Modelling a global biogeochemical nitrogen cycle in terrestrial ecosystems

An integrated global model developed mechanistically based on nitrogen transformation processes and nitrogen fluxes between the terrestrial biosphere and the atmosphere is described in this paper. This model was designed in conjunction with our previous global carbon model on the compartments of vegetation and organic-soil, a third compartment, inorganic-soil, comprising ammonium and nitrate was also incorporated. We divided the global terrestrial biosphere into 60156 grid cells, each cell being 0.5° latitude by 0.5° longitude in size, whereas we simplified the atmosphere by regarding it as a well-mixed reservoir. Each grid cell was fixed as one of the five types of ecosystems: tropical forest, temperate forest, boreal forest, crops/grassland, or no vegetation (desert, ice). Geographical information system (GIS) data on climate, soils, and vegetation for each grid cell was used to drive the model in monthly time steps. Almost all of the key processes governing nitrogen cycling in the natural ecosystem were defined based on their respective mechanisms and specific relationships to environmental factors, then integrated into a highly aggregated dynamic model. Parameters required by the model were derived from published information or determined by model calibrations such as curve fitting. Our model-based estimate of potential nitrogen storage in vegetation was 16 Pg, which is of similar magnitude to estimates found in other studies (e.g. 10, 11–14 and 12–15 Pg), while nitrogen storage in global soil was 280 Pg according to our model, which also shows good agreement with previously reported values, which range from 70 to 820 Pg, with intermediate estimates of 170, 175, 300 and 760 Pg. Of the 280 Pg of nitrogen in soil, 25 Pg was in inorganic forms such as ammonium and nitrate, and 255 Pg was in organic forms such as detritus and humus. Thus, more than 90% of the soil nitrogen was present in organic forms, a finding that agrees very well with other fields research. A sensitivity analysis on the uncertainties of the initial conditions of the model and the parameter values determined by model calibrations was also performed. Although additional work on model validation is still necessary, this modelʹs ability to simulate the nitrogen cycle in the natural ecosystem and to quantitatively estimate potential nitrogen storage in global vegetation and soil has been documented. We therefore can conclude at this stage that the basic framework of a global biogeochemical nitrogen cycle, which can be used as a tool for quantitative evaluation of anthropogenic disturbances, has been developed.