Competing Vegetation Effects on Soil Carbon and Nitrogen in a Douglas-fir Plantation
2014
Knight, Erika | Footen, Paul | Harrison, Robert | Terry, Thomas | Holub, Scott
Application of herbicides to control competing vegetation and improve crop tree growth is a common silvicultural practice. Vegetation control has the potential to change pools of soil C and N and thus affect soil quality and C sequestration. In this study, the effects of vegetation control (primarily for herbaceous vegetation) on soil C and N were compared for a bole-only harvest with and without 5 yr of annual herbicide application (+VC and −VC, respectively). Soil C and N were measured in six depth increments (forest floor and 0–15, 15–30, 30–45, 45–60, and 60–100 cm) in a 12-yr-old Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] plantation at the Fall River Long-Term Soil Productivity site in western Washington. Deep-soil (60–100-cm) C concentration was significantly higher (α = 0.10) with vegetation control (14.7 g kg⁻¹ for +VC, 10.4 g kg⁻¹ for −VC). Nitrogen concentration was significantly higher in the forest floor treatment without vegetation control (11.2 g kg⁻¹ N for +VC, 12.7 g kg⁻¹ N for −VC); however, the N content of the −VC 0- to 15-cm mineral soil was significantly lower than the +VC (2920 kg N ha⁻¹ for +VC, 2720 kg N ha⁻¹ for −VC). The root concentration (kg roots kg soil⁻¹) was higher in the +VC treatment at both the 30- to 45- and 45- to 60-cm depth intervals. Despite these differences, there were no significant differences in total C or N content to 100 cm with and without vegetation control. The longer term impact of the greater root concentration at 30 to 60 cm on soil C and N pools needs to be assessed.
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