EFFECTS OF ENHANCED ATMOSPHERIC CO2 AND NUTRIENT SUPPLY ON THE QUALITY AND SUBSEQUENT DECOMPOSITION OF FINE ROOTS OF BETULA-PENDULA ROTH AND PICEA-SITCHENSIS (BONG) CARR

Fine root litter derived from birch (Betula pendula Roth.) and Sitka spruce (Picea sitchensis (Bong.) Carr.) plants grown under two CO2 atmospheric concentrations (350 ppm and 600 ppm) and two nutrient regimes was used for decomposition studies in laboratory microcosms. Although there were interactions between litter type, CO2/fertiliser treatments and decomposition rates, in general, an increase in the C/N ratio of the root tissue was observed for roots of both species grown under elevated CO2 in unfertilized soil. Both weight loss and respiration of decomposing birch roots were significantly reduced in materials derived from enriched CO2, whilst the decomposition of spruce roots showed no such effect. A parallel experiment was performed using Betula pendula root litter grown under different N regimes, in order to test the relationship between C/N ratio of litter and root decomposition rate. A highly significant (p < 0.001) negative correlation between C/N ratio and root litter respiration was found, with an r(2) = 0.97. The results suggest that the increased C/N ratio of plant tissues induced by elevated CO2 can result in a reduction of decomposition rate, with a resulting increase in forest soil C stores.