Nearly all of Earth's plant life responds favorably to increases in the air's CO2 content by exhibiting enhanced rates of photosynthesis and biomass production. Consequently, these phenomena tend to increase soil carbon contents by increasing root exudation of organic compounds and the amount of plant litter returned to the soil. Thus, one might expect CO2-mediated increases in soil carbon content to impact soil bacterial communities; and this summary explores the findings of several scientists who have studied the effects of elevated CO2 on bacteria.
In the case of beneficial terrestrial bacteria, Ronn et al. (2003) grew wheat (Triticum aestivum L. cv. Minaret) in open-top chambers fumigated with either ambient air or air enriched with an extra 320 ppm of CO2; and on two occasions during the growing season, they assessed various plant and soil characteristics, as well as total protozoan numbers and numbers of culturable bacteria. This work revealed, in their words, that there were "higher numbers of bacterivorous protozoa in soil under plants grown at elevated CO2 and larger amounts of root-derived substrates in the soil at plant maturity." As for the significance of this finding, Ronn et al. note that "protozoan grazing generally enhances carbon and nitrogen mineralization in soil," which typically results in more nitrogen being made available to plants. This phenomenon, in turn, enables plants to significantly increase their biomass (as was observed in the CO2-enriched plants in this experiment) while not suffering reductions in tissue nitrogen concentration (as was also observed in the CO2-enriched plants in this experiment). The end result of these linked phenomena was thus more high-quality wheat production in response to atmospheric CO2 enrichment, which bodes well for the still-expanding human population of the planet.
Similar results were obtained by Montealegre et al. (2002), who grew white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.) in free-air CO2 enrichment (FACE) plots maintained at atmospheric CO2 concentrations of 350 and 600 ppm for three years before the soil was sampled to determine the effects of elevated CO2 on its bacterial populations. Although elevated CO2 increased the total number of bacteria and respiring bacteria in the bulk soil beneath white clover by 40 and 70%, respectively, it had no significant impact on bulk-soil bacterial numbers beneath perennial ryegrass. However, when the total bacterial numbers in the rhizosphere soil - which lies within about 1.5 mm of plant roots and is characterized by heightened biological activity and chemical weathering of minerals - were expressed on a per unit land area basis, it was found that elevated CO2 increased the total number of bacteria and respiring bacteria beneath white clover by about 100 and 250%, respectively, while it increased the total number of bacteria and respiring bacteria beneath perennial ryegrass by approximately 85 and 125%, respectively.
In a related study from the same FACE clover and ryegrass plots, Marilley et al. (1999) reported that atmospheric CO2 enrichment also altered the profile of bacterial communities in a plant species-dependent manner. In ryegrass, for example, elevated CO2 increased the dominance of Pseudomonas species, which enhance plant growth by many different mechanisms; while in white clover, it increased the dominance of Rhizobium species, which enhance plant growth by making atmospheric nitrogen available for their utilization. Furthermore, after three-years of differential CO2 treatment in the same FACE experiment, Montealegre et al. (2000) determined the genetic structure of 120 isolates of the symbiotic bacterium Rhizobium leguminosarum associated with roots of white clover, finding that atmospheric CO2 enrichment favored some of the isolates over others. And when these isolates were mixed with isolates favored in ambient air and the resulting combination was exposed to CO2-enriched air, the isolates favored by elevated CO2 produced 17% more nodules on roots than the isolates favored in ambient air.
In another experiment, Zak et al. (2000) grew six genotypically different aspen (Populus tremuloides) cuttings in open-top chambers for 2.5 growing seasons in Michigan, USA, at atmospheric CO2 concentrations of 350 and 700 ppm under adequate and inadequate supplies of soil nitrogen, reporting the effects of elevated CO2 and soil nitrogen on soil microbial composition, biomass, and functioning. Results indicated that, although atmospheric CO2 enrichment had no effect on soil microbial biomass, even after 2.5 years of treatment, high soil nitrogen supply increased it five-fold over that observed in low soil nitrogen plots. Similarly, elevated CO2 did not significantly impact microbial community composition, whereas high soil nitrogen supply did. Moreover, atmospheric CO2 did not influence microbial rates of nitrogen mineralization, nor did it alter the microbial demand for inorganic nitrogen.
In totality, the several observations of Zak et al. suggest that the increased fine root biomass and turnover, which led to greater carbon inputs to the soils of the CO2-enriched plots, were not significant enough to elicit any responses in microbial community composition, biomass, and functioning, due to the enormous amount of background organic carbon present in the experimental soils, which was approximately 1000-fold greater than that contributed by the aspen roots. Notwithstanding this observation, the authors conducted an eloquent review of the scientific literature pertaining to this topic; and they concluded that when root-associated soil carbon inputs are sufficiently large, relative to native soil organic carbon contents, they can influence microbial community composition, biomass, and functioning. Therefore, as the atmospheric CO2 concentration continues to rise, it is likely that aspen trees will exhibit significant increases in growth, regardless of soil nitrogen availability. These growth increases will occur both above- and below-ground, thus stimulating greater carbon inputs to soils. Because most forest soils are already relatively rich in organic carbon, however, it is likely that the extra carbon inputs, resulting from the increasing CO2 content of the air, will have little impact on soil microbial composition, biomass, and functioning. Thus, it is likely that future increases in the air's CO2 concentration will continue to maintain soil microbial diversity beneath regenerating aspen stands.
With respect to the benefits of atmospheric CO2 enrichment on aquatic bacteria, Fu et al. (2008) employed semi-continuous culturing methods that used filtered, microwave-sterilized surface Sargasso seawater that was enriched with phosphate and trace nutrients to examine the physiological responses of steady-state iron (Fe)-replete and Fe-limited cultures of the biogeochemically critical marine unicellular diazotrophic cyanobacterium Crocosphaera watsonii at 380 ppm and 750 ppm CO2 levels. The results they obtained indicated that when the seawater was replete with iron, daily primary production at 750 ppm CO2 was 21% greater than it was at 380 ppm; but when the seawater was iron-limited, daily primary production at 750 ppm CO2 was 150% greater than it was at 380 ppm. With respect to N2 fixation, rates varied little between the two CO2 treatments when the seawater was iron-limited; but when the seawater was replete with iron, N2 fixation at 750 ppm CO2 was 60% greater than it was at 380 ppm.
In discussing their findings, Fu et al. write that "several studies examining the marine diazotrophic cyanobacterium Trichodesmium have shown significant increases in N2 fixation and photosynthesis in response to elevated CO2 concentration (Hutchins et al., 2007; Levitan et al., 2007; Ramos et al., 2007)," and they say that their data "extend these findings to encompass the marine unicellular N2-fixing cyanobacterium Crocosphaera," which group, they add, "is now recognized as being perhaps equally as important as Trichodesmium to the ocean nitrogen cycle (Montoya et al., 2004)." Consequently, they conclude that "anthropogenic CO2 enrichment could substantially increase global oceanic N2 and CO2 fixation," which two-pronged phenomenon would be a tremendous boon to the marine biosphere.
In more of a hybrid terrestrial/aquatic environment, Feng et al. (2009) measured a number of characteristics of purple phototrophic bacteria (PPB) within the rhizosphere and bulk soils of a rice/wheat rotation system at the Nianyu Experimental Station in Jiangsu Province, China, under two CO2 treatments. This they did because rice fields, in their words, "represent the most important agricultural ecosystems in Asia, since rice and wheat are the main source for food supply, and more than 90% of rice fields around the world are located in Asia," and they indicate that "purple phototrophic bacteria (PPB) are thought to be crucial in the nutrient cycling of rice fields." Indeed, the Chinese researchers say that PPB "thrive in the anaerobic portions of all kinds of aquatic environments, and have long been recognized as one of the key players in global carbon and nitrogen cycles." Thus, the researchers set out to grow rice plants (Oryza sativa L.) under standard paddy culture at two levels of soil nitrogen (N) fertility (low and high) and two levels of atmospheric CO2 concentrations (ambient and ambient +200 ppm), throughout which period they measured a number of PPB characteristics.
Among their several findings, Feng et al. report that (1) "based on denaturant gradient gel electrophoresis (DGGE) analysis of pufM gene encoding the M subunit of anoxygenic PPB light reaction center, elevated CO2 appeared to enhance the biodiversity of PPB in flooded paddy soils," that (2) "this was further supported by canonical correspondence analysis (CCA) of DGGE fingerprinting pattern of pufM genes in paddy soils as well as Shannon diversity indices," that (3) "real-time quantitative PCR analysis of pufM gene further indicated that PPB abundance was stimulated by elevated CO2 in bulk soil," and that (4) "N fertilization enhanced the biodiversity of PPB under elevated atmospheric CO2." The significance of these findings is brought into perspective by Feng et al.'s noting that it has been found that "PPB inoculation into the flood water [in rice paddy culture] could lead to grain yield increase by 29% (Elbadry et al., 1999; Harada et al., 2005)," as well as the fact that "PPB are thought to be capable of fixing nitrogen." Similar sentiments were expressed by Feng et al. (2011) who, in discussing the results of the same experiment, acknowledge the importance of elevated CO2 concentrations on PPB to "enhance the microbial food chain and promote the growth and yield of crops."
Taken together, the above observations suggest that rising atmospheric CO2 levels will likely allow similar or greater numbers of bacteria to exist in both terrestrial and aquatic environments, enhancing carbon and nitrogen sequestration, which in turn will stimulate the growth and productivity of the surrounding environments.
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Zak, D.R., Pregitzer, K.S., Curtis, P.S. and Holmes, W.E. 2000. Atmospheric CO2 and the composition and function of soil microbial communities. Ecological Applications 10: 47-59.Last updated 12 September 2012