Harvest intensity and competing vegetation control have little effect on soil carbon and nitrogen pools in a Pacific Northwest Douglas-fir plantation

Increasing demand for timber as well as current interest in the use of woody biomass for energy and chemical production may result in higher quantities of organic matter removal from plantation forests than currently occurs during harvesting. Two practices that can increase the yield of woody biomass from a harvest site are (1) the application of herbicides to control competing vegetation and improve crop tree growth and (2) the removal of branches and foliage (slash) in addition to the bole during harvest. The potential of these practices to change pools of soil carbon and nitrogen necessitates an evaluation of how management practices affect soil quality and carbon sequestration. In this study, soil carbon and nitrogen were measured to a depth of one meter in a 12-year-old Douglas-fir (Pseudotsuga menziesii) plantation at the Fall River Long-term Soil Productivity site in western Washington. The effects of vegetation control (bole-only harvest with versus without annual herbicide application, BO+VC vs. BO-VC) and harvest intensity (bole-only harvest with vegetation control versus total tree plus harvest with vegetation control, BO+VC vs. TTP+VC) on soil carbon and nitrogen were compared. Forest floor and mineral soil samples in six depth increments (forest floor, 0-15 cm, 15-30 cm, 30-45 cm, 45-60 cm, and 60-100 cm) were collected at 12 years following planting of seedlings. Carbon and nitrogen concentrations for the forest floor and the fraction of mineral soil <4.75 mm were obtained and contents calculated. Deep soil (60-100 cm) carbon concentration was significantly higher in the treatment with vegetation control (1.60% C for BO+VC, 1.17% C for BO-VC, α=0.10), and forest floor nitrogen concentration was greater in the treatment without vegetation control (1.21% N for BO-VC vs. 1.06% N for BO+VC), however surface mineral soil (0-15 cm) nitrogen content was higher in plots with vegetation control than those without (2890 kg N ha-1 for BO+VC vs. 2760 kg N ha-1 for BO-VC). No other significant differences were found for the vegetation control, bole-only harvest comparison. In the harvest intensity comparison, forest floor carbon and nitrogen concentrations and contents were lower in total tree plus compared to bole-only harvest (5.28 Mg C ha-1 for TTP+VC vs. 9.52 Mg C ha-1 and 133 kg N ha-1 for TTP+VC vs. 247 kg N ha-1 for BO+VC), but no differences existed in the mineral soil. There were no significant differences in total carbon or nitrogen content to 100 cm for either the vegetation control or the harvest intensity comparisons, which suggests that, at this site, neither the use of vegetation control nor whole-tree harvest is likely to have a negative impact on forest productivity or soil quality in the long-term management of the plantation.