Nitrogen Fixation and Biomass Accumulation in Plant Communities Dominated by Cytisus scoparius L. in Oregon and Scotland

Abstract

(1) The nitrogenase activity of Cytisus scoparius (L.) Link measured at indigenous soil temperatures at intervals during the year, showed two apparent maxima, early and late spring, which coincided with flowering and with late fruit maturation, respectively. 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. (2) Nitrogenase activity was eliminated by frosting during the winter of 1979, unlike 1978 when virtual absence of frost permitted nitrogenase activity throughout the winter. (3) The age structure of Cytisus scoparius (L.) Link plants on two study sites in Oregon and Scotland differed by about 1 year (predominantly 2-4and 5-6-year-old plants, respectively). Biomass accumulation in the former site was 44 000 and in the latter 84 000 kg dry weight top growth ha-'. (4) Plants in both areas were poorly modulated (1 2 to 2 9 g m-2 in Oregon and 0 3 g m-2 in Scotland) and analysis of the total N content of soil of the Scottish site showed an accretion of N only half that of an adjacent area colonized by Alnus glutinosa (L.) Gaertn. Low soil pH could limit the modulation of broom and inoculation of plants with acidtolerant strains of Rhizobium could be one way to improve nitrogen fixation on many sites. (5) Selection and use of non-flowering strains of Cytisus scoparius may also improve accretion of fixed nitrogen, by eliminating the mid-season trough in nitrogenase activity, and would reduce the nuisance value of the species due to its free-seeding habit.