Stimulation of r- vs. K-selected microorganisms by elevated atmospheric CO(2) depends on soil aggregate size.

Increased root exudation under elevated atmospheric CO(2) and the contrasting environments in soil macro- and microaggregates could affect microbial growth strategies. We investigated the effect of elevated CO(2) on the contribution of fast- (r-strategists) and slow-growing (K-strategists) microorganisms in soil macro- and microaggregates. We fractionated the bulk soil from the ambient and elevated (for 5 years) CO(2) treatments of FACE-Hohenheim (Stuttgart) into large macro- (>2 mm), small macro- (0.25-2.00 mm), and microaggregates (<0.25 mm) using 'optimal moist' sieving. Microbial biomass (C(mic)), the maximum specific growth rate (mu), growing microbial biomass (GMB) and lag-period (t(lag)) were estimated by the kinetics of CO(2) emission from bulk soil and aggregates amended with glucose and nutrients. Although C(org) and C(mic) were unaffected by elevated CO(2), mu values were significantly higher under elevated than ambient CO(2) for bulk soil, small macroaggregates, and microaggregates. Substrate-induced respiratory response increased with decreasing aggregate size under both CO(2) treatments. Based on changes in mu, GMB and lag period, we conclude that elevated atmospheric CO(2) stimulated the r-selected microorganisms, especially in soil microaggregates. Such an increase in r-selected microorganisms indicates acceleration of available C mineralization in soil, which may counterbalance the additional C input by roots in soils in a future elevated atmospheric CO(2) environment.