Abstract
Most evapotranspiration over land occurs through vegetation. The fraction of net radiation balanced by evapotranspiration depends on stomatal controls. Stomates transpire water for the leaf to assimilate carbon, depending on the canopy carbon demand, and on root uptake, if it is limiting. Canopy carbon demand in turn depends on the balancing between visible photon-driven and enzyme-driven steps in the leaf carbon physiology. The enzymedriven component is here represented by a Rubisco-related nitrogen reservoir that interacts with plant‐soil nitrogen cycling and other components of a climate model. Previous canopy carbon models included in GCMs have assumed either fixed leaf nitrogen, that is, prescribed photosynthetic capacities, or an optimization between leaf nitrogen and light levels so that in either case stomatal conductance varied only with light levels and temperature. A nitrogen model is coupled to a previously derived but here modified carbon model and includes, besides the enzyme reservoir, additional plant stores for leaf structure and roots. It also includes organic and mineral reservoirs in the soil; the latter are generated, exchanged, and lost by biological fixation, deposition and fertilization, mineralization, nitrification, root uptake, denitrification, and leaching. The root nutrient uptake model is a novel and simple, but rigorous, treatment of soil transport and root physiological uptake. The other soil components are largely derived from previously published parameterizations and global budget constraints. The feasibility of applying the derived biogeochemical cycling model to climate model calculations of evapotranspiration is demonstrated through its incorporation in the Biosphere‐Atmosphere Transfer Scheme land model and a 17-yr Atmospheric Model Inter comparison Project II integration with the NCAR CCM3 GCM. The derived global budgets show land net primary production (NPP), fine root carbon, and various aspects of the nitrogen cycling are reasonably consistent with past studies. Time series for monthly statistics averaged over model grid points for the Amazon evergreen forest and lower Colorado basin demonstrate the coupled interannual variability of modeled precipitation, evapotranspiration, NPP, and canopy Rubisco enzymes.
Highlights
The balancing of absorbed solar energy by evapotranspiration (ET) is a major determinant of land surface temperature and other features of climate
N does not occur in fixed amounts in any given leaf but rather responds to overall plant–soil nitrogen cycling processes
It can be argued that neither plant photosynthetic capacities nor stomatal function parameters should be specified empirically in a climate model but rather both should be jointly derived with N-cycling processes as they interact with the climate system and soil processes
Summary
The more recent formulation in terms of leaf carbon assimilation (as reviewed by Sellers et al 1997) provides an interface for calculation of carbon budgets and dependences on the concentrations of atmospheric carbon dioxide, but otherwise can be viewed as an improved version of the same model with a single basic parameter required to be specified, since a constant Vmax is functionally equivalent to the earlier specified minimum stomatal resistances, and either set of parameters can be given as a table for the different model land covers Measurements of both sets of parameters have shown substantial variation even for the same plant species. Our analysis has not attempted to provide further evidence for that assertion but rather to show that given the importance of the stomatal functioning for climate models, inclusion of nitrogen controls makes a quantitative difference and improves the realism of the treatment
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