Jason James, School of Environmental and Forest Sciences, University of Washington, Seattle, WA, Robert Harrison, University of Washington, Seattle, WA, Warren Devine, U.S. Army, Fort Lewis, WA and Thomas A. Terry, Weyerhaeuser Co. (retired) and USDA Forest Service Pacific Northwest Research Station, Olympia, WA
Soil is the primary sink for C in forest ecosystems. Nonetheless, soil C is overlooked in ecosystem C budgets and underreported in the literature. N is one of the principle limiting nutrients for forests in the Pacific Northwest. Due to its mobility, considerable amounts of N may be found in deep soil layers. This study examined the systematic sampling depth for ecosystem C and N analyses in the Pacific Northwest, and compared best-fit models of C and N in deep soil layers with laboratory measurements. We gathered forest floor samples and mineral soil bulk density samples at regular intervals down each profile to 2.5 or 3.0 m from 22 sites across the coastal Pacific Northwest Douglas-fir zone. Soil samples were screened to 4.7 mm and analyzed for C and N content. We found that systematic soil sampling shallower than 1.0 m significantly underestimated total soil C and N. On average, 21% of total C and 34% of total N was below 1.0 m. Models predicting total C and N to 2.5 m given only data to 1.0 m were reliable for 20 of 22 sites; the sites that could not be accurately modeled carried the greatest C and N at depth and contained noncrystalline minerals. Shallow soil sampling at best provides a biased estimate and at worst leads to misleading conclusions regarding soil C and N. Researchers seeking to quantify soil nutrient pools or measure change over time should sample deep soil to create a more complete picture of soil organic matter and nutrient dynamics.