Soil Carbon and Nutrient Pools: Responses to Competing Vegetation Control and Harvest Residual Retention after Ten Years in Two Douglas-Fir Stands of the Pacific Northwest.

Intensive forest management techniques such as whole-tree harvesting and use of herbicides to control weeds can have an adverse effect on nutrient cycling and pool sizes in the mineral soil. This study was conducted to determine if there were differences in C and nutrient pools associated with different harvest intensity (bole-only vs. whole-tree harvest) and weed control treatments (initial weed control vs. five years of annual weed control) ten years after initial treatments.  Soil total C, N, and P, and extractable Ca, K, Mg were measured at 0-15, 15-30, and 30-60 cm soil depths at treatment establishment and after ten years in two Douglas-fir (Pseudotsuga menziesii) plantations in the Pacific Northwest near Matlock, WA (sandy-skeletal, mixed, mesic, Dystric Xerorthents) and Molalla, OR (fine-loamy, isotic, mesic Andie Dystrudepts). We observed increases in most nutrient concentrations with initial weed control compared with annual weed control at Matlock, with opposite trends at Molalla. However, only Ca, and Mg pools showed these responses to weed control treatments. Harvesting treatments had no significant effects on soil C and nutrient pools at either site. Ten-year responses suggest that harvesting intensity has not impacted potential for maintaining soil C and nutrient pools for long-term productivity.  However, effects of weed control varied by site, likely related to differences in vegetative communities between the two sites.