Root dynamics in a semi-natural grassland in relation to atmospheric carbon dioxide enrichment, soil water and shoot biomass

Plant responses to increasing atmospheric CO2 concentrations have been stud ied intensively. However, the effects of elevated CO2 on root dynamics, whi ch is important for global carbon budgets as well as for nutrient cycling i n ecosystems, has received much less attention. We used minirhizotrons insi de open-top chambers to study the effects of elevated atmospheric carbon di oxide concentration on root dynamics in a nutrient-poor semi-natural grassl and in central Sweden. We conducted our investigation over three consecutiv e growing seasons during which three treatments were applied at the site: E levated (approximate to 700 mu mol mol(-1)) and ambient (approximate to 360 mu mol mol(-)1) chamber levels of CO2 and a control, without a chamber. Du ring 1997, a summer with two dry periods, the elevated treatment compared w ith ambient had 25% greater mean root counts, 65% greater above-ground biom ass and 15% greater soil moisture. The chambers seemed responsible for chan ges in root dynamics, whereas the elevated CO2 treatment in general increas ed the absolute sum of root counts compared with the ambient chamber. In 19 98, a wet growing season, there were no significant differences in shoot bi omass or root dynamics and both chamber treatments had lower soil moisture than the control. We found that as seasonal dryness increased, the ratio of elevated - ambient shoot biomass production increased while the root to sh oot ratio decreased. We conclude that this grasslands response to elevated CO2 is dependent on seasonal weather conditions and that CO2 enrichment wil l most significantly increase production in such a grassland when under wat er stress.