Description, calibration and sensitivity analysis of the local ecosystem submodel of a global model of carbon and nitrogen cycling and the water balance in the terrestrial biosphere

Abstract

We have developed a geographically-distributed ecosystem model for the carbon, nitrogen, and water dynamics of the terrestrial biosphere TERRA. The local ecosystem model of TERRA consists of coupled, modified versions of TEM and DAYTRANS. The ecosystem model in each grid cell calculates water fluxes of evaporation, transpiration, and runoff; carbon fluxes of gross primary productivity, litterfall, and plant and sou* respiration; and nitrogen fluxes of vegetation uptake, litterfall, mineralization, immobilization, and system loss. The state variables are soil water content; carbon in live vegetation; carbon in soil; nitrogen in live vegetation; organic nitrogen in soil and litter; available inorganic nitrogen aggregating nitrites, nitrates, and ammonia; and a variable for allocation. Carbon and nitrogen dynamics are calibrated to specific sites in 17 vegetation types. Eight parameters are determined during calibration for each of the 17 vegetation types. At calibration, the annual average values of carbon in vegetationC v show site differences that derive from the vegetation-type specific parameters and intersite variation in climate and soils. From calibration, we recover the average C v of forests, woodlands, savannas, grasslands, shrublands, and tundra that were used to develop the model initially. The timing of the phases of the annual variation is driven by temperature and light in the high latitude and moist temperate zones. The dry temperate zones are driven by temperature, precipitation, and light In the tropics, precipitation is the key variable in annual variation. The seasonal responses are even more clearly demonstrated in net primary production and show the same controlling factors.We have found the sensitivities of the total ecosystem, total carbon storage, and net primary production to changes in model parameters. With only a few exceptions, the systems are ultra sensitive to the parameters controlling the effect of soil moisture on soil decomposition and soil respiration from the tundra to the tropics. The calibration parameters are important in all 17 vegetation types in determining the total system sensitivity; of these, usually Kd (parameter for soil respiration) is the most important and K r (parameter for plant respiration), the least The most common ordering in total system sensitivity for the eight calibration parameters is K& Nio SS (loss of nitrogen from soil), Kf a u (rate of carbon transfer by litterfall), Nmax (rate of nitrogen uptake by vegetation), Cmax (gross primary productivity), Lnc (nitrogen transfer by litterfall), N up (immobilization of nitrogen by bacteria in litter), and K r , from the most sensitive to the least. This suggests that immobilization and plant respiration are less important on a relative basis for the total system, and soil respiration and nitrogen losses from soils are the most important processes. The parameter that controls the respiration response to temperature <2io is a key parameter in total system sensitivity, total carbon sequestration and net...