Effects of vegetation control on soil carbon and nitrogen in Pacific Northwest Douglas-fir

There is considerable interest in the effects of forest management practices on soil properties. Soil carbon (C) and nitrogen (N) pools are particularly important because of their ties to sustainable site productivity and the role of soil C in forest C budgets. While there have been many studies of forest harvesting effects on soil properties, far fewer have addressed the use of herbicides to control competing vegetation, another common forestry practice.

A recent study found no significant effect of an initial five years of vegetation control on soil C and N in a 12-year-old Douglas-fir plantation compared to a no vegetation control treatment. An important finding of the study was that vegetation control increased concentrations of deep soil (60-100cm) C and root biomass (at 3-45 cm and 45-60 cm depths), which may be linked to higher forest productivity in the vegetation control treatment.

The paper was written by Erika Knight, Paul Footen, and Robert Harrison of the University of Washington, Thomas Terry of Sustainable Solutions, and Scott Holub of Weyerhaeuser Company.  The abstract follows.

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.

  

Reference 

Knight, E., P. Footen, R. Harrison, T. Terry, and S. Holub. 2014. Competing vegetation effects on soil carbon and nitrogen in a Douglas-fir plantation. Soil Science Society of America Journal. http://dx.doi.org/10.2136/sssaj2013.07.0320nafsc