Effects of soil nitrogen reduction on nonnative plants in restored grasslands

We studied the cumulative effects of 3 years of carbon amendments on previo usly disturbed mixed-grass prairie sites near Boulder, Colorado. Analysis o f soil inorganic nitrogen during the third field season indicated statistic ally significant but short-term nitrogen reduction in response to addition of a combination of sugar and sawdust treatments. Plant foliage production was significantly reduced by these carbon amendments and averaged 377 g/m(2 )/year on control plots versus 219 g/m(2)/year on treated plots. Undesirabl e species such as Centaurea diffusa (diffuse knapweed) exhibited a similar biomass response. But after three years of treatment there is little eviden ce to suggest a relative increase in desirable, reseeded species such as Ag ropyron smithii (western wheatgrass). We suggest that the carbon amendment treatment alone is an inadequate remediation technique in areas exposed to extensive seed rain by exotic species.