An alternative explanation for the post-disturbance NO3- flush in some forest ecosystems

The appearance of soil NO3- after forest disturbance is commonly ascribed t o a higher availability of NH4+ to autotrophic nitrifiers, or to a reductio n in available-C resulting in lower microbial assimilation of NO3-. Alterna tivelv, it has been proposed that increasing NH4+ pools following disturban ce could increase net nitrification by reducing microbial assimilation of N O3-. Forest floor material was collected from shelterwood harvest plots whi ch displayed both low available-C and low NH4+ pools, and where previous ex periments had suggested the prevalence of heterotrophic nitrification. Subs amples were amended with incremental rates of glucose-C or NH4+, and gross NO3- transformation rates were measured by isotope dilution. Glucose-C addi tions had little effect on the net difference between gross NO3- production and consumption rates. On the other hand, NH4+ additions caused gross NO3- consumption processes to decrease sharply, while gross NO3- production pro cesses remained constant. The results suggest that NH4+ can have an immedia te positive effect on net nitrification rates by suppressing NO3- assimilat ion and uptake systems.