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

Abstract

Many assessments of soil C have been restricted to the <2-mm fraction, but C has recently been identified in >2-mm fractions of forest mineral soils. Our objective was to determine the importance of the >2mm fraction to whole-soil C pools in Pacific Northwest old-growth coniferous forests. Seventy-nine pedons in 18 western Washington and Oregon forests were sampled to o depth of 100 cm. The <2-mm fraction was separated f'rom the >2-mm fractioo by airdrying, physicalty rrushing soil, and sieving; C was determined by Leco combustion. The >2-mm fraction contained up to 46% of the whole-soil C and averaged 23% for the seven forests that had C in that fraction. Following treatment with sodium hexametaphosphate to disaggregate soil material, up to 20% of whole-soil C remained in the >2-mm frsction Thus, the >2-mm fraction C appears to be in stable and unstable aggregates, as well as concretions. The whole-soil C in the surEsce 100 em of mineral soil ranged from 30 to 400 Mg C ha-'. Multiple regression analysis indicated this C pool was positively related to available water capacity, annual precipitation, and coarse woody debris (rZ = 0.63 to 0.66, n = 18 forests). Similar results were obtained with only the <2-mm soil C, which is the basis of previous regional evalmtions. This suggests consideration of the >2-mm fraction does not alter our understanding of the importance of dimate and soil texture as controls of soil C pools, but it does affect the quantificatiun of soil C pools lo many *ld-growth forests ie the Pocific Northwest. SOIL IS A MAJOR POOL in regional budgets and the global cycles of C (Jobbagy and Jackson, 2000;Zinke and Stangenberger, 2000;Bernoux et al., 2002; Goodale et al., 2002). Soil C content varies considerably within landscapes and between regions. It is influenced by climate, soil texture, topography, vegetation, and land management (Homann et al., 1995; Jobbigy and Jackson, 2000;Parker et al., 2001). There is considerable uncertainty associated with estimates of soil C pools and their association with controlling variables, because of sampling and analytical difgculties, soil spatial variability, and methods of extrapolation to regions (Homann et al., 1995(Homann et al., , 1998(Homann et al., , 2001 Brejda et al., 2001).Most studies of soil C pools have measured only fine fractions of the soil, such as €2, <3, or <4 mm (Grigal and Ohmann, 1992;Canary et al., 2000;Parker et al., 2001; Brejda et al., 2001;Homann et al., 2001). 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 b...

Soils of temperate rainforests of the North American Pacific Coast

“…SOC is positively associated with soil water holding capacity, coarse woody debris, and clay content (Homann et al, 2004(Homann et al, , 2007. Vegetation and forest type have been observed to correlate with SOC (Edmonds and Chappell, 1994), as has parent material, with higher soil C concentration found on sedimentary rock in the Pacific Northwest (Littke et al, 2011).…”

“…The values reported in Table 5 are likely underestimates of actual SOC, as they generally do not include soil C from the N2 mm fraction of soils, which has been found contain up to 46% additional SOC in oldgrowth forests in the Pacific Northwest (Homann et al, 2004). Broadscale studies of soil C in Pacific temperate rainforests consistently report higher SOC values than the estimate of Post et al (1985) for cool temperate rainforest globally (154 Mg/ha; Table 5), as well as for wet tropical forests (132 Mg/ha).…”

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).…”

Carbon stocks of western redcedar and western hemlock stands in Canada's inland temperate rainforests.

“…These estimates of mineral soil C stocks are close to the range of old-growth forests of Douglas-fir, western hemlock, and western redcedar in the Pacific Northwest, USA (100 to 150 Mg C ha" 1 ) (Homann et al 2004) and western redcedar and hemlock in coastal BC (47 to 154 Mg C ha" 1 ) (Trofymow and Blackwell 1998;Trofymow et al 2008). The additional C stocks (46 to 160 Mg C ha' 1 ) for mineral soil suggests that our estimates of total forest C stocks (including mineral soil) in uncut old-growth stands could range from 501 to 615 Mg C ha" 1 .…”