CARBON STORAGE AND PRODUCTIVITY OF A CARBON-DIOXIDE ENRICHED NITROGENLIMITED GRASS SWARD AFTER ONE YEARS GROWTH

Determining the response of nitrogen restricted ecosystems to carbon d ioxide enrichment is important in evaluating the role of the terrestri al biosphere in the unidentified sink in global carbon cycle models. S wards of the C3 grass Danthonia richardsonii (Cashmere) were establish ed in large pots filled with a soil of low C and N content. The swards were continuously supplied with N at rates of 2, 6 and 18 g m(-2) yr( -1), and exposed to atmospheric CO2 concentrations of either 357 or 71 2 mu L L(-1). After 1 year's growth the high CO2 treatments gained 19, 53 and 43% more C than at low CO2 concentrations for the low, medium and high N treatments, respectively. This extra C gain was found in al l plant and soil pools at the medium N level. At the low N level no ex tra C was found in the roots. At the high N level no extra carbon was found in the soil. Leaf area index was not affected by growth at high CO2. The extra C was gained with the same total N investment in green leaf in the two lowest N treatments, and with 30% less N in green leaf at the highest N level. Growth at the high CO2 concentration resulted in all C pools having a higher C:N ratio. Total water use was decreas ed and water use efficiency increased by growth at the high CO2 concen tration. It was noted that if these results were transferable to the h eld, and if the higher C:N ratios do not reduce longer term productivi ty by reducing N-mineralization rates, grasslands could form a substan tial part of the unidentified C sink. The potential feedback of decrea sed N availability in the longer term is being investigated in the fin al 3 years of the experiment.