Competitive control of invasive vegetation:: a native wetland sedge suppresses Phalaris arundinacea in carbon-enriched soil

1. Invasive plants pose a major threat to native plant communities around the globe. Current methods of controlling invasive vegetation focus on eradication of existing populations, and are often effective only in the short term. Manipulating resource availability to give native species a competitive advantage over invasive species could reduce ecosystem vulnerability to invasion and might more effectively control invasive vegetation. We evaluated this approach for controlling invasions of sedge meadow communities by Phalaris arundinacea, a widespread invasive grass in North American wetlands. 2. To test whether lowering nitrogen (N) availability would allow a wetland sedge, Carex hystericina, to suppress Phalaris competitively, we examined Carex and Phalaris competition under a range of inorganic N concentrations (25-400 mg kg(-1)) in a glasshouse. We lowered N availability in wetland soil using carbon enrichment and repeated harvests of a cover crop, and then created a N gradient by applying NH4-N to the N-depleted soil. 3. In soil without carbon added, competition with Phalaris reduced Carex biomass by 91%, while competition with Carex did not influence Phalaris, as is commonly observed in sedge meadows. Phalaris biomass was five times Carex biomass in mixed stands. Conversely, in soil depleted of available N via carbon enrichment, competition with Carex reduced Phalaris biomass by 82%, while competition with Phalaris reduced Carex biomass by only 32%, indicating that Carex is the superior competitor for N. Carex biomass was six times Phalaris biomass in mixed stands in the carbon-enriched soil. 4. Carbon enrichment lowered soil inorganic N by 10-30 mg kg(-1). NH4-N addition mitigated the negative effects of carbon on Phalaris growth and competitive ability, indicating that carbon enrichment altered competitive outcomes by lowering N availability. Greater Carex N uptake efficiency under N-poor conditions appeared to account for the Carex competitive ability for N. 5. Synthesis and applications. Carex dominance in carbon-enriched soil strongly suggests that lowering soil inorganic N to < 30 mg kg(-1) in restored wetlands would allow establishing sedge meadow communities to suppress Phalaris invasions. Low-N soils might be achieved via carbon enrichment, vegetation harvests and reduced N inputs. Reducing community vulnerability to invasion by manipulating resource availability appears to be a promising approach to invasive species management.