Nitrification in forested ecosystems

Abstract Nitrification, the microbial oxidation of NH4+-N, plays a key role in the cycling of N in forested and other terrestrial ecosystems. Solution losses of nitrate and gaseous losses of N2 and nitrous oxides are important vectors of N loss from many forested systems and are directly or indirectly controlled by the activity of the nitrifiers. These losses can also have important consequences for downstream ecosystems, groundwater quality, and atmospheric concentrations of ozone. Relative nitrification (the proportion of the total mineral N that is nitrate at the end of an incubation period) provides an independent means of evaluating the general importance of site factors thought to regulate nitrification in situ. Regressions of relative nitrification against soil pH, C:N, and percentage N, with the use of data from previously published studies, suggest that although these factors may be important regulators of nitrification in particular sites, they are not good predictors of nitrification across a wide range of sites. Reasons for their low predictive ability may include limitations of current measurement techniques or the capacity of nitrifiers to adapt to relatively extreme conditions.