Microbes in Soil - Helping a Carbon Source or Sink?

Elevated atmospheric CO2 can have indirect effects on soil microbial communities via altered plant inputs (litter, exudates, and rhizodeposition). Carney et al [6] reported that soils from elevated CO2 area demonstrated higher rate of microbial respiration. At elevated CO2, the “extra” carbon fixed by plants, deposited to soils through increased leaf litter fall, root exudation, or root turnover grown is quickly metabolized by the microorganisms [7]. The response of microorganisms to the extra carbon input will mediate critical carbon transformations in soil and influence how much carbon can be stored in soils over the long term. For example, at elevated CO2, the increased in plant carbon may suppress the decomposition of the organic matter in the soil. This could be that the simple organic compounds of the root exudates are easier for the microbes to use than the recalcitrant organic materials found in soil [8], as also demonstrated in short-term laboratory or greenhouse experiments [9].

Elevated CO2 also altered soil microbial communities in the field such that they degraded soil organic matter more rapidly, an approximately of 52% loss of soil carbon over a 6 year period in an elevated CO2 site [6]. The isotopic composition of microbial fatty acids confirmed that elevated CO2 increased microbial utilization of soil organic matter. Work by Hooebeek et al [10] also found that such increase of extra carbon to soil may stimulate microbial degradation of soil organic matter for nutrients or change in microbial activity or community composition. Indication of whether specific microbial groups were preferentially using soil carbon was traced by using isotope signature into the microbial fatty acids via phospholipid fatty acid (PLFA) analysis.

Furthermore, elevated CO2 promote the growth of fungal in soil as there is lower nitrogen availability [11]. Fungi tend to have higher carbon: nitrogen ratios than bacteria, which lessen fungal demands for nitrogen [12]. The higher root turnover at elevated CO2 area also helps to promote fungal growth. At elevated CO2, there are also higher activities of a soil carbon degrading enzyme phenol oxidase [6], which led to more rapid rates of soil organic matter degradation than soils exposed to ambient CO2. Thus, elevated CO2 both accelerated the oxidation of soil organic carbon to CO2 by soil microorganisms and increased their use of this carbon source as a substrate from biomass production.

If elevated CO2 promote the growth of fungal in soil and alter the soil microorganism activities, does it help in bacterial diversity? Sadly, the mutualism relationship of elevated CO2 and bacterial diversity has not yet to be determined. Several studies found a positive relationship between elevated CO2 and bacterial richness [13], whereas others found a negative effect [14]. Furthermore, Griffiths et al. [15] or Zak et al [16] reported that elevated CO2 concentrations have little effect on soil microbial composition. In contrast, aboveground plant diversity significantly affected the bacterial composition in the soil, suggesting that the soil microbial composition is mainly related to plant diversity (assuming that different plant species might harbor specific microbial populations) rather than altered soil carbon fluxes induced by elevated atmospheric CO2 and subsequently increased photosynthetic activities. Moreover, plant diversity had a significant effect on belowground microbial community composition on a genetic level. Bacterial community structure in soil can be differentiated in relation to different plant diversity levels as reported by Stephan and his colleagues [17] that plant diversities exhibit their own bacterial environment in 3 different plants. As also stated by other authors [18], the soil type might be a key determinant for soil microbial communities. This finding suggest elevated CO2 does not lead to quantitative alteration (bacterial richness) in soil whereas plant diversity affect bacterial composition and diversity (their frequency of occurrence and bacterial types)

These effects of elevated CO2 on the microbial communities can theoretically occur in any terrestrial ecosystem. However, it will require a study of colossal scale on many ecosystems to confirm it. In conclusion, while elevated CO2 does not lead to the diversity of microorganism, it can cause soil microbes to increase the oxidation of soil organic carbon to CO2 and increased their use of this carbon source as a substrate from biomass production. While no studies to date have demonstrated an explicit link between changes in soil microbial activity and composition and long term carbon storage at elevated CO2, findings indicated that microbial community responses to elevated CO2 will constrain the potential for net gains in soil carbon storage by enhancing the decomposition of soil carbon. This can cause a potential carbon sink to become a carbon source, which do not help in the efforts in slowing global warming. There is only a limited capacity in Earth’s ecosystem to stabilize atmospheric CO2 and slow global warming. We as human would have to do more than to rely and hope that the Earth can recover on its own or with minimum help.

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