QUANTIFICATION OF SOIL CARBON INPUTS UNDER ELEVATED CO2-C-3 PLANTS INA C-4 SOIL

The objective of this investigation was to quantify the differences in soil carbon stores after exposure of birch seedlings (Betula pendula Roth.) over one growing season to ambient and elevated carbon dioxide concentrations. One-year-old seedlings of birch were transplanted to p ots containing 'C-4 soil' derived from beneath a maize crop, and place d in ambient (350 mu L L-1) and elevated (600 mu L L-1) plots in a fre e-air carbon dioxide enrichment (FACE) experiment. After 186 days the plants and soils were destructively sampled, and analysed for differen ces in root and stem biomass, total plant tissue and soil C contents a nd delta(13)C values. The trees showed a significant increase (+50%) i n root biomass, but stem and leaf biomasses were not significantly aff ected by treatment. C isotope analyses of leaves and fine roots showed that the isotopic signal from the ambient and elevated CO2 supply was sufficiently distinct from that of the 'C-4 soil' to enable quantific ation of net root C input to the soil under both ambient and elevated CO2. After 186 days, the pots under ambient conditions contained 3.5 g of C as intact root material, and had gained an additional 0.6 g C ad ded to the soil through root exudation/turnover; comparable figures fo r the pots under elevated CO2 were 5.9 g C and 1.5 g C, respectively. These data confirm the importance of soils as an enhanced sink for C u nder elevated atmospheric CO2 concentrations. We propose the use of 'C -4 soils' in elevated CO2 experiments as an important technique for th e quantification of root net C inputs under both ambient and elevated CO2 treatments.