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

摘要

The importance of heterotrophic nitrification was studied in soil from a mixed-conifer forest (Abies concolor, Libocedrus decurrens, Pseudotsuga menziesii, Pinus ponderosa, P. lambertiana and Quercus kelloggii) in California, USA. Three sites in theforest were sampled all on Alfisols: a clear felled 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 by15NH4+ 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 treatedwith 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 felled area, autotrophic nitrification was the most important NO3- generating process with total nitrification (45μg N/kg per h) accounting for approx. one-third of gross N mineralization (140μg N/kg per h). 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～ 5% of gross mineralization (gross mineralization rates of 90μg N/kg per h mineral; 550μg N/kg per h organic). The gross nitrification rate decreased from the clear felled area to the young forest area to the mineral soil of the mature forest (45; 17; 4.5μg/kg per h 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.