Integrating mycorrhiza in a complex model system: effects on ecosystem C and N fluxes

During the last decades, ectomycorrhiza has been identified to be of major importance for ecosystem carbon (C) and nitrogen (N) cycling and tree growth. Despite this importance, mycorrhiza has largely been neglected in ecosystem models or regarded only implicitly by a static mycorrhiza term. In order to overcome this limitation, we integrated the dynamic mycorrhiza model MYCOFON (Meyer et al. in Plant Soil 327:493-517, 2010a, Plant Soil 327:519, 2010b) into the ecosystem modelling framework MoBiLE (Modular Biosphere simuLation Environment) and coupled it to available forest growth and development process models. Model testing was done for different beech and spruce forest sites in Germany. Simulation results were compared to a standard model set-up, that is, without explicit consideration of mycorrhiza. Parameters were set in order not to violate previous findings about C partitioning into aboveground and belowground biomasses. Nevertheless, the explicit consideration of mycorrhiza let to considerable differences between sites and deposition scenarios with respect to simulated root biomass, plant nitrogen supply, and gaseous soil C and N emissions. The latter was mainly a result of differences in soil N concentration and dynamics. Our simulation results also show that the C supply to mycorrhizal fungi by plants as well as the importance of mycorrhizal fungi for plant N uptake, that is, the allocation of C and N between plants and fungi, depends on the magnitude of N deposition. This effect is neglected by standard model approaches so far. Therefore, explicit consideration of mycorrhiza in ecosystem models has a high potential to improve model simulations of ecosystem C and N cycling and associated biosphere-hydrosphere-atmosphere exchange processes and consequently simulation of soil CO2 and N trace gas emissions from forest sites.