CAPABILITY OF APSIM-ORYZA TO SIMULATE LOWLAND RICE-BASED FARMING SYSTEMS UNDER NITROGEN TREATMENTS IN A TROPICAL CLIMATE Ahmad Suriadi 1,4 , Donald Gaydon 2 , Yahya Abawi 3 , and Rabi Misra 1 1) Faculty of Engineering and Surveying, The University of Southern Queensland, Toowoomba, QLD 4350, Australia, Suriadi@usq.edu.au 2) CSIRO Sustainable Ecosystem, St Lucia QLD 4068, Australia 3) Queensland Climate Change Centre of Excellence, Department of Environment and Resource Management, Toowoomba, QLD 4350, Australia 4) Assessment Institute for Agricultural Technology (BPTP) PO Box:1017 NTB 83010, Indonesia INTRODUCTION Rice is the most important crop in Asia and the staple food for most of the world’s population. Due to the overwhelming importance of this crop, modelling rice-based farming systems will provide valuable help to compare experimental research findings across regions, extrapolate field experimental data to wider environments, develop management recommendations and decision-support systems, explore effects of climate change and adaptation options, and prediction of crop yield. There is an increasing demand for the capability to simulate rice-based cropping systems, especially in Asia. Such a system capability will allow expanded investigation of nitrogen dynamics, crop sequencing, intercropping, crop residue management and soil and water management. Incorporation of the ORYZA2000 rice model (Bouman and van Laar, 2006) into APSIM (Agricultural Production Systems Simulator (APSIM-Oryza) together with recent work on carbon and nitrogen dynamics in transitional flooded/non-flooded systems (Gaydon et al., 2009) has facilitated long-term simulation of lowland rice-based farming systems scenarios. However, the capability of APSIM-Oryza to simulate rice-based crop sequences involving other crops has undergone limited testing to this point and under a variety of crop management practices and cropping systems. In this paper, we detail testing of the APSIM-Oryza simulation model against an experimental dataset involving lowland rice-rice-soybean crop rotation in West Nusa Tenggara Province (NTB) Indonesia. DESCRIPTION OF MODEL The APSIM farming systems simulation framework is described in detail by Keating et al. (2003), and the key processes of ORYZA2000 have been well documented (Bouman and van Laar, 2006). Simulation of the transitional (flooded-non-flooded) soil environment and pond processes within the APSIM framework has also been recently described (Gaydon et al., 2009). RESULTS AND DISCUSSION The performance of APSIM-ORYZA with and without nitrogen limitation was evaluated using rice- rice-soybean crop sequence data from a field experiment conducted at the Assessment Institute for Agricultural Technology (BPTP) NTB Indonesia in 2007-2008. Three rates of N fertiliser were applied to rice; 0 kg N ha − 1 (F0), 70 kg N ha -1 (F1) and 140 kg N ha − 1 (F2) and three rates for soybean; 0 kg N ha − 1 (S0), 12 kg N ha -1 (S1) and 24 kg N ha − 1 (S2). In general, the model satisfactorily simulated the dynamics of crop variables measured (phenological stages, yield and biomass), soil and water variables (ponded water depth, pH, temperature and daily infiltration rate). Simulated biomass matched the pattern of rice growth when nitrogen was not limiting factor (F2), with slight over-prediction under both F1 and F0 treatments for both the first (wet) and second (dry) rice seasons (Fig. 1). A similar pattern was also -267- Farming Systems Design 2009, Monterey, California, USA