Scotch broom (Cytisus scoparius) invasion has increased nitrate levels in the naturally dry, nutrient-poor soils of the south Puget Sound prairies (Washington state, USA). Although the broom has been successfully controlled, the higher soil nitrate level has allowed many other non-native species to invade and competitively exclude the natives. We tested the responses of native and non-native plant species to two methods to reduce soil nitrogen in intact, but invaded, remnant prairies: biomass removal and carbon addition. In a stratified random design, we clipped and removed biomass, or added 2.4 kg/m2 of sucrose (1 kg/m2 carbon) on 144 1-m2 plots. Sugar was sprinkled on the soil surface in three applications in March and April, 2006, and the cover of each species was assessed in June 2006 after most species could be identified. We clipped biomass immediately after the cover assessment. Only three months later, the sucrose treatments had reduced the total cover of non-native species significantly and markedly, up to 50%, whereas the total cover of native species showed only non-significant reductions in cover (up to 16%). Individual species, however, deviated from this overall pattern: of particular concern was a six-fold increase in the cover of the invasive grass, Agrostis capillaris. The biomass removal results will be assessed in June 2007. These early results suggest that sucrose, through microbially-mediated nitrogen sequestration, osmotic interference, or some other mechanism, has given the native species a competitive advantage against many, but not all, non-native species.
