Stimulation of r- vs. K-selected microorganisms by elevated atmospheric CO2 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 (μ), growing microbial biomass (GMB) and lag-period ( tlag ) 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 , μ 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 μ, 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.