Carbon Storage and Productivity of a Carbon Dioxide Enriched Nitrogen Limited Grass Sward After One Year's Growth

Abstract

Determining the response of nitrogen restricted ecosystems to carbon dioxide enrichment is important in evaluating the role of the terrestrial biosphere in the unidentified sink in global carbon cycle models. Swards of the C3 grass Danthonia richardsonii (Cashmore) were established 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 712 ,uL L1. 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 all plant and soil pools at the medium N level. At the low N level no extra 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 decreased and water use efficiency increased by growth at the high CO2 concentration. It was noted that if these results were transferable to the field, and if the higher C:N ratios do not reduce longer term productivity by reducing N-mineralization rates, grasslands could form a substantial part of the unidentified C sink. The potential feedback of decreased N availability in the longer term is being investigated in the final 3 years of the experiment.