CARBON BALANCE OF TALL FESCUE (FESTUCA-ARUNDINACEA SCHREB) - EFFECTS OF NITROGEN FERRITIZATION AND THE GROWING-SEASON

The C balance of a tall fescue sward grown under different rates of N fertilization in summer, autumn, and spring was calculated using model s derived from measurements of shoot growth, canopy gross photosynthes is, shoot respiration and of C partitioning to the roots. Under the di verse growing conditions associated with the seasons and the N fertili zation, C utilization for above- and below-ground biomass accumulation never exceeded 39 and 14% of the canopy gross photosynthesis, respect ively. Carbon losses attributed to root respiration and exudation, whi ch were estimated by difference between canopy net photosynthesis and total growth, ranged between 3 and 30% of canopy gross photosynthesis. Seasonal differences in shoot growth could be attributed to the amoun t of intercepted radiation, the radiation-use efficiency and the C par titioning to the roots. The effect of N deficiency on shoot growth can be attributed to its effects on canopy photosynthesis (principally re sulting from changes in intercepted photosynthetically active radiatio n) and C partitioning. In comparison with the effect on shoot growth, the effect of the N deficiency on the canopy gross photosynthesis per unit of light intercepted over the regrowth cycle was limited. It is c oncluded that most of the effect of N fertilization on shoot growth is due to changes in C partitioning which result in faster leaf area dev elopment and greater light interception.