Effects of free-air CO2 enrichment on microbial populations in the rhizosphere and phyllosphere of cotton

Abstract

Cotton (Gossypium hirsutum L.) plants were exposed to free-air CO2 enriched (FACE = 550 μmol mol−1) or ambient (CONTROL = 370 μmol mol−1) levels of atmospheric CO2 and to wet (100% of evapotranspiration replaced) or dry (67% of ET replaced) soil water content treatments. Foliar, soil and root samples were collected in June and August 1991 to determine the effects of elevated CO2 on selected groups of phyllosphere and rhizosphere microorganisms. Foliage and rhizosphere soil were analyzed for bacteria and/or fungi using dilution plating. Mycorrhizal colonization of cotton roots was assessed. Root-zone soil was analyzed for populations of nematodes, microarthropods and Rhizoctonia using various extraction methods. A dehydrogenase assay for total microbial respiration and a bioassay for cotton root infecting organisms were also conducted using root-zone soil. Populations of fungi on cotton leaves varied, by genera, in response to CO2 enrichment, but none was affected by soil water content treatments; populations of foliar bacteria were not affected by either CO2 or soil water content treatments. In August, higher total numbers of rhizosphere fungi were found under the wet compared with the dry soil water treatment, but differences related to CO2 were not detected. There was a trend for infestation by Rhizoctonia solani to be higher under FACE in the August sample, but the soil bioassay demonstrated no increase in damping-off potential. There was a significant interaction between CO2 concentration and soil water content for populations of saprophagous nematodes; populations were different between the CO2 levels in the dry soil treatment only, with higher numbers under FACE. Microarthropod numbers were low; however, there was a trend for Collembola populations to be higher under FACE in the August sample and more fungi were isolated from Collembola in June. Total microbial activity was higher under FACE at both sample dates. Effects of elevated atmospheric CO2 on plant microbe interactions could have profound influence on the productivity of agro-ecosystems, and deserve further research.