Above-ground biomass accumulation and nitrogen fixation of broom ( Cytisus scoparius L.) growing with juvenile Pinus radiata on a dryland site

Abstract

Annual above-ground biomass growth and nitrogen accumulation was investigated in a mixed stand of juvenile Pinus radiata and broom ( Cytisus scoparius L.), growing on a dryland site in Canterbury, New Zealand. Nitrogen fixation by broom, and the extent of fixed nitrogen transfer to the P. radiata was assessed using the 15 N natural abundance method. Broom biomass increased five-fold over the course of the year to 4250 kg ha −1. Values of δ 15 N were highest in P. radiata growing without broom, intermediate in P. radiata growing with broom, and lowest in field-grown broom. Broom was an effective fixer, deriving 81% of nitrogen in above-ground tissues from the atmosphere, which was equivalent to 111 kg N ha −1 per year. The intermediate values of δ 15 N found for trees growing with broom suggest that there was some transfer of fixed nitrogen from the broom to the P. radiata. Relatively small differences in δ 15 N between nitrogen derived from the non-fixer and fixation, combined with a lack of information on mycorrhizal associations and sources of nitrogen suggested that estimates of broom nitrogen fixation could be subject to some error. Despite these uncertainties there was close agreement between estimates of nitrogen fixation obtained from sequential harvests with that obtained using the 15 N natural abundance technique. The comparatively low values of above-ground biomass growth exhibited by broom at this drought prone site highlight the importance of root-zone water content in regulating growth in this species. Competition by broom for water and soil nitrogen completely outweighed any beneficial influence to the tree resulting from fixed nitrogen transfer. However, the high rates of nitrogen fixation documented in this study suggest that the presence of broom may enhance long-term growth of P. radiata on wet, nitrogen-deficient sites.