The relative importance of autotrophic and heterotrophic nitrification in a conifer forest soil as measured by 15N tracer and pool dilution techniques |
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Authors: | Henning Pedersen Kristie A Dunkin Mary K Firestone |
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Institution: | (1) Department of Microbial Ecology, University of Aarhus, DK-8000 Aarhus C, Denmark;(2) Department of Environmental Science, Policy and Management, 151 Hilgard Hall, University of California, 94720 Berkeley, California, USA |
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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 by15NH
4
+
and15NO
3
–
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 with15NH
4
+
and treated with acetylene, no15NO
3
–
was detectable showing that the acetylene treatment effectively blocked the autotrophic nitrification, and that NH
4
+
was not a substrate for heterotrophic nitrification. In the clear cut area, autotrophic nitrification was the most important NO
3
–
generating process with total nitrification (45 ug N kg–1h–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 NO
3
–
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). The15N 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. |
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Keywords: | autotrophic nitrification heterotrophic nitrification conifer forest N-mineralization |
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