Carbon accumulation, distribution and water use of Danthonia richardsonii swards in response to CO2 and nitrogen supply over four years of growth

Microcosms of Danthonia richardsonii (Cashmore) accumulated more carbon when grown under CO₂ enrichment (719 μL L^–1 cf. 359 μL L^–1) over a four-year period, even when nitrogen availability severely restricted productivity (enhancement ratios for total microcosm C accumulation of 1.21, 1.14 and 1.29 for mineral N supplies of 2.2, 6.7 and 19.8 g N m^–2 y^–1, respectively). The effect of CO₂ enrichment on total system carbon content did not diminish with time. Increased carbon accumulation occurred despite the development over time of a lower leaf area index and less carbon in the green leaf fraction at high CO₂. The extra carbon accumulated at high CO₂ in the soil, senesced leaf and leaf litter fractions at all N levels, and in root at high-N, while at low-and mid-N less carbon accumulated in the root fraction at high CO₂. The rate of leaf turnover was increased under CO₂ enrichment, as indicated by increases in the carbon mass ratio of senesced to green leaf lamina. Microcosm evapotranspiration rates were lower at high CO₂ when water was in abundant supply, resulting in higher average soil water contents. The higher soil water contents at high CO₂ have important implications for microcosm function, and may have contributed significantly to the increased carbon accumulation at high CO₂. These results indicate that CO₂ enrichment can increase carbon accumulation by a simple soil–plant system, and that any increase in whole system carbon accumulation may not be evident from snapshot measurements of live plant carbon.