Prediction of the effects of management practices on discharge and mineral nitrogen yield from paddy fields under future climate using APEX-paddy model

This study is to evaluate the BMPs in the reduction of surface discharge and mineral nitrogen yield from paddy cultivation for three future time slices (e.g., the 2010s, 2040s, and 2070s) using APEX-Paddy (Agricultural Policy/Environmental eXtender-Paddy) model. The model was calibrated and validated for surface discharge and mineral nitrogen yield using 3-year monitoring data (2013–2015) from the conventional paddy management field (CMP-1). For surface discharge and mineral nitrogen yield estimates, the future projections of 29 GCMs (General Circulation Model) were bias-corrected and applied to the calibrated APEX-Paddy model. We investigated five specific management strategies related to paddy drainage outlet regulation and new fertilization methods, as the BMPs minimize the mineral nitrogen yield and surface discharges due to climate change. The modeling results indicated that the effects of BMPs would vary by future climate scenarios (i.e., RCP4.5, RCP8.5) and periods (i.e., the 2010s, 2040s, 2070s). It was generally expected that the surface discharge and mineral nitrogen yields would increase in the future. The combination of raising drainage outlets and soil test-based fertilization (DOR-STF) showed a substantial reduction in surface discharge in both scenarios (RCP4.5 and 8.5); the highest reduction rate was observed in the 2010s and was estimated at 21.9 % under RCP4.5. Soil test-based fertilization (STF) showed a substantial reduction in mineral nitrogen yield by 31.0 and 28.3 % during the 2010s under RCP8.5 and RCP4.5, respectively followed by DOR-STF, as compared to conventional management practice (CMP-1). However, the combination of drainage outlet raising, and fertilizer application before outlet weir installation (DOR-FABWI) management resulted in increased mineral nitrogen yield of up to 31.0 % under RCP4.5 and 36.7 % under RCP8.5. The study findings indicate that climate change will increase exports of mineral nitrogen from paddy fields. Nevertheless, appropriate BMPs may play a vital role in reducing the mineral nitrogen yields for the production of paddy rice in future climates, and these effects may vary according to future climate conditions.