Modelling the role of algae in rice crop nutrition and soil organic carbon maintenance

Photosynthetic aquatic biomass (PAB - algae and other floodwater flora) is a significant source of organic carbon (C) in rice-based cropping systems. A portion of PAB is capable of fixing nitrogen (N), and is hence also a source of N for crop nutrition. To account for this phenomenon in long term simulation studies of rice-based cropping systems, the APSIM modelling framework was modified to include new descriptions of biological and chemical processes responsible for loss and gain of C and N in rice floodwater. We used well-tested algorithms from CERES-Rice, together with new conceptualizations for algal dynamics, in modelling the contribution of PAB to maintenance of soil organic C and soil N-supplying capacity in ricebased cropping systems. We demonstrate how our new conceptualization of PAB growth, turnover, and soil incorporation in flooded rice systems facilitates successful simulation of long-term soil fertility trials, such as the IRRI Long Term Continuous Cropping Experiment (35+ years), from the perspectives of both soil organic carbon levels and yield maintenance. Previous models have been unable to account for the observed maintenance of soil organic C in these systems, primarily due to ignoring inputs from PAB as a source of C. The performance of long-term rice cropping system simulations, with and without inclusion of t hese inputs, is shown to be radically different. Details of our modifications to APSIM are presented, together with evidence that the model is now a useful tool to investigate sustainability issues associated with management change in rice-based cropping systems.

光合水生生物量（Photosynthetic Aquatic Biomass，简称PAB，即藻类与其他淹水植被）是稻作系统中有机碳（C）的重要来源。部分PAB具备固氮能力，因此亦可作为作物养分的氮素（N）来源。为在稻作系统的长期模拟研究中纳入该现象，研究人员对APSIM建模框架进行了修改，新增了稻田淹水层中碳、氮增减相关生物与化学过程的描述。本研究采用经充分验证的CERES-Rice算法，并结合藻类动力学的全新概念框架，以模拟PAB对稻作系统土壤有机碳维持及土壤供氮能力的贡献。本研究阐明了针对淹水稻作系统中PAB的生长、周转及土壤归还过程所构建的全新概念框架，如何从土壤有机碳水平与产量维持两个维度，成功模拟长期土壤肥力试验——如国际水稻研究所（International Rice Research Institute，简称IRRI）的长期连作试验（已开展35年以上）——的结果。此前的模型无法复现观测到的此类系统中土壤有机碳维持现象，主要原因在于未将PAB作为碳源的输入纳入考量。对比纳入与未纳入此类碳输入的长期稻作系统模拟结果，二者的模拟表现存在根本性差异。本文详述了针对APSIM模型的修改细节，并提供证据表明，修改后的模型现已成为探究稻作系统管理变革相关可持续性问题的有效工具。