BIOMASS PRODUCTION IN A NITROGEN-FERTILIZED, TALLGRASS PRAIRIE ECOSYSTEM EXPOSED TO AMBIENT AND ELEVATED LEVELS OF CO2

Increased biomass production in terrestrial ecosystems with elevated a tmospheric CO2 may be constrained by nutrient limitations as a result of increased requirement or reduced availability caused by reduced tur nover rates of nutrients. To determine the short-term impact of nitrog en (N) fertilization on plant biomass production under elevated CO2, w e compared the response of N-fertilized tallgrass prairie at ambient a nd twice-ambient CO2 levels over a 2-year period. Native tallgrass pra irie plots (4.5 m diameter) were exposed continuously (24 h) to ambien t and twice-ambient CO2 from 1 April to 26 October. We compared our re sults to an unfertilized companion experiment on the same research sit e. Above- and belowground biomass production and leaf area of fertiliz ed plots were greater with elevated than ambient CO2 in both years. Th e increase in biomass at high CO2 occurred mainly aboveground in 1991, a dry year, and belowground in 1990, a wet year. Nitrogen concentrati on was lower in plants exposed to elevated CO2, but total standing cro p N was greater at high CO2. Increased root biomass under elevated CO2 apparently increased N uptake. The biomass production response to ele vated CO2 was much greater on N-fertilized than unfertilized prairie, particularly in the dry year. We conclude that biomass production resp onse to elevated CO2 was suppressed by N limitation in years with belo w-normal precipitation. Reduced N concentration in above-and belowgrou nd biomass could slow microbial degradation of soil organic matter and surface litter, thereby exacerbating N limitation in the long term.