The impact of increased atmospheric carbon dioxide on microbial community dynamics in the rhizosphere

Abstract

Rising atmospheric CO2 levels are predicted to have major consequences upon carbon cycle feedbacks and the overall functioning of terrestrial ecosystems. Photosynthetic activity and the structure of terrestrial macrophytes is expected to change, but it remains uncertain how this will affect soil-borne communities dependent on plant-derived carbon, and their feedbacks on ecosystem function. Our main objective is to assess the impact of increased atmospheric carbon dioxide on microbial community dynamics in the rhizosphere. Using a controlled growth system, we examined the short-term and long-term impact of elevated atmospheric CO2 on soil-borne microbial communities by comparing belowground community responses associated with plants grown under ambient versus double ambient CO2 environments. Results on the structure and dynamics of broad and specific microbial groups provided insight into the plant-microbe interactions of the r hizosphere under elevated CO2. We also showed that the specific microbial groups are affected by elevated CO2 and demonstrate that presumably rhizo-competent bacteria and fungi are most highly affected by increased atmospheric CO2. These patterns were consistent with observed changes in the density of antibiotic production genes as well as changes in exudation patterns. The results demonstrate that elevated CO2 influenced different parts of the soil microbial community, but that the effects depend on the plant species and soil type. Pulse labelling studies demonstrates that elevated atmospheric CO2 increases translocation of plant-fixed carbon, via arbuscular mycorrhizal fungi (AMF), and that distinct microbial populations incorporate plant-derived carbon under different levels of atmospheric CO2. As opposed to simply increasing the activity of soil-borne microbes resident at ambient CO2 conditions, elevated atmospheric CO2 clearly selects for opportunistic plant-associated microbial communities, with a shift in dominant AMF species, as well as rhizosphere bacterial and fungal populations. These experiments also showed that AMF are the main conduit in the transfer of carbon between plants and soil. The microbial carbon dynamic model derived from our results provides a general framework for reappraising our view of carbon flow paths in soils and their effects on soil biodiversity under elevated atmospheric CO2 concentration.