EFFECTS OF ELEVATED ATMOSPHERIC CO2 ON SOIL MICROBIOTA IN CALCAREOUS GRASSLAND

Microbial responses to three years of CO2 enrichment (600 mu L L-1) in the field were investigated in calcareous grassland. Microbial biomas s carbon (C) and soil organic C and nitrogen (N) were not significantl y influenced by elevated CO2. Microbial C:N ratios significantly decre ased under elevated CO2 (-15%, P = 0.01) and microbial N increased by + 18% (P = 0.04). Soil basal respiration was significantly increased o n one out of 7 sampling dates (+ 14%, P = 0.03; December of the third year of treatment), whereas the metabolic quotient for CO2 (qCO(2) = b asal respiration/microbial C) did not exhibit any significant differen ces between CO2 treatments. Also no responses of microbial activity an d biomass were found in a complementary greenhouse study where intact grassland turfs taken from the field site were factorially treated wit h elevated CO2 and phosphorus (P) fertilizer (1 g P m(-2) y(-1)). Prev iously reported C balance calculations showed that in the ecosystem in vestigated growing season soil C inputs were strongly enhanced under e levated CO2. It is hypothesized that the absence of microbial response s to these enhanced soil C fluxes originated from mineral nutrient lim itations of microbial processes. Laboratory incubations showed that sh ort-term microbial growth (one week) was strongly limited by N availab ility, whereas P was not limiting in this soil. The absence of large e ffects of elevated CO2 on microbial activity or biomass in such nutrie nt-poor natural ecosystems is in marked contrast to previously publish ed large and short-term microbial responses to CO2 enrichment which we re found in fertilized or disturbed systems. It is speculated that the absence of such responses in undisturbed natural ecosystems in which mineral nutrient cycles have equilibrated over longer periods of time is caused by mineral nutrient limitations which are ineffective in dis turbed or fertilized systems and that therefore microbial responses to elevated CO2 must be studied in natural, undisturbed systems.