GLOBAL ATMOSPHERIC CHANGE EFFECTS ON TERRESTRIAL CARBON SEQUESTRATION- EXPLORATION WITH A GLOBAL C-CYCLE AND N-CYCLE MODEL (CQUESTN)

A model of the interacting global carbon and nitrogen cycles (CQUESTN) is developed to explore the possible history of C-sequestration into the terrestrial biosphere in response to the global increases (past an d possible future) in atmospheric CO2 concentration, temperature and N -deposition. The model is based on published estimates of pre-industri al C and N pools and fluxes into vegetation, litter and soil compartme nts. It was found necessary to assign low estimates of N pools and flu xes to be compatible with the more firmly established C-cycle data. Ne t primary production was made responsive to phytomass N level, and to CO2 and temperature deviation from preindustrial values with sensitivi ties covering the ranges in the literature. Biological N-fixation coul d be made either unresponsive to soil C:N ratio, or could act to tend to restore the preindustrial C:N of humus with different N-fixation in tensities. As for all such simulation models, uncertainties in both da ta and functional relationships render it more useful for qualitative evaluation than for quantitative prediction. With the N-fixation respo nse turned off, the historic CO2 increase led to standard-model seques tration into terrestrial ecosystems in 1995AD of 1.8 Ct C yr(-1). With N-fixation restoring humus C:N strongly, C sequestration was 3 Ct yr( -1) in 1995. In both cases C:N of phytomass and litter increased with time and these increases were plausible when compared with experimenta l data on CO2 effects. The temperature increase also caused net C sequ estration in the model biosphere because decrease in soil organic matt er was more than offset by the increase in phytomass deriving from the extra N mineralised. For temperature increase to reduce system C pool size, the biosphere ''leakiness'' to N would have to increase substan tially with temperature. Assuming a constant N-loss coefficient, the h istoric temperature increase alone caused standard-model net C sequest ration to be about 0.6 Gt C in 1995. Given the disparity of plant and microbial C:N, the modelled impact of anthropogenic N-deposition on C- sequestration depends substantially on whether the deposited N is init ially taken up by plants or by soil microorganisms. Assuming the latte r, standard-model net sequestration in 1995 was 0.2 Ct C in 1995 from the N-deposition effect alone. Combining the effects of the historic c ourses of CO2, temperature and N-deposition, the standard-model gave C -sequestration of 3.5 Ct in 1995. This involved an assumed weak respon se of biological N-fixation to the increased carbon status of the ecos ystem. For N-fixation to track ecosystem C-fixation in the long term h owever, more phosphorus must enter the biological cycle. New experimen tal evidence shows that plants in elevated CO2 have the capacity to mo bilize more phosphorus from so-called ''unavailable'' sources using me chanisms involving exudation of organic acids and phosphatases.