Impacts of Nitrogen Fertilization and Herbicide Use on Forest Soil Bacterial Community and Selected Soil Enzyme Activity.

Beginning in the mid-twentieth century, American Foresters began incorporating the use of herbicides in forestry management as a means to regulate species composition and control competing vegetation. Today, herbicides are widely used in forestry management   practices as a silviculture technique, aiming to achieve greater productivity by improving and increasing forest stands, controlling tree species competition and selecting species best suited for the desired location. Though the use of herbicides is effective in agricultural and forestry management practices; there have been addressed concerns in reference to the long-term environmental effects of using such chemical in a soil habitat. Respectable indicators to evaluate the various changes occurring spontaneously within the soil include soil enzymes, which are protein catalysts that increase the rate of a reaction without experiencing any permanent changes themselves. Enzymes within the soil are usually identified from the microorganisms living in the soil environment, these soil enzymes possess the ability to respond to slight changes within the soil, due to management practices and environmental influences, making them conveniently useful for detecting biological changes. Enzymatic assays are quick, simple and an operative way to measure activities within soil, in addition to detecting different groups of soil organisms. This study was conducted to assess the impacts of herbicides and nitrogen fertilization [NPK] on soil microbial communities and their accompanying activities. Soil samples were obtained from the Escambia Experimental Forest, located in Brewton, AL. Soil treatments consisted of,a total of three herbicide treatments: including hexazonine [ULW], triclopyr [garlon XRT; CH], imazapyr [chopper EC] and two fertilizer regimes:  (+NPK  -NPK) under a randomized complete block design.  Soil samples were evaluated for selected soil enzyme activities: phosphodiesterase [PD] and phosphomonoesterases [PM] [acid and alkaline], in addition to soil chemical (total organic carbon and soil pH) and biological parameters  (microbial diversity). Microbial diversity was determined by extraction of whole DNA followed by PCR optimization and 454 pyrosequencing of the 16S rRNA gene with subsequent bioinformatic analysis. In the assessment of PM (alkaline) the only significant difference among herbicides was triclopyr CH having greater activity than imazapyr chopper EC (p<0.01) and a significant difference was also observed between fertilized and unfertilized plots for PD and PM (acid) enzyme activity.  Also, enzyme activity was observed as being higher (p<0.05) in soils where no fertilizer was applied compared to soil containing fertilizer treatment.