Root biomass dynamics in a semi-natural grassland exposed to elevated atmospheric CO2 for five years

摘要

The effects of elevated atmospheric CO2 on root dynamics were studied in a semi-natural grassland in central Sweden during five consecutive summer seasons. Open-top chambers were used for ambient and elevated ( + 350 mumol mol(-1)) concentrations of CO2, and chamberless rings were used for control. Root dynamics were observed in situ with minirhizotrons during the five summers and root biomass production was measured with root in growth cores during the last two years, from which total root biomass was estimated for each of the five years. The elevated CO2 treatment showed both a greater increase in root numbers during the early summer and a greater decline in root numbers during autumn and winter than the ambient CO2 treatment. Mean root production under elevated CO2 was 50% greater than ambient CO2 during the five years, and the difference increased from + 25% in the first year to + 80% in the last two years. Conversely, during the same period, the elevated to ambient CO2 difference in shoot biomass decreased from + 50% to + 5%. This resulted in a dramatic change in root to shoot ratios in elevated CO2 compared with the ambient treatment, which increased from - 15% in 1996 to + 70% in 2000. Similar differences were seen between elevated CO2 and the chamberless grown control plants, where root to shoot ratios increased steadily from - 47% in 1996 to + 27% in 2000. Less dynamically, the root to shoot ratios of ambient CO2 grown plants compared with the chamberless control plants were consistently - 29% +/- 96% during the experimental period. In conclusion, during the 5 years this grassland was studied, there was a clear shift in plant biomass partitioning from above to below ground for plants exposed to elevated CO2.