Responses of soil biota to elevated atmospheric carbon dioxide |
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Authors: | Elizabeth G O'Neill |
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Institution: | (1) Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA |
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Abstract: | Increasing concentrations of atmospheric CO2 could have dramatic effects upon terrestrial ecosystems including changes in ecosystem structure, nutrient cycling rates,
net primary production, C source-sink relationships and successional patterns. All of these potential changes will be constrained
to some degree by below ground processes and mediated by responses of soil biota to indirect effects of CO2 enrichment. A review of our current state of knowledge regarding responses of soil biota is presented, covering responses
of mycorrhizae, N-fixing bacteria and actinomycetes, soil microbiota, plant pathogens, and soil fauna. Emphasis will be placed
on consequences to biota of increasing C input through the rhizosphere and resulting feedbacks to above ground systems. Rising
CO2 may also result in altered nutrient concentrations of plant litter, potentially changing decomposition rates through indirect
effects upon decomposer communities. Thus, this review will also cover current information on decomposition of litter produced
at elevated CO2.
Summary Predictably, the responses of soil biota to CO2 enrichment and the degree of experimental emphasis on them increase with proximity to, and intimacy with, roots. Symbiotic
associations are all stimulated to some degree. Total plant mycorrhization increases with elevated CO2. VAM fungi increase proportionately with fine root length/mass increase. ECM fungi, however, exhibit greater colonization
per unit root length/mass at elevated CO2 than at current atmospheric levels. Total N-fixation per plant increases in all species examined, although the mechanisms
of increase, as well as the eventual benefit to the host relative to N uptake may vary. Microbial responses are unclear. The
assumption that changes in root exudation will drive increased mineralization and facilitate nutrient uptake should be examined
experimentally, in light of recent models. Microbial results to date suggest that metabolic activity (measured as changes
in process rates) is stimulated by root C input, rather than population size (measured by cell or colony counts). Insufficient
evidence exists to predict responses of either soil-borne plant pathogens or soil fauna (i.e., food web responses). These
are areas requiring attention, the first for its potential to limit ecosystem production through disease and the second because
of its importance to nutrient cycling processes. Preliminary data on foliar litter decomposition suggests that neither nutrient
ratios nor decomposition rates will be affected by rising CO2. This is another important area that may be better understood as the number of longer term studies with more realistic CO2 exposures increase. Evidence continues to mount that C fixation increases with CO2 enrichment and that the bulk of this C enters the belowground component of ecosystems. The global fate and effects of this
additional C may affect all hierarchical levels, from organisms to ecosystems, and will be largely determined by responses
of soil biota. |
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Keywords: | CO2 enrichment decomposition mycorrhizae nitrogen fixation rhizosphere soil biota soil fauna |
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