Chemoautotrophic nitrification in acidic forest soils along an urban-to-rural transect

We studied N-mineralization and nitrification in oak forest soils along a 130 km urban-to-rural transect in the New York City metropolitan area. We found most urban and suburban stands had a substantial amount of extractable soil nitrate from July to December 1996, but nitrate pools in the rural stands were negligible. Lab incubations showed that urban and suburban soils nitrified rapidly, but the rural soils did not. Although soil pH was acidic in all stands, nitrification was totally inhibited by acetylene, indicating control of this process by chemoautotrophic rather than by heterotrophic microbes. When we amended soils with NH4+ or P, or raised their pH, nitrification in rural soils was not stimulated, nor was it further elevated in urban soils. Raising pH did, however, increase NH4+ production in all soils. We hypothesize that exotic earthworms, which are present in the urban and suburban stands but not in the rural stands, may create soil conditions that favor autotrophic nitrifiers.