MAGE, a dynamic model of alkaline grassland ecosystems with variable soil characteristics

An area-based process model for alkaline grassland ecosystem, MAGE, was developed to address the problems associated with the soil alkalization/dealkalization processes coupled with surface vegetation on Songnen Plain, northeast China. The model gave special consideration to the variation of soil characteristics such as water retentivity and hydraulic conductivity as functions of surface vegetation. Soil within 1 m depth was divided into two layers, a surface layer on top of a core layer at bottom. The amount of non-capillary pores and hence the hydraulic conductivity and water retentivity characteristics of the surface layer were considered to be dependent on surface vegetation status. The model is able to handle multiple plant species succession, with competition between species reflected as soil water sharing and species niche overlapping along soil water and alkali axes. The model was parameterized using published data and field observations on soil water content, soluble sodium and calcium cation concentrations, and aboveground and belowground biomass. The model was used to evaluate the effects of variable soil characteristics, the harvesting intensity and the core layer soil alkaline status on surface soil alkalization/dealkalization processes. Surface vegetation dynamics and optimal harvesting control in light of maximum harvest subject to system stability were also simulated. The results showed that surface soil alkalization process was primarily determined by the chemical status of the core soil. Increasing vegetation and the non-capillary soil water capacity can reduce the rate and extent of soil alkalization or increase the rate and extent of soil dealkalization. Hence for given chemical conditions in deep soil, surface vegetation and soil non-capillary porosity are indeed crucial factors for soil alkalization/dealkalization. The results also indicated that there existed an optimal harvest intensity which renders the total harvest a maximum while maintaining system stability; and that the maximum total harvest and the associated optimal harvest control level decreased with core soil alkali but increased with the maximum non-capillary water capacity in surface soil.