Asymbiotic nitrogen fixation and denitrification in a mature forest in coastal British Columbia

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Abstract
Gaseous nitrogne inputs due to asymbiotic nitrogen fixation and outputs due to biological denitrification were estimated for a mature mid-successional Pseudotsuga menziesii (Mirb.) Franco-Thuja plicata Donn-Tsuga heterophylla (Raf.) Sarg. forest in southwestern British Columbia. Forest floor material, mineral soil, decaying wood, foliage, and bark were incubated in an atmosphere of 10 kPa c2H2 to allow the simultaneous measurement of N2O production by denitrifying bacteria and C2H2 reduction by free-living bacteria and blue-green algae. Forest floor material accounted for 70% of an estimated total annual nitrogen fixation of 0.3 kg N .cntdot. ha-1 .cntdot. year-1. Relatively small amounts of nitrogen were fixed in mineral soil, decaying wood, and foliage, and no indication of nitrogen fixation activity in bark was detected. Some denitrification was found; it was essentially negligible, although possibly underestimated. The net gaseous nitrogen input into the mid-successional forests of the study area is likely to be < 5% of the total net nitrogen inputs, which are primarily from precipitation.