EQUILIBRIUM-ANALYSIS OF INTEGRATED PLANT-SOIL MODELS FOR PREDICTION OF THE NUTRIENT LIMITED GROWTH-RESPONSE TO CO2 ENRICHMENT

Although higher ambient CO, concentration is known to promote increase d plant productivity under optimal growing conditions, it is not obvio us if there will be a sustained growth response in natural and plantat ion ecosystems, where other resources, such as nutrients, may become l imiting. Comins and McMurtrie (1993, Ecol. Applic. 3, 666-681) have co nstructed the G'DAY (Generic Decomposition A nd Field) integrated plan t-soil model to investigate this CO2-nutrient interaction, and have de scribed an analytic method for predicting the long-term response of th eir model to a step change in CO2 concentration, using the analytic '' two timing'' approximation. This analysis gives insights into the inte ractions of the numerous parameters in a comprehensive plant-soil mode l, and may be generalizable to other such models. The current paper ex plores the accuracy of the approximation, and discusses various genera lizations of the basic model to which the analytic model can still be applied. The very long-term CO2 response of G'DAY was predicted by con sidering the dynamics of the passive soil organic matter pool in the ' 'two timing'' approximation. It was found that the two-timing approxim ation underestimates the 50-100 year CO2 response in systems that lose a very small proportion of nitrogen per recycling cycle. The other ar eas considered here are as follows. (i) More complex relationships bet ween N:C ratios and carbon allocation fractions for plant organs, incl uding variable heartwood N:C ratio (which has been identified as an im portant determinant of long-term CO2 response). Typical results are pr esented for a range of sensitivities of heartwood N:C ratio to changes in foliar N:C ratio. (ii) Variants of the CENTURY soil model were exa mined, having variable N:C ratios in the soil organic matter pools and /or carbon flux partition fractions influenced by N:C ratios. (iii) Re sults are presented for a preliminary analysis of variable nitrogen fi xation.