Soil carbon and nitrogen response to thinning and fertilization treatments in a coastal Pacific Northwest forest

Background/Question/Methods:  Intensive forest management can impact soil nutrition by shortening rotation intervals, degrading soil structure, and decreasing organic matter retention at the site. This study examines soil carbon (C) and nitrogen (N) 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, thinning treatments, and fertilization treatments – were randomly assigned to nine 0.2-ha plots (within an area of 5 ha) established in 1989 in a juvenile stand with ≥90% (by stem count) Douglas-fir. Fertilized plots received a total of 1120 kg N ha^-1 as urea over 16 years. Thinning treatments were based on relative stand density. Soil bulk density and chemical analysis samples were taken in 2015 in the middle of succeeding soil layers at depths of 0.1, 0.2, 0.5, 1.0, and 1.5 m. Three pits were sampled per plot and averaged to account for within plot variation.

Results/Conclusions:  Thinning treatments significantly (Tukey’s HSD, P < 0.1) reduced total soil C (Mg ha^-1) and N (kg ha^-1) compared to no treatment by 31% and 32%, respectively. Most of this loss (65% and 73%, respectively) occurred in the subsoil (below 20 cm in depth). Potential mechanisms that may explain lower soil C and N stocks due to thinning treatments, as well as differential changes in surface soil compared to deeper soil C and N, include: (1) reduced root density with thinning (fewer total trees) decreasing root turnover and root exudates; (2) reduced competition for water and nutrients with thinning allowing trees to allocate less gross productivity to underground growth; and (3) increased surface soil moisture and temperature with thinning increasing soil organic C decomposition rates. Fertilization treatments tended to reduce soil pH throughout the entire soil profile compared to both thinning treatments and no treatment, indicating the potential for nitrate leaching. Across all management regimes, the subsoil contained over 50% of total soil C and N. This study shows: (1) over a relatively short period (< 30 years), thinning treatments significantly reduced soil C and N stocks; and (2) accurately assessing soil C and N stocks requires sampling deep soil.