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

Abstract

Dinitrogen fixation was measured in a 17-year-old snowbrush, Ceanothus velutinus var. velutinus (Dougl.), stand in the Western Oregon Cascades. Diurnal and seasonal rates of nitrogenase activity were measured in the field using the C2H2 reduction technique. Snowbrush had a total biomass, estimated with equations developed, of 42 680 kg∙ha−1, including 750 kg∙ha−1 of nodule biomass. Snowbrush fixed N2 for approximately 240 days annually. Except during precipitation events or periods of low xylem 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 at approximately 101 kg N∙ha−1 (C2H2). Sustained periods of precipitation suppressed nitrogenase activity and reduced the estimate by about 19%. The annual N2 fixation rate is higher than previously reported for other mature stands and primarily is attributed to the maintenance of a large nodule biomass.