Model Development – The Ecosys Modelling Project

At this time, the overall architecture of ecosys has been established and many of its higher resolution processes have been tested in collaboration with experimental research programs in the U.S., Europe and Canada. The testing of other processes are in progress. Examples of model development and testing follow.

Photosynthesis

Submodels of biochemical CO₂ fixation in leaf chloroplasts and of stomatal resistance to diffusive CO₂ and H₂O transfer through leaf surfaces have been coupled and their combined behavior tested against data at both the leaf and canopy levels under different atmospheric CO₂ concentrations (C_a) recorded in the Soil-Plant-Atmosphere Research Units at the Univ. of Florida. The testing of this submodel builds upon earlier work in the simulation of the biochemistry and physics of CO₂ fixation at both leaf and canopy levels, and has been extended to elevated C_a in the first reported testing of photosynthesis submodels under field conditions as part of the FACE project. In this project, the photosynthesis submodels enabled ecosys to reproduce important interactions on plant growth between C_a and water stress and between C_a and nitrogen. Testing of the photosynthesis submodel at both leaf and canopy scales has been extended to different forest stands in the BOREAS and Fluxnet-Canada projects, and to semiarid grasslands and arctic tundra in the AMERIFLUX project.

As part of ecosys, this coupled submodel of CO₂ fixation and gas exchange has a critical role in the simulation of photosynthetic responses to altered C_a under different soil and management conditions. The rigorous testing of the photosynthesis submodel under elevated C_a and field conditions, as done in the FACE project, is a critical prerequisite for the use of ecosystem models to predict plant behavior under elevated C_a. This submodel has been used to estimate changes in net ecosystem productivities of forests, grasslands and tundra under hypothesized changes in climate (IS92a).

Mass and Energy Exchange

The submodel described above has been extended to include a solution to the first-order closure of the energy balance between the atmosphere and plant canopies. Results have been tested with eddy correlation and Bowen ratio data collected over agricultural crops, forests, grasslands and tundra. The solution to the energy balance features explicit linkages of water status among the soil, root, canopy and atmosphere during radial and axial movement of water in the liquid phase through heterogeneous soil and root systems, and during diffusive transfer of water in the vapor phase through leaves and boundary layers in the canopy. This solution allows ecosys to reproduce the dynamic effects of soil water deficits on plant water status and growth under current or hypothesized climates. It will therefore make an important contribution to the study of how soil water relations altered by climate change will affect ecosystem primary productivity.

This work builds upon earlier work in the simulation of soil-plant water relations at both the hourly and seasonal levels, and has been extended to ambient vs. elevated C_a under adequate vs. inadequate irrigation and N as part of the FACE project. This work is of direct relevance to understanding the exchange of mass and energy between atmospheres and terrestrial ecosystems under current or altered climates. The testing of mass and energy exchange, as conducted in the FACE, BOREAS and Fluxnet-Canada projects, is a necessary prerequisite for the use of ecosystem models to predict ecosystem behavior under hypothesized climate changes.

Plant Growth

All plant activity in ecosys is simulated at the organ, population and community levels to allow dynamic representation of complex biomes with competing plant species. Temporal and spatial patterns of organ growth in plant shoots and roots are explicitly represented in the model and are used to calculate access to irradiance, water and nutrients among competing plant populations. Partitioning of carbon, nitrogen and phosphorus among different organs in the shoot and root is controlled by interaction between phenology and environment. The sensitivity of resulting plant growth to C_a, water deficits, fertilization, weeds and erosion has been tested. This testing is being extended in the FACE project to examine interactions among C_a, water stress and nitrogen limitations in their effects on shoot and root growth. Spatial and temporal patterns of root length density and N content reported from soil core and minirhizotron studies are used to test model hypotheses for the distribution of C, N and P in root systems under different soil and atmospheric conditions. Spatial and temporal patterns of size, mass and N content of leaves, sheaths, internodes and grains reported from plant analyses are used to test model hypotheses for the distribution of C and N in shoot systems. The accurate reproduction of interactions among soil and atmospheric conditions on shoot and root growth is critical to the sucessful use of ecosystem models to predict effects of climate change and soil management on ecosystem behavior.

Water and Energy Transfer

A comprehensive solution to the general heat flux equation in soils has been developed and tested against field data at hourly, daily and seasonal scales. This solution allows calculation of short and long-term phase changes and transport of water through snowpacks, surface covers and soils. This solution is necessary when using ecosys to examine the effects of altered soil properties and management practices on soil temperature and water content, especially in temperate and boreal ecosystems. This work builds upon earlier work in which a solution to the heat transfer equation was used to reproduce diurnal temperature cycles in soil under different soil managements⁷. Water transfer in ecosys is fully coupled to infiltration and runoff through an adaptation of the Richard’s and Green-Ampt equations which has been tested on agricultural and forest soils. Detailed measurements of soil water content have been used to test the water transfer scheme in ecosys under different C_a. Tested algorithms and parameters for C and energy exchange from ecosys are being adapated for use in the terrestrial C component of the Canadian Global Coupled Carbon Climate Model.

Solute and Gas Transfer

All solutes, mineral and gaseous, undergo convective-dispersive transport through the simulated soil profile as determined by water transport and soil properties. Algorithms for solution pH, ion speciation and exchange , and precipitation-dissolution reactions (Al, Fe, Ca, Mg, Na, K, Cl, S, P, H and OH) have been incorporated into ecosys and coupled through pH and osmotic potential to the biological activity of roots and microbes. Algorithms for mineral ion speciation, exchange and transport have been tested with data from soil columns. These algorithms have been used in ecosys to study the dynamics of soil salinity, and of P fixation, runoff and leaching, and how these dynamics affect water and P uptake during plant growth.

Microbial Activity

An integrated set of hypotheses is included in ecosys in which the concurrent activity of several soil microbial populations is represented as a parallel set of substrate-microbe complexes that include the rhizosphere, plant and animal residues²⁵, and native organic matter. The activity of each microbial population in the model is driven by the energetics of the oxidation-reduction reactions that it conducts. These hypotheses have been tested with data recorded from several incubation studies in which different soil types were amended with different ¹⁴C an ¹⁵N-labelled substrates. These hypotheses allow ecosys to reproduce the dynamics of microbial activity, including mineralization-immobilization, and heterotrophic growth and decay, at different soil temperatures and water contents over time scales ranging from hours to years. An autotrophic nitrifier population is also represented in the model and tested against rates of NO₃^– formation and N2O evolution³¹. Simulated microbial activity is coupled to the transport and sequential reduction of O₂, NO₃^–, NO₂^– and N₂O during C oxidation and tested with data for N₂O fluxes recorded over soils with different fertility managements. The simulation of CH₄biochemistry in methanotrophic⁴⁰ and methanogenic³⁷ microbial populations has been tested under laboratory conditions, and has been tested with methane fluxes recorded under field conditions in the BOREAS project.

These tests are a necessary prerequisite for the use of ecosys in predicting trace gas exchange between soils and the atmosphere as part of global climate change studies. By coupling microbial activity to the exchange and transfer of C, O, N and P in aqueous and gaseous phases,ecosys can reproduce the effects of soil alterations, such as compaction, erosion or tillage, on the exchange of gaseous C and N with the atmosphere during microbial activity under natural or disturbed soil conditions This work has been extended to the modelling of land use effects on long-term changes in soil C