Changes in Soil Nitrogen Pools As a Result of Organic Matter Removal and Competing Vegetation Control in a Pacific Northwest Douglas-Fir.

The effects of forest management on soil carbon and nitrogen are important tools to understand the variability in soil fertility and growth of trees, reflecting in cicling and uptaking of nutrients for timber production along the years. The increasing interest in energy production from woody biomass may possibly affect the uptake of these nutrients in soil, mainly N, due to the removal of great quantities of  branches and foliage during the timber harvesting when comparing to the conventional harvest process – it can lead to a decrease in organic matter contents in soil and, consequently, to a lack of nutrients for the next years. Moreover, the decrease of N contents in soil may affect the balance of the nutrient during the growth of the plants resulting in  decrease in final forest productivity. This study aimed to evaluate the soil nitrogen pool in a 12 year old Douglas-fir plantation to a depth of one meter at the Fall River long-term soil productivity site located in Washington. The treatments were composed by: compare bole-only harvest with and without competing vegetation control (B0+VC vs. BO-VC) and bole-only harvest with vegetation control to total tree plus harvest with vegetation control (TTP+VC vs. BO+VC). Four plots per treatment were each sampled in six random subplot locations. It were taken five depth increments for mineral soil sampling : 0-15 cm, 15-30 cm, 30-45 cm, 45-60 cm, and 60-100 cm. Forest floor samples were also collected. Samples were dried at 60°C and mineral soils separated into >4.75 mm and <4.75 mm fractions. Material <4.75 mm was ground and processed in a CHN analyzer. According to primary analysis, it can be infered that there are differences in forest floor N content, however there are few differences in mineral soil.