicate the coarse fractions can the fine fraction would substantially underestimate the whole-soil C pool. For example, the <2-mm fraction contains only 63% of whole-soil C in a coastal Oregon forest (Cromack et al, 1999), 53 to 80% in three Australian forest sites (Bauhus et al, 2002), and 37% in one western Washington forest but 97% in another (Harrison et al, 2003).Some >2-mm C may be associated with charcoal or rocks (Bauhus et al, 2002;Harrison et al, 2003). Alternatively, it may be contained in aggregates that are not broken down during routine sample processing.…”

“…Recent investigations of forests indicate the coarse fractions can the fine fraction would substantially underestimate the whole-soil C pool. For example, the <2-mm fraction contains only 63% of whole-soil C in a coastal Oregon forest (Cromack et al, 1999), 53 to 80% in three Australian forest sites (Bauhus et al, 2002), and 37% in one western Washington forest but 97% in another (Harrison et al, 2003).…”

“…Some >2-mm C may be associated with charcoal or rocks (Bauhus et al, 2002;Harrison et al, 2003). Alternatively, it may be contained in aggregates that are not broken down during routine sample processing.…”

Organic carbon, organic matter and bulk density relationships in boreal forest soils

“…However, it should be noted that they only apply to the fine earth fraction of soils (B2 mm). Coarse fraction analysis was not included in this paper, although this compartment may represent substantial amounts of organic C (Harrison et al 2003;Homann et al 2004).…”