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

Gaseous nitrogen inputs due to asymbiotic nitrogen fixation and outputs due to biological denitrification were estimated for a mature mid-successional Pseudotsugamenziesii (Mirb.) Franco–Thujaplicata Donn–Tsugaheterophylla (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 C 2 H 2 to allow the simultaneous measurement of N 2 O production by denitrifying bacteria and C 2 H 2 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•ha −1 •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.