Stimulation of r- vs. K-selected microorganisms by elevated atmospheric CO2 depends on soil aggregate size

Increased root exudation under elevated atmospheric CO2 and the contrasting environments in soil macro- and microaggregates could affect microbial growth strategies. We investigated the effect of elevated CO2 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) CO2 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 CO2 emission from bulk soil and aggregates amended with glucose and nutrients. Although C-org and C-mic were unaffected by elevated CO2, mu values were significantly higher under elevated than ambient CO2 for bulk soil, small macroaggregates, and microaggregates. Substrate-induced respiratory response increased with decreasing aggregate size under both CO2 treatments. Based on changes in mu, GMB and lag period, we conclude that elevated atmospheric CO2 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 CO2 environment.