Characterisation and microbial utilisation of exudate material from the rhizosphere of Lolium perenne grown under CO2 enrichment

The effects of elevated atmospheric CO2 concentration on alterations, both qualitatively and quantitatively, of exuded compounds from the roots of Lolium perenne seedlings were investigated by growing plants in a sterilised sand microcosm unit. In addition, the effect of CO2 treatment on carbon substrate utilisation of microbial populations extracted from the rhizosphere of L. perenne seedlings grown in soil microcosm units was examined and alterations on microbial activity and diversity assessed using a commercially-available redox-based sole C source utilisation test (Biolog(R)) including additional exudate compounds. Both types of microcosm units (sand and soil) were maintained at specific growth conditions under two CO2 regimes (450 and 720 mu mol mol(-1)). Growth of L. perenne seedlings from both types of microcosm units was enhanced under elevated atmospheric CO2 although the root-to-shoot ratios were not significantly altered, indicating no gross change in dry matter partitioning. Cumulative total organic carbon (TOC) release in the exudate material over the duration of the experiment was significantly (P less than or equal to 0.05) higher from ambient-grown seedlings despite a significant (P less than or equal to 0.05) increase in the dry weight of roots of the elevated CO2 grown seedlings as determined at harvest. Over the individual sampling periods TOC release was significantly (P less than or equal to 0.05) higher from elevated CO2 grown seedlings on only one occasion (21 d). Qualitative differences, measured between d 1-6 and 14-18, also occurred with elevated CO2 treatment decreasing the amount of phenolic acids and total sugars at the latter sampling period compared to ambient CO2 seedlings. Total numbers of bacteria were significantly (P less than or equal to 0.05) decreased under elevated CO2 although culturable numbers significantly (P less than or equal to 0.05) increased. This increase in culturable microorganisms may explain the faster carbon source utilisation rates of the elevated CO2 treatment. No change in morphotypes of microbial colonies were observed suggesting a quantitative difference due to elevated CO2 treatment only. (C) 1998 Elsevier Science Ltd. All rights reserved.