Impact of soil nitrogen saturation rate on nitrogen and carbon fluxes in Japan

Recently the nitrogen deposition has been increasing since industrial revolution in Japan. As a result of the soil nitrogen saturation, some existing studies reported that the nitrogen exceeding the holding facility of ecosystem flowed out to streams. And then, although nitrogen is the key nutrient for the ecosystem growth, the excessive nitrogen input induces the forest decline and eutrophication as negative factor. Understanding the effect of the increased nitrogen input on terrestrial ecosystem production, we need comprehending a rate of the progression of nitrogen saturation in regional scale. However, the existing studies about the soil nitrogen saturation rate in Japan is not enough to evaluate the relationship with climate change. Here, we tried to analyze the effect of the increasing deposition on the terrestrial ecosystem via the soil nitrogen saturation rate in Japan. We developed a new nitrogen cycle submodel and focused on four processes as a key nitrogen process; 1) using of nitrogen deposition estimated by a chemical transport model as the model input; 2) calculation of nitrogen demands by vegetation and microbes; 3) calculation of retranslocation by using LAI and optimal leaf nitrogen content at maximal photosynthesis; 4) calculation of immobilization from both ammonium and nitrate. And the nitrogen submodel was integrated into the existing biosphere model, Biosphere model integrating Eco-physiological and mechanistic approaches using Satellite data, BEAMS (Sasai et al., 2017). The model validation at multiple flux tower sites showed that the carbon to nitrogen ratio in leaf, soil nitrogen pools and fluxes, carbon fluxes (GPP, NEP), and net radiation flux had good agreements with measurements. For regional simulation in Japan, we analyzed spatial and temporal patterns in terrestrial carbon exchange derived from the soil nitrogen saturation rate. We found that the soil nitrogen saturation rate exceeded 70% in most Japanese ecosystem. The cropland and urban area were over 90% because of fertilization in cropland and the low nitrogen demands by vegetation in urban. We conclude that understanding the spatial and temporal change of the soil nitrogen saturation rate provides important information relevant to mitigation for climate change and ecosystem conservation. As future plan, we need to improve the hydrological processes, which plays an important role on the soil nitrogen dynamics because of the abundant water resources.