Nitrogen dynamics of Centaurea maculosa and native species

Abstract

Centaurea maculosa has invaded millions of hectares of disturbed and undisturbed semi-arid grasslands. Its success may be due to an ability to conserve nitrogen (N) or use N more efficiently than native species. It may also be able to acquire more N from early spring pulses than native vegetation. Growth response, N acquisition, allocation, resorption and use efficiency of C. maculosa was compared with Pseudoroegneria spicata, Pascopyrum smithii, and Rudbeckia hirta by growing plants in high and low N supplies, which mimic disturbed and undisturbed sites. 15Nitrogen acquired per unit of root mass was highest in R. hirta. Centaurea maculosa had the lowest C:N ratio. Resorption proficiency was lower in C. maculosa for Harvest 2 and 3, and similar among all species at Harvest 4. Centaurea maculosa had the highest mean residence time (MRT) and lowest N productivity. Centaurea maculosa had the highest nitrogen use efficiency (NUE) for plants receiving the high N supply whereas NUE was similar for all species receiving the low N supply.

Centaurea maculosa's success in disturbed sites may be due to a high NUE, which can be attributed to a high MRT. Uptake and growth response of C. maculosa receiving an early spring, late spring or steady N treatment were determined in a glasshouse and a field study. Centaurea maculosa's response was compared with P. spicata, Festuca idahoensis and R. hirta in the glasshouse, and P. spicata, F. idahoensis and Balsamorhiza sagittata in the field. In the glasshouse, with plants receiving the steady N treatment produced the most biomass whereas plants receiving the late N treatment produced the least. Grasses receiving the early N treatment had the highest root mass ratio (RMR), whereas RMRs of C. maculosa were similar among N treatments. Forbs acquired more N than grasses. In the field, biomass was similar among N treatments. Plants took up more N from the early treatments than the late or steady treatments. Centaurea maculosa may be able to acquire more N from pulses than grasses in disturbed sites when plants are small, but as the grasses grow, C. maculosa may lose its advantage in acquiring N.