Testing the Suitability of the DNDC Model for Simulating Long-Term Soil Organic Carbon Dynamics in Japanese Paddy Soils

The suitability of the DeNitrification-DeComposition (DNDC) model for simulating long-term changes in the content of soil organic carbon (SOC) was validated using 5 sites for long-term experiments related to Japanese paddy soils. Since the model could not simulate crop growth adequately, several crop growth parameters provided by the model as default were changed to adjust crop growth to the observation. Overall, the changes in the content of SOC with time simulated by DNDC using adjusted crop parameters, agreed well with the observation in 9 plots from 5 experimental sites during the 16 to 22-year period of the experiment. The good performance of the decomposition sub-model in the DNDC was verified in the long-term SOC decomposition in paddy soils as well as in upland soils reported in previous studies. However, the simulated SOC did not agree well with the observation in some plots, especially in soils with a very low SOC content, suggesting that care should be exercised when applying the model to soils with a very low SOC content. Moreover, careful tuning of crop growth parameters should be promoted for better simulation, and detailed information about farm management required for input parameters is often difficult to obtain, especially in long-term experiments. In conclusion, the DNDC model is an effective tool for simulating long-term SOC dynamics in paddy soils. The unique kinetic scheme "anaerobic balloon" in the model may play an important role in successful simulation of SOC dynamics in paddy soils that are water-logged during the rice cropping period. This scheme may be helpful for modifying the other turnover models of soil organic matter for use for paddy soils, too.