Elevated atmospheric CO₂ increases microbial growth rates in soil: results of three CO₂ enrichment experiments

Abstract

Increasing the belowground translocation of assimilated carbon by plants grown under elevated CO 2 can cause a shift in the structure and activity of the microbial community responsible for the turnover of organic matter in soil. We investigated the long-term effect of elevated CO 2 in the atmosphere on microbial biomass and specific growth rates in root-free and rhizosphere soil. The experiments were conducted under two free air carbon dioxide enrichment (FACE) systems: in Hohenheim and Braunschweig, as well as in the intensively managed forest mesocosm of the Biosphere 2 Laboratory (B2L) in Oracle, AZ. Specific microbial growth rates (l) were determined using the substrateinduced respiration response after glucose and/or yeast extract addition to the soil. For B2L and both FACE systems, up to 58% higher l were observed under elevated vs. ambient CO 2 , depending on site, plant species and N fertilization. The l-values increased linearly with atmospheric CO 2 concentration at all three sites. The effect of elevated CO 2 on rhizosphere microorganisms was plant dependent and increased for: Brassica napus 5 Triticum aestivumoBeta vulgarisoPopulus deltoides. N deficiency affected microbial growth rates directly (N limitation) and indirectly (changing the quantity of fine roots). So, 50% decrease in N fertilization caused the overall increase or decrease of microbial growth rates depending on plant species. The l-value increase was lower for microorganisms growing on yeast extract then for those growing on glucose, i.e. the effect of elevated CO 2 was smoothed on rich vs. simple substrate. So, the r/K strategies ratio can be better revealed by studying growth on simple (glucose) than on rich substrate mixtures (yeast extract). Our results clearly showed that the functional characteristics of the soil microbial community (i.e. specific growth rates) rather than total microbial biomass amount are sensitive to increased atmospheric CO 2 . We conclude that the more abundant available organics released by roots at elevated CO 2 altered the ecological strategy of the soil microbial community specifically a shift to a higher contribution of fast-growing r-selected species was observed. These changes in functional structure of the soil microbial community may counterbalance higher C input into the soil under elevated atmospheric CO 2 concentration.

Plant diversity effects on soil food webs are stronger than those of elevated CO 2 and N deposition in a long-term grassland experiment

“…These findings agree with a recent metaanalysis (15) and are most likely due to elevated soil moisture content (23), increased belowground translocation of assimilated carbon by plants (20,24,25), and increased above-and belowground litter input (9).…”

Fungal Communities Respond to Long-Term CO 2 Elevation by Community Reassembly

“…As consequences, increased carbon input in turn significantly changed bacterial diversity, composition, and structure and increased the functional potential of bacterial communities for carbon degradation and nutrient cycling, although such effects differed across various ecosystems (6,(11)(12)(13)(14)(15)(16)(17)(18)(19)(20). In contrast, fungal biomass and relative abundance of total microbial biomass did not change significantly under eCO 2 in this BioCON (biodiversity, CO 2 , and N deposition) experimental site (6,21).…”

Distinct responses of soil microbial communities to elevated CO2 and O3 in a soybean agro-ecosystem

“…For example, no detectable effects of eCO 2 on microbial community structure, microbial activity, potential soil N mineralization or nitrification were observed at a sweetgum free-air CO 2 enrichment (FACE) experiment in TN, USA (Austin et al, 2009), whereas in a no-till wheat-soybean rotation agro-ecosystem, the community composition and structure significantly affected by eCO 2 but not by eO 3 or eCO 2 þ eO 3 (Cheng et al, 2011). Recently, more studies suggest that eCO 2 and/or eO 3 significantly alter microbial community composition, structure, functional potential/activity, interaction network and/or dynamics (Lesaulnier et al, 2008;Blagodatskaya et al, 2010;Drigo et al, 2010;Feng et al, 2010;Zhou et al, 2010Zhou et al, , 2011He et al, 2010bHe et al, , 2012bDeng et al, 2012;Drigo et al, 2013;Li et al, 2013). In addition, it has been shown that the response of soil microbial comunities to global change factors may be directly or indirectly mediated by plant genotypes/cultivars, the diversity of plant assemblages and/or other environmental factors (Talhelm et al, 2009;Singh et al, 2010;Drigo et al, 2013;Li et al, 2013).…”