Carbon Fluxes in Plant‐Soil Systems at Elevated Atmospheric CO2 Levels

Abstract

The flow of carbon from photosynthesizing tissues of higher plants, through the roots and into the soil is one of the key processes in terrestrial ecosystems. An increased level of CO in the atmosphere will likely result in an increased input of organic carbon into the soil due to the expected increase in primary production. Whether this will lead to accumulation of greater amounts of organic carbon in soil depends on the flow of carbon through the plant into the soil and its subsequent transformation in the soil by microorganisms. In this paper the major controls of carbon translocation via roots into the soil as well as the subsequent microbial turnover of root-derived carbon are reviewed. We discuss possible consequences of an increased CO level in the atmosphere on these processes.

Influence of an Elevated Atmospheric CO 2 Content on Soil and Rhizosphere Bacterial Communities Beneath Lolium perenne and Trifolium repens under Field Conditions

“…1) showed under CO 2 enrichment a consistent increase in the most probable numbers of heterotrophs inhabiting rhizospheric soil surrounding L. perenne roots. This result is explained by an increased amount of CO 2 -induced rhizodeposition [47,52,66]. Qualitative changes in the composition of rhizodeposition may also be an explanation, but this feature is not well documented.…”

“…Nevertheless, the major influence of a doubling of CO 2 atmospheric concentration on soil microbial communities is indirect because CO 2 concentration in soil is greater than 0.1 kPa [66], whereas the current atmospheric content is 0.035 kPa. Consequently, the main influence is thought to occur by the intermediation of the plants.…”

“…Lekkerkerk et al [32] showed that the root-derived, easily biodegradable compounds of wheat roots increased under a CO 2 -enriched atmosphere. Van Veen et al [66] suggested quantitative and qualitative changes in rhizodeposition linked to CO 2 enrichment. Since rhizodeposition consists mainly of organic matter, these changes may influence the biomass, activity, and structure of the bacterial community, leading to changes in soil processes involving plant roots, microorganisms, and soil physico-chemical properties.…”

Carbon Fluxes in the Rhizosphere

“…If a large proportion of root exudates is used by diazotrophs in the rhizosphere, the assimilation efficiency of root exudates will be much lower than if it is being used by non-nitrogen-fixing microbes. Several studies (Liljeroth et al 1990;Van Veen et al 1991) have shown that the assimilation efficiency of root-derived materials is higher when more nitrogen fertilizer has been used. It is likely that nitrogen fertilization suppresses diazotrophic activity in the rhizosphere, which contributes to higher assimilation efficiency.…”

“…Because of the abundant supply of available carbon in the form of exudates, microbial growth in the rhizosphere may be highly limited by mineral nutrients. For example, microbial respiration rate in the young wheat rhizosphere is not stimulated by addition of glucose (Cheng et al 1994), but the assimilation efficiency of root-derived materials is higher when more nitrogen fertilizer has been used (Liljeroth et al 1990;Van Veen et al 1991).…”