Soil Carbon and Nitrogen Stocks: Underestimation with Common Sampling Methods, and Effects of Thinning and Fertilization Treatments in a Coastal Pacific Northwest Forest

Thesis (Master's)--University of Washington, 2017-06Soil contains substantially more carbon (C) than the atmosphere and vegetation combined. Consequently, the fate of soil organic C (SOC) in response to changes in climate, land use, and management is of great concern. The ability to accurately quantify and compare SOC stocks over time is an important part of understanding carbon-climate feedbacks and constraining climate models, particularly because soils with larger standing SOC stocks are most at risk to release considerable C to the atmosphere. The objectives of this study are to 1) review the global importance of SOC and deep soil biogeochemical processes; 2) compare soil sampling methods and approaches to quantify and compare SOC stocks over time; and 3) examine SOC and nitrogen (N) response to thinning and fertilization treatments in a coastal Pacific Northwest forest. Three soil sampling methods for estimating SOC stocks were compared: clod, core, and excavation. As it is considered the least biased method, the excavation method was used as the standard by which other methods were compared. Soil was sampled at an intensively managed Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantation in northwestern Oregon, USA. Management regimes – no treatment (control), thinning treatments, and fertilization treatments – were randomly assigned to nine 0.2-ha plots established in 1989 in a juvenile stand. Prior to harvest in 2015, soil bulk density and chemical analysis samples were collected in the middle of soil depth layers 0-10, 10-20, 20-50, 50-100, and 100-150 cm. Forest floor samples were also collected. Soil was generally non-rocky (