Soil Carbon Response to Thinning and Fertilization Treatments in a Coastal Pacific Northwest Forest.

Forests provide valuable ecosystem and societal services, including the sequestration of carbon (C) from the atmosphere. The majority of forest ecosystem C stocks are contained in the soil. Management practices can impact the balance of C inputs and losses from soil, affecting the ability of forests to act as a C sink and sustainably produce natural resources. This study examines soil organic carbon (SOC) response to thinning and fertilization treatments. 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. Fertilized plots received a total of 1120 kg N ha^-1 as urea over 16 years. Thinning treatments were based on relative stand density. 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. Three pits were sampled per plot and averaged to account for within plot variation. During a single rotation of ~40 years, thinning treatments significantly (Tukey’s HSD, P < 0.1) reduced SOC (Mg ha^-1) by 25% compared to no treatment. Most of this loss occurred in deeper soil layers (below ~20 cm). Thinning effects on the soil environment (e.g., soil temperature increases) and complex interactions between microorganisms, roots, and nutrients enhancing SOC decomposition and nitrogen mineralization potentially explain the observed decrease in SOC. Across all management regimes, deeper soil layers comprised the majority of SOC stocks. This study shows that 1) accurately quantifying and comparing SOC stocks requires sampling deep soil; and 2) forest management can significantly impact both surface and deep SOC on decadal timescales.