Soil nitrogen transformations under alternative management strategies in Appalachian forests

Once subject to frequent fire and strongly N limited, the forests of the Appalachian Mountain region of eastern North America have experienced almost a century of fire suppression, and changes in tree species composition, understory density and composition, and accumulations of detritus have paralleled the changes in fire frequency In an effort to restore these ecosystems and reduce wildfire hazard, alternative management strategies (prescribed fire, mechanical fuel reduction, and a combination of both) were implemented at study sites in Ohio and North Carolina as part of the Fire and Fire Surrogate Network Study. As changes in N cycling resulting from these treatments could have potential negative impacts on forest health that might limit their usefulness as management alternatives, we monitored treatment effects on extractable total inorganic N (TIN), net N mineralization, net nitrification, and proportional nitrification during the growing season before treatment, the first growing season after treatment, and the third or fourth growing season after treatment. Mechanical treatment resulted in increases in extractable TIN, N mineralization, and nitrification during the first post-treatment year in Ohio and TIN was greater in the mechanical treatment and the mechanical + fire treatment during the third post-treatment year in North Carolina. Net N mineralization was reduced by fire alone in Ohio, but not in North Carolina. Nitrogen transformation rates were 2- to 10-fold higher in Ohio than North Carolina, a difference that may partly result from of the ericaceous shrub understory present in North Carolina but not in Ohio. Impacts of these treatments on N transformations were modest and for the most part transient, and would therefore not eliminate these treatments as viable management strategies for wildfire hazard reduction or ecosystem restoration.