Effects of intensive biomass harvesting on the soil microbial community in a northern hardwood forest

Background/Question/Methods

If forest-derived biofuel is going to be a significant part of our current and future energy portfolio, we need to understand how intensive harvesting practices affect soil quality and health, which are integral to ecosystem stability and sustainable forestry practices. A rapid-response soil indicator metric, such as microbial community phospholipid fatty-acid (PLFA) analysis, is a useful tool for assessing the below-ground effects of woody biomass harvests. Biomass harvesting differs from traditional harvesting because not only are trees removed, resulting in canopy gaps, but woody debris, which otherwise would be considered non-valuable, is also removed, resulting in less organic inputs to the soil system. To test the significance of canopy gap creation and woody debris removal due to intensive biomass harvest on the soil microbial community (SMC), we implemented a research study in a second-growth northern hardwood forest in the Flambeau River State Forest, Wisconsin, USA. We used a replicated, 2 x 2 factorial treatment design of canopy gap creation and whole-tree addition, resulting in four treatment levels; gap*whole-tree addition (GWD), non-gap*whole-tree addition (CWD), gap*no whole-tree addition (Gap), non-gap*no whole-tree addition (Control). We sampled the SMC in the 4^th and 5^thyears following treatments (May and August of 2010 and 2011). 

Results/Conclusions

Response variables include microbial guilds (actinomycetes, gram-positive bacteria, gram-negative bacteria, arbuscular mycorrhizal fungi (AMF), saprotrophic/ectomycorrhizal fungi), biomass, and stress-response ratios. Seasonal changes in the SMC composition and biomass were highly significant (p<.001). Stress response ratios indicate that the SMC was most stressed in 2011 overall, and in August of both years (p<.0001). These seasonal changes in the SMC are most likely due to changing microenvironment, including soil temperature and moisture, and differences in carbon and nutrient availability. Actinomycetes, which are a slow-growing, hyphal bacteria that compete poorly with other microbes, were significantly more abundant in control than in CWD and Gap treatments (p<0.04). AMF populations were significantly larger in control than in Gap and GWD (p<0.02); most likely due to an extensive and intact root system of symbiotic AMF trees in the uncut controls versus the harvested Gap and GWD treatments. Finally, the gram-positive : gram-negative ratio was significantly higher in gaps than non-gaps (p<0.06) due to gram-positive bacteria having a thicker cell wall than gram-negative bacteria, allowing them to better withstand physical stresses, such as temperature or moisture. These data emphasize both the temporal variability and the effects of biomass harvesting on the SMC.