CARBON USE IN ROOT RESPIRATION AS AFFECTED BY ELEVATED ATMOSPHERIC O-2

The use of fossil fuel is predicted to cause an increase of the atmosp heric CO2 concentration, which will affect the global pattern of tempe rature and precipitation. It is therefore essential to incorporate eff ects of temperature and water supply on the carbon requirement for roo t respiration of plants to predict effects of elevated [CO2] on the ca rbon budget of natural and managed systems. There is insufficient info rmation to support the contentention that an increase in the concentra tion of CO2 in the atmosphere will enhance the CO2 concentration in th e soil to an extent that is likely to affect root respiration. Moreove r, there is no convincing evidence for a direct effect of elevated atm ospheric [CO2] on the rate of root respiration per unit root mass or t he fraction of carbon required for root respiration. However, there ar e likely to be indirect effects of elevated [CO2] on the carbon requir ement of plants in natural systems. Firstly, it is very likely that th e carbon requirement of root respiration relative to that fixed in pho tosynthesis will increase when elevated [CO2] induces a decrease in nu trient status of the plants. Although earlier papers have emphasized t hat elevated [CO2] favours investment of biomass in roots relative to that in leaves, these are in fact indirect effects. The increase in ro ot weight ratio is due to the more rapid depletion of nutrients in the root environment as a consequence of enhanced growth. This will decre ase the specific rate of root respiration, but increase the carbon req uirement as a fraction of the carbon fixed in photosynthesis. It is li kely that these effects will be minor in systems where the nutrient su pply is very high, e.g, in many managed arable systems, and increase w ith decreasing soil fertility, i.e. in many natural systems. Secondly, a decrease in rainfall in some parts of the world may cause a shortag e in water supply which favours the carbon partitioning to roots. Wate r stress is likely to reduce rates of root respiration per unit root m ass, but enhance the fraction of total assimilates required for root r espiration, due to greater allocation of biomass to roots. Increased t emperatures are unlikely to affect the specific rate of root respirati on in all species. Broadly generalized, the effect of temperature on b iomass allocation is that the relative investment of biomass in roots is lowest at a certain optimum temperature and increases at both highe r and lower temperatures. The root respiration of some species acclima tes to growth temperature, so that the effect of global temperature ri se is entirely accounted for by the effect of temperature on biomass a llocation. The specific rate of root respiration of other species will increase with global warming. In response to global warming the carbo n requirement of roots is likely to decrease in temperate regions, whe n temperatures are suboptimal for the roots' capacity to acquire water . Here global warming will induce a smaller biomass allocation to the roots. Conversely, the carbon requirements are more likely to increase in mediterranean environments, where temperatures are often supraopti mal and a rise in temperature will induce greater allocation of biomas s to the roots.