BayCEER Workshop 2008

Microbial growth strategies in soil aggregates under elevated CO2 as evaluated by Substrate Induced Growth Respiration approach (SIGR)

Microbial decomposition of soil organic matter is controlled by physico-chemical soil properties and by the amount and composition of plant deposits. Elevated atmospheric CO₂ can alter plant residues composition and its amount, hence affecting the rates of microbial metabolism in soil. We investigated the effects of elevated CO₂ on growth rates of soil microorganisms in bulk soil and in three aggregate-size classes.Soil samples from FACE plots (Hohenheim, Germany) under ambient (380 ppm) and elevated CO₂ (540 ppm; for 5 years) were separated to aggregates using dry sieving. Bulk soil and isolated large macro- (>2 mm), small macro- (0.25-2) and microaggregates (<0.25) were amended with glucose and nutrients to stimulate unlimited microbial growth. Soil respiration was approximated based on microbial growth kinetic, and the maximal specific growth rates (μ_max) and microbial biomass were calculated. The distribution of soil aggregates (>2 mm: 30%, 0.25-2 mm: 60%, and <0.25 mm: 10%) has not changed after 5 years of elevated CO₂. μ_max were significantly higher under elevated compared to ambient CO₂. Under both CO₂ treatments μ_max increased in the following order: large macroaggregates < small macroaggregates < microaggregates. Total microbial biomass increased with decreasing aggregate size for both CO₂ treatments. Increase of μ_max under elevated CO₂ indicates stimulation of fast growing microorganisms. This stimulation was especially pronounced in microaggregates.