Response of the invasive Centaurea maculosa and two native grasses to N-pulses

Nitrogen is often a limiting resource on semi-arid grasslands. During the growing season, N is often only available during short-term pulses associated with wetting events. The Eurasian forb Centaurea maculosa Lam. has invaded millions of hectares of semi-arid grasslands in western North America. C. maculosa's success could be attributed to greater use of N-pulses, or more efficient use of N supplied in those pulses compared with native grasses. In a glasshouse, C. maculosa and two native grasses, the caespitose Pseudoroegneria spicata [Scribn. and Smith] A. Love and the rhizomatous Pascopyrum smithii [Rybd.] A. Love, were established in mixed- and monoculture combinations, and then conditioned to weekly N-pulses of 8, 24, or 72 h for 8 weeks. These pulse durations are typical on semi-arid grasslands. At the end of the 8 weeks, plants were exposed to N-15-labeled nitrate ((NO3-)-N-15) for 8 h and harvested 16 h later to compare short-term root uptake of (NO3-)-N-15. C. maculosa did not have greater enrichment (atom % N-15), rate of N-15-uptake (mumol g(-1) h-(1)), or N-15 acquired (relative to N-15 applied) than the grasses. C. maculosa's N-15-uptake per unit mass was relatively consistent across pulse durations, whereas N-15-uptake was lower at the longer pulse durations for the grasses. In general, C. maculosa acquired more of the applied N-15 than P. spicata but less than P. smithii. N-15 acquired was often influenced by the neighbour's identity. Regarding growth responses, C. maculosa produced more total biomass than the grasses, except for P. smithii plants growing with C. maculosa conditioned to 72 h pulses of N. Root mass ratios varied depending on the neighbor. Overall, C. maculosa used nitrogen less efficiently than the grasses. C. maculosa's success as an invasive species cannot be explained wholly by a greater response to N-pulses or more efficient use of N-pulses compared with native grasses with which it competes.