Dinitrogen fixation by a mature Ceanothus velutinus (Dougl.) stand in the western Oregon Cascades

MCNABB, D. H., and K. CROMACK, JR. 1983. Dinitrogen fixation by a mature Ceanothus velutinus (Dougl.) stand in the WesternOregon Cascades. Can. J. Microbiol. 29: 1014-1021.
Dinitrogen fixation was measured in a 17-year-old snowbrush, Ceanothus velutinus var velutinus (Doug! ). stand in theWestern Oregon Cascades. Diurnal and seasonal rates of nitrogenase activity were measured in the field using the C2H, reductiontechnique. Snowbrush had a total biomass, estimated with equations developed. of 42 680 kg • ha-I, including 750 kg • ha-1 ofnodule biomass. Snowbrush fixed N2 for approximately 240 days annually. Except during precipitation events or periods of lowxylem pressure potentials, C2H2 reduction rates in the summer and fall were significantly correlated with soil temperature (R2 =0.93**, n = 6). A diurnal variation in nitrogenase activity also was measured. The annual N2 fixation rate was estimated atapproximately 101 kg N • ha-1 (C2H2). Sustained periods of precipitation suppressed nitrogenase activity and reduced theestimate by about 19%. The annual N2 fixation rate is higher than previously reported for other mature stands and primarily isattributed to the maintenance of a large nodule biomass.