Phoenix, a model of the dynamics of carbon and nitrogen in grassland soils

A simulation model - "Phoenix" - has been developed to examine carbon (C) and nitrogen (N) cycling both between plants and soils and within soil, between microorganisms and the humic component. One of the main purposes of Phoenix is to examine, through simulation modelling, the interrelationship of C and N and their recycling within soil. Plant components considered aje living roots, living shoots and standing dead. Dead organisms are subdivided into a rapidly recycling N-rich metabolic component and a structural component which decomposes slowly. The microbial component considers both bacteria and fungi. The effects of soil animals are included implicitly through a prédation function. Soil organic matter is subdivided into a "humads" component with a turnover time of about 20 years and a resistant soil organic matter fraction with a turnover time of about 500 years. Unique features of Phoenix include concurrent mineralization and immobilization of N, population density effects on decomposition when the substrate is also the habitat, density-dependent death of microorganisms, fluctuating C/N ratio of microorganisms, the manner in which litter is partitioned, which implies faster internal recycling of N than of C from fresh litter, a cascading system of soil organic matter cycling, inclusion of two populations of microorganisms and microbial death through predation, and the high degree of interaction between plants and microorganisms. The model and the background ideas and concepts used in its development are described in detail. Phoenix has been tested qualitatively and semiquantitatively using both laboratory and field data. The model appears to accurately reflect the qualitative responses of grassland systems to perturbation. Further work is needed to rigorously test its quantitative behaviour.