AUTOTROPHIC AND HETEROTROPHIC NITRIFICATION IN RESPONSE TO CLEAR-CUTTING NORTHERN HARDWOOD FOREST

Acid forest soils, pH 3.9, from hardwood forests of Hubbard Brook lose NO3-N in streamwater following clear-cutting. We evaluated the importance of autotrophic and heterotrophic nitrification in NO3-N production. Forest floor samples collected from an undisturbed forest (fresh forest floor-FFF) and a forest clear-cut 6 yr previously, where regrowth was suppressed for the first 3 yr (aged forest floor-AFF), were incubated in laboratory microcosms with and without nitrapyrin, an inhibitor of autotrophic ammonium-oxidizers. Autotrophic and heterotrophic nitrifiers were about equally important in NO3-N production in FFF, but heterotrophic nitrifiers were almost solely responsible in AFF. When pH was increased to 4.4 in FFF by adding lime, autotrophic nitrification almost doubled while heterotrophic nitrification was not affected. The addition of the same amount of lime to AFF increased pH to 4.6 and neither increased NO3-N production nor changed the process from heterotrophic nitrification. When pH was increased to 6.6 in FFF by heavier applications of lime, heterotrophic nitrification was eliminated and NO3-N production was solely autotrophic. Total NO3-N production increased 3-fold. Heavy applications of lime to AFF eliminated heterotrophic nitrification and activated resting cells of autotrophs to make the process wholly autotrophic. Total NO3-N production increased 2-fold. Acidification to pH 3.5 reduced NO3-N production for both FFF and AFF by completely inhibiting autotrophic nitrification and reducing heterotrophic nitrification by about one-half of the unamended treatment for FFF and one-quarter for AFF. Population counts for autotrophic nitrifiers supported conclusions about the relative importance of autotrophic and heterotrophic nitrification. Ecological implications for these forests suggest that soon after cutting NO3-N production is greatly accelerated and is mediated by both autotrophic and heterotrophic nitrifiers in about equal proportions. As forest regrowth proceeds nitrification rates decline, NO3-N production shifts first toward dominance by heterotrophic nitrifiers but later the process moves back to a balance between autotrophic and heterotrophic nitrifiers.