Implications for ecosystem N loss
Understanding the mechanisms controlling soil nitrification in response to N addition and N cessation is important for accurately predicting ecosystem potential N losses accompanying changed anthropogenic N inputs. Results from our study showed that although elevated inorganic N concentrations in the soil had declined to control levels after treatments ceased, their effects on ammonia oxidizers were maintaining high nitrification even 7 years after cessation of nutrient addition. These results resulted from two pathways. First, soil microbial biomass N predominantly controlled mineralization variability and soil nitrogen availability for ammonia oxidizers after the cessation of N inputs. Mobile N in recovering ecosystems has not the same fate as N in unimpacted ecosystems. Second, legacy effects of the ammonia oxidizer community due to N addition may have contributed to lasting changes in N cycling. Together, our results indicate a high nitrification environment induced by N enrichment is not possible simply by reducing N input, and it requires a long recuperation time.