The Effects of Pile Burning and Charcoal on Carbon and Nitrogen Transformations in Eastern Sierra Nevadan Forested Soils
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Over a century of fire prevention and suppression in the western United States has led to overstocked and diseased forests with increased fuel loading and vertical continuity of fuels, thus increasing the risk of severe and stand-replacing wildfire. Mechanical thinning followed by pile burning is a commonly used technique to reduce fuel loads. Prescribed pile burning and products of pile burning, such as ash and charcoal, may influence nutrient dynamics within forest soils. Specifically, charcoal-rich environments have been shown to enhance nitrogen cycling. Pile burning also generates a pulse of biologically available nutrients through the mineralization of organic matter and deposition of ash and charcoal. Lab incubations of soil amended with varying amounts of charcoal (0.5, 1, 2.5, and 5% by weight) were conducted to assess the importance of charcoal on microbial activity and nitrogen cycling in forest soils from Little Valley, approximately 30 km south of Reno, Nevada. Carbon respiration and potential nitrification rate, proxies for microbial activity and size of the nitrifying microbial community respectively, were monitored over a two month period. Nitrogen (N), phosphorous (P), and potassium(K), were also monitored in natural pile burns with ages ranging from three to eight years and at varying points within a pile burn between 0 to 10 cm of the surface horizon. In general, microbial respiration and potential nitrification were highest at 2.5% charcoal additions. Also, microbial respiration rates decreased but potential nitrification rates increased over time across most treatments. Polyphenol sorption onto charcoal-amended soil was also analyzed and showed that extractable polyphenol concentrations decreased with increasing amounts of charcoal which may partly explain patterns in potential nitrification rates. Pile burns displayed decreasing nutrient concentration with age and lower levels of N and K compared to unburned soils whereas P was greater within pile burns. Prescribed fire is a powerful tool at the disposal of forest managers and can be used to increase forest health and productivity. Managing prescribed fires for increased charcoal production can benefit soil microbial activity but may stimulate nitrogen losses through increased nitrate production.