Equilibrium analysis of integrated plant-soil models for prediction of the nutrient limited growth response to CO2 enrichment

Although higher ambient CO2 concentration is known to promote increased plant productivity under optimal growing conditions, it is not obvious if there will be a sustained growth response in natural and plantation ecosystems, where other resources, such as nutrients, may become limiting. Comins & McMurtrie (1993, Ecol. Applic. 3, 666 681) have constructed the G'DAY (Generic Decomposition A nd Yield) integrated plant-soil model to investigate this CO2-nutrient interaction, and have described an analytic method for predicting the long-term response of their model to a step change in CO2 concentration, using the analytic "two timing" approximation. This analysis gives insights into the interactions of the numerous parameters in a comprehensive plant-soil model, and may be generalizable to other such models. The current paper explores the accuracy of the approximation, and discusses various generalizations 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 considering the dynamics of the passive soil organic matter pool in the "two timing" approximation. It was found that the two-timing approximation underestimates the 50-100 year CO2 response in svstems that lose a very small proportion of nitrogen per recycling cycle. The other areas considered here are as follows. (i) More complex relationships between N:C ratios and carbon allocation fractions for plant organs, including variable heartwood N:C ratio (which has been identified as an important determinant of long-term CO2 response). Typical results are presented 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 examined. having variable N:C ratios in the soil organic matter pools and/or carbon flux partition fractions influenced by N:C ratios. (iii) Results are presented for a preliminary analysis of variable nitrogen fixation.