Deep Soil Carbon and Nitrogen Changes With Organic Matter Removal and Competing Vegetation Control in a Pacific Northwest Douglas-Fir Plantation.

The yield of woody biomass from forest harvesting can be increased through application of herbicides to control competing vegetation, thereby increasing crop tree growth, as well as through the removal of branches and foliage in addition to the bole during harvest. In this study, the effects of vegetation control (bole-only harvest with and without competing vegetation control, 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 measured to a depth of one meter in a Douglas-fir plantation at the Fall River Long-term Soil Productivity site in Washington. 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. There were no significant differences in total carbon or nitrogen content to 100 cm for either the vegetation control or the harvest intensity comparisons, however the results of our study suggest that the soil has changed at depths deeper than 20 cm. For the vegetation control comparison, the BO+VC and BO-VC treatment averages suggest that BO+VC content to 100 cm was 16 Mg C ha^-1 greater than BO-VC content, with 14.6 Mg C ha^-1 of this difference occurring in soil deeper than 20 cm. Similarly, the observed harvest intensity treatment differences in mineral soil carbon content to 20 cm was 7.5 Mg ha^-1 while the difference in carbon content to 100 cm was 18 Mg ha^-1. The Fall River results also show treatment differences in nitrogen content of the deep soil, with 20-100 cm soil nitrogen contents of 8400 kg ha^-1 in the BO+VC treatment, 8860 kg ha^-1 in BO-VC, and 8800 kg ha^-1 in TTP+VC.