A key issue attached to the use of post-harvest forest residuals (slash) to make biojet fuel and other chemical products is how slash removal impacts the productivity and ecology of a working forest. The USDA-NIFA, through the NARA project, funds research to evaluate the impacts of slash removal on soil, wildlife, air, and water quality. NARA researchers have published multiple peer-reviewed reports that evaluate how soil nutrient levels are maintained and effectively measured, and additional studies will be published soon that focus on wildlife, soil temperature and water quality.
Recently a peer-reviewed study, funded by the USDA-NIFA through NARA, was published that evaluates how slash removal treatments affect soil microbial populations and diversity. It is anticipated that changes in soil microbial ecology could provide an early warning system for potential long-term impacts on forest watersheds and food webs.
Experiment and results
The authors obtained soil samples from the NARA long-term soil productivity (LTSP) site located on Weyerhaeuser land near Springfield Oregon. This site contains 28 one-acre plots that were harvested for timber and treated with different compaction intensities and varied levels of slash removal. The treatments are used to evaluate how soil compaction and slash removal affect soil temperature, tree growth, pollinators, and microbial diversity.
Multiple soil samples were obtained from each treatment plot plus from a non-harvested section near by, and the DNA contained in each soil sample was extracted and used to identify the number of microbial communities in each sample. The results indicated no statistical difference in the number of microbial populations between treatments.
The authors also observed that the total amount of DNA from each sample varied significantly. This variation was observed even among samples taken in plots that contain a similar treatment. Additional evaluations of soil type are underway to explain the variable DNA concentrations.
The results presented in this publication suggest that the varied compaction and slash removal treatments did not affect microbial diversity. The authors intend to develop the microbial DNA testing method further so that it could be used to predict long-term changes to water quality and nutrient levels. To do this, they intend to use the DNA in the samples to identify individual microbes with the soil community and note those microbes that are responsible for nitrogen and phosphorus cycling. Soils with abundant or deficient numbers of these microbe types should affect water quality in different ways.