Soil Carbon, Nutrients, and Phosphorus Fractions: Responses to Weed Control and Harvest Residual Retention in two 10-Year-Old Douglas-fir (Pseudotsuga menziesii) Stands of the Pacific Northwest

Abstract

Intensive forest management techniques such as whole-tree harvesting and use of herbicides to control weeds can have an effect on nutrient cycling and nutrient pool sizes in the mineral soil. This study was conducted to measure differences in soil carbon (C), nutrient pools, or phosphorus (P) fractions associated with harvest-intensity (bole-only vs. whole-tree harvest) and weed-control treatments (initial weed control vs. five years of annual weed control) after ten years. Soil C, nitrogen (N), and P, and exchangeable calcium (Ca), potassium (K), magnesium (Mg), and Hedley P fractions were measured prior to treatment establishment and after ten years in two Douglas-fir (Pseudotsuga menziesii) plantations in the Pacific Northwest. We observed increases in most nutrient concentrations with initial weed control compared with annual weed control at Matlock, with opposite trends at Molalla. Exchangeable Ca and Mg content showed responses to weed control treatments. Harvesting treatments had no significant effects on soil C and nutrient pools at either site. Total P and many of the Hedley P fractions decreased over time, but these effects were ameliorated with the less-intensive initial-weed-control-only treatment at Matlock and the more-intensive annual-weed-control at Molalla. Ten-year responses suggest that harvesting intensity has not impacted potential for maintaining soil C and nutrient pools, with the exception of P, for long-term productivity. Knowledge of the on-site vegetative communities and their role in nutrient cycling is required to anticipate what effect weed control might have on nutrient pools through ensuing stand development.