Effects of mycorrhizal colonization and elevated atmospheric carbon dioxide on carbon fixation and below-ground carbon partitioning in Plantago lanceolata

Abstract

Plantago lanceolata with or without the mycorrhizal fungus Glomus mosseae were grown over a 100 d period under ambient (380±50 μmol mol -1 ) and elevated (600 ± 150 μmol mol -1 ) atmospheric CO 2 conditions. To achieve similar growth, non-mycorrhizal plants received phosphorus in solution whereas mycorrhizal plants were supplied with bonemeal. Measures of plant growth, photosynthesis and carbon input to the soil were obtained. Elevated CO 2 stimulated plant growth to the same extent in mycorrhizal and non-mycorrhizal plants, but had no effect on the partitioning of carbon between shoots and roots or on shoot tissue phosphorus concentration. Mycorrhizal colonization was low, but unaffected by CO 2 treatment. Net photosynthesis was stimulated both by mycorrhizal colonization and elevated CO 2 , and there was a more than additive effect of the two treatments on net photosynthesis. Colonization by mycorrhizal fungi inhibited acclimation, in terms of net carbon assimilation, of plants to elevated CO 2 . 13 C natural abundance techniques were used to measure carbon input into the soil, although the results were not conclusive. Direct measurements of below-ground root biomass showed that elevated CO 2 did stimulate carbon flow below-ground and this was higher in mycorrhizal than non-mycorrhizal plants. For the four treatment combinations, the observed relative differences in amount of below-ground carbon were compared with those expected from the differences in net photosynthesis. A considerable amount of the extra carbon fixed both as a result of mycorrhizal colonization and growth in elevated CO 2 did not reveal itself as increased plant biomass. As there was no evidence for a substantial increase in soil organic matter, most of this extra carbon must have been respired by the mycorrhizal fungus and the roots or by the the plants as dark-respiration. The need for detailed studies in this area is emphasized.