Nitrogen fixation and biomass accumulation in plant communities dominated by Cytisus scoparius L. in Oregon and Scotland.
The nitrogenase activity of C. scoparius measured at indigenous soil temp. at intervals during the year showed 2 apparent maxima, in early and late spring, which coincided with flowering and with late fruit maturation, resp. The final decline in nitrogenase activity in early summer coincided with a period of drought which increased soil moisture stress, as measured by an increase in pre-dawn xylem pressure potential, to about -5 bars. Nitrogenase activity was eliminated by frosting during the winter of 1979, unlike 1978 when virtual absence of frost permitted nitrogenase activity throughout the winter. The age structure of C. scoparius plants on 2 study sites in Oregon and Scotland differed by about 1 year. Biomass accumulation in the former site was 44 000 and in the latter 84 000 kg DW shoot growth/ha. Plants in both areas were poorly nodulated (1.2-2.9 g/m2 in Oregon and 0.3 g/m2 in Scotland) and analysis of the total N content of soil of the Scottish site showed an increase in N only half that of an adjacent area colonized by Alnus glutinosa.