The relative importance of autotrophic and heterotrophic nitrification in a conifer forest soil as measured by N-15 tracer and pool dilution techniques

The importance of heterotrophic nitrification was studied in soil from a mi xed-conifer forest. Three sites in the forest were sampled: a clear cut are a, a young stand and a mature stand. In the mature stand, the mineral soil (0-10 cm) and the organic layer were sampled separately. Gross rates of N m ineralization and nitrification were measured by (NH4+)-N-15 (NO3-)-N-15 is otopic pool dilution, respectively. The rates of autotrophic and heterotrop hic nitrification were distinguished by use of acetylene as a specific inhi bitor of autotrophic nitrification. In samples supplemented with (NH4+)-N-1 5 and treated with acetylene, no (NO3-)-N-15 was detectable showing that th e acetylene treatment effectively blocked the autotrophic nitrification, an d that NH4+ was not a substrate for heterotrophic nitrification. In the cle ar cut area, autotrophic nitrification was the most important NO3- generati ng process with total nitrification (45 ug N kg(-1) h(-1)) accounting for a bout one-third of gross N mineralization (140 ug N kg(-1) h(-1)). In the yo ung and mature forested sites, gross nitrification rates were largely unaff ected by acetylene treatment indicating that heterotrophic nitrification do minated the NO3- generating process in these areas. In the mature forest mi neral and organic soil, nitrification (heterotrophic) was equal to only abo ut 5% of gross mineralization (gross mineralization rates of 90 ug N kg(-1) h(-1) mineral; 550 ug N kg(-1) h(-1) organic). The gross nitrification rat e decreased from the clear cut area to the young forest area to the mineral soil of the mature forest (45; 17; 4.5 ug kg(-1) h(-1) respectively). The N-15 isotope pool dilution method, combined with acetylene as an inhibitor of autotrophic nitrification provided an effective technique for assessing the importance of heterotrophic nitrification in the N-cycle of this mixed- conifer ecosystem.