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

Abstract

The importance of heterotrophic nitrification was studied in soil from a mixed-conifer forest. Three sites in the forest were sampled: a clear cut area, 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 mineralization and nitrification were measured by 15NH4+ and 15NO3- isotopic pool dilution, respectively. The rates of autotrophic and heterotrophic nitrification were distinguished by use of acetylene as a specific inhibitor of autotrophic nitrification. In samples supplemented with 15NH4+ and treated with acetylene, no 15NO3- was detectable showing that the acetylene treatment effectively blocked the autotrophic nitrification, and that NH4+ was not a substrate for heterotrophic nitrification. In the clear cut area, autotrophic nitrification was the most important NO3- generating process with total nitrification (45 ug N kg- 1 h-1) accounting for about one-third of gross N mineralization (140 ug N kg-1 h-1). In the young and mature forested sites, gross nitrification rates were largely unaffected by acetylene treatment indicating that heterotrophic nitrification dominated the NO3- generating process in these areas. In the mature forest mineral and organic soil, nitrification (heterotrophic) was equal to only about 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 rate 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 15N 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.