MODIFICATIONS OF THE CARBON AND NITROGEN ALLOCATIONS IN THE PLANT (TRITICUM-AESTIVUM L) SOIL SYSTEM IN RESPONSE TO INCREASED ATMOSPHERIC CO2 CONCENTRATION

The aim of this work was to examine the response of wheat plants to a doubling of the atmospheric CO2 concentration on: (1) carbon and nitro gen partitioning in the plant, (2) carbon release by the roots; and (3 ) the subsequent N uptake by the plants. The experiment was performed in controlled laboratory conditions by exposing fast-growing spring wh eat plants, during 28 days, to a (CO2)-C-14 concentration of 350 or 70 0 muL L-1 at two levels of soil nitrogen fertilization. Doubling CO2 a vailability increased total plant production by 34% for both N treatme nt. In the N-fertilized soil, the CO2 enrichment resulted in an increa se in dry mass production of 41% in the shoots and 23% in the roots; w ithout N fertilization this figure was 33% and 37%, respectively. In t he N-fertilized soil, the CO2 increase enhanced the total N uptake by 14% and lowered the N concentration in the shoots by 23%. The N concen tration in the roots was unchanged. In the N-fertilized soil, doubling CO2 availability increased N uptake by 32% but did not change the N c oncentrations, in either shoots or roots. The CO2 enrichment increased total root-derived carbon by 12% with N fertilization, and by 24% wit hout N fertilization. Between 85 and 90% of the total root derived-C-1 4 came from respiration, leaving only 10 to 15% in the soil as organic C-14. However, when total root-derived C-14 was expressed as a functi on of root dry weight, these differences were only slightly significan t. Thus, it appears that the enhanced carbon release from the living r oots in response to increased atmospheric CO2, is not due to a modific ation of the activity of the roots, but is a result of the increased s ize of the root system. The increase of root dry mass also resulted in a stimulation of the soil N mineralization related to the doubling at mospheric CO2 concentration. The discussion is focused on the interact ions between the carbon and nitrogen allocation, especially to the roo t system, and the implications for the acquisition of nutrients by pla nts in response to CO2 increase.