MODELING THE BELOWGROUND RESPONSE OF PLANTS AND SOIL BIOTA TO EDAPHICAND CLIMATIC-CHANGE - WHAT CAN WE EXPECT TO GAIN

As atmospheric CO2 concentrations continue to increase, so too will th e emphasis placed on understanding the belowground response of plants to edaphic and climatic change. Controlled-exposure studies that addre ss the significance of an increased supply of carbon to roots and soil biota, and the consequences of this to nutrient cycling will play a p rominent role in this process. Models will also contribute to understa nding the response of plants and ecosystems to changes in the earth's climate by incorporating experimental results into conceptual or quant itative frameworks from which potential feedbacks within the plant-soi l system can be identified. Here we present five examples of how model s can be used in this analysis and how they can contribute to the deve lopment of new hypotheses in the areas of root biology, soil biota, an d ecosystem processes. Two examples illustrate the role of coarse and fine roots in nitrogen and phosphorus uptake from soils, the respirato ry costs associated with this acquisition of nutrients, and the signif icance of root architecture in these relationships. Another example fo cuses on a conceptual model that has helped raise new ideas about the effects of elevated CO2 on root and microbial biomass, and on nutrient dynamics in the rhizosphere. Difficulties associated with modeling th e contribution of mycorrhizal fungi to whole-plant growth are also dis cussed. Finally several broad-scale models are used to illustrate the importance of root turnover, litter decomposition, and nitrogen minera lization in determining an ecosystem's response to atmospheric CO2 enr ichment. We conclude that models are appropriate tools for use both in guiding existing studies and in identifying new hypotheses for future research. Development of models that address the complexities of belo wground processes and their role in determining plant and ecosystem fu nction within the context of rising CO2 concentrations and associated climate change should be encouraged.