Effects of irrigation and phosphorus fertilization on physiology, growth, and nitrogen-accumulation of Scotch broom (Cytisus scoparius)

We tested the effects of phosphorus (P) fertilization and soil water on the growth, physiology, and total nitrogen (N) accumulation in N-fixing Scotch broom in Olympia, WA. We manipulated soil water and P availability via irrigation and fertilization, respectively, in a completely randomized 2 × 2 factorial on potted one-year old Scotch broom seedlings (n = 20) in an N-deficient sand. There was substantial evidence that increased-irrigation and P-fertilization had similar positive effects on N accumulation in Scotch broom approximately equally. High-irrigation rates were more often associated with positive physiological and growth responses in Scotch broom than fertilization, however. Although the irrigation × fertilization interaction was not significant, there were additive effects of high-irrigation and fertilization on biomass and N content as both were 50% greater in the fertilized-and-high-irrigation treatment relative to the respective fertilized and high-irrigation treatments. We noted an accumulation of N and P in the plant tissues. Analyses indicated a pattern of decreasing function and growth with increasing N and P concentrations in Scotch broom biomass, suggesting plant growth and physiology were limited by some other resource. Total plant N content values ranged from 7.0 ± 1.1 g plant⁻¹ in the control and 23.4 g ± 9.0 plant⁻¹ in the fertilized-and-high-irrigation treatment. Extrapolated to typical densities of comparably sized Scotch broom plants on invaded sites in the western Pacific Northwest, these findings suggest that, at least, 12–65 kg N ha⁻¹ would be found in Scotch broom plants in the field.