THE EFFECT OF PLANT-MATERIAL GROWN UNDER ELEVATED CO2 ON SOIL RESPIRATORY ACTIVITY

The biodegradability of aerial material from a C4 plant, sorghum grown under ambient (345 mu mol mol(-1)) and elevated (700 mu mol mol(-1)) atmospheric CO2 concentrations were compared by measuring soil respiratory activity. Initial daily respiratory activity (measured over 10 h per day) increased four fold from 110 to 440 cm(3) CO2 100g dry weight soil(-1) in soils amended with sorghum grown under either elevated or ambient CO2. Although soil respiratory activity decreased over the following 30 days, respiration remained significantly higher (t-test; p>0.05) in soils amended with sorghum grown under elevated CO2 concentrations. Analysis of the plant material revealed no significant differences in C:N ratios between sorghum grown under elevated or ambient CO2. The reason for the differences in soil respiratory activity have yet to be elucidated. However if this trend is repeated in natural ecosystems, this may have important implications for C and N cycling.