Soil nutrients and microbial biomass following weed-control treatments in a Christmas tree plantation

Control of competing vegetation in Christmas tree plantations is an important part of management, as uncontrolled weed growth can affect the growth and form of lower branches. Herbicide application is the most common weed-control strategy, but some Christmas tree growers prefer organic mulches. Although both herbicides and other weed-control strategies are available, there is little information about their effects on soil nutrients and microbial biomass, which determine long-term site productivity. A field experiment was conducted in a Christmas tree plantation in eastern Kentucky to examine the effects of four weed-management strategies-sulfometuron methyl herbicide (methyl 2[[[[(r,6-dimethyl-2-pyrimidinyl)amino]carbonyl]amino]sulfonyl]benzoate) (Oust) at two application rates, sawdust mulch, and rubber-tire mulch - on soil nutrients and microbial biomass. We tested the cumulative (3 yr) and re-treatment effects of the four treatments. Soil organic matter, moisture content, and total N were significantly higher in the sawdust-treated soils than in all other treatments. Inorganic N and extractable P were higher in the Oust-treated soils. The Oust and rubber-tire mulch treatments generally had negative cumulative and re-treatment effects on soil cations and pH. Significantly higher microbial biomass occurred in the sawdust treatment. This was most likely due to increased soil water content under the sawdust mulch and coincided with higher total N and soil organic matter for the cumulative and re-treatment periods. Sawdust was the best of the four weed-control strategies for long-term Christmas tree production because of the positive effects on soil physical, chemical, and biological characteristics, an important consideration for growers choosing a weed-control strategy for use on land expected to produce Christmas trees for many years.