Analysis of the spatial variation in the net ecosystem production of rice paddy fields using the diagnostic biosphere model, BEAMS

To realistically understand spatial and temporal variations in the net ecosystem production (NEP) of rice paddies in Japan, we enhanced the existing diagnostic-type biosphere model with the addition of agricultural processes. Validated with a comparison of seasonal and annual variations over 5 years, the NEP estimations bore good agreement with the measurements. In regional-scale analyses, we targeted all rice paddy fields in Japan from January 2001 to December 2009. Before and after the model enhancement, the NEP showed very different spatial variations and absolute values, suggesting that the most effective improvements were the introduction of harvest and soil oxidation–reduction processes. In annual anomaly analyses covering a 9-year period, the NEP was found to be negative in 2003 and 2006 (−0.11 and −0.25 TgC/year) and positive in 2004 and 2007 (+0.02 and +0.21 TgC/year). We found that the negative values were caused by decreases in solar radiation and air temperature and the positive values were caused by increases in these two parameters and precipitation. In response to the NEP, our obtained harvested grain biomass values in 2003 and 2006 were lower than usual. The results were in good agreement with the results from reports of the Japan Meteorological Agency and the Ministry of Agriculture, Forestry, and Fisheries.