长期石灰施用对稻田镉迁移的影响机制与模型预测

China faces great challenges in remediating and utilizing its rice paddies from cadmium (Cd) contamination. Liming material is cost-effective and practical and its application can potentially immobilize soil Cd and reduce Cd uptake by rice plants. However, the efficiency of such materials usage varied with application types, fields, and times. The conventional observations fall to determine the plus or minus effect of long-term liming application on rice uptake Cd. These situations postpone the widely application of such remediation measures. The methods to address this problem draw a lot of attention. It was against this background that pot trails and field investigations (instead of temporal samples with spatial samples) were conducted using common liming materials and contaminated rice paddies in southern China. The variations of parameters of soil pH, soil leaching, solid-solution partitioning of soil Cd, and plant uptake factors of Cd, and their interactions in response to liming application were determined. We sought to characterize the liming effects on Cd accumulation and transfer in the paddy ecosystem. Based on the methods of Monte Carlo simulation and uncertainty analysis, we developed a mass-balanced model for the fate and transfer of Cd in the paddy ecosystem. Then the accumulation and transfer of Cd in rice paddies in different scenarios was predicted, and the mechanisms by which liming lowers the phytoavailability of Cd to rice was systematically elucidated. Coupling with the trade-off analysis and mode simulation, we evaluated the sustainability of such measures application over a longer period of time, and optimized the strategies of risk management on Cd-contaminated rice paddies based on the characteristics of local agriculture production and soil environment. The research can provide scientific and technology supports for the risk management of paddy ecosystem in China from Cd contamination.

如何安全利用镉（Cd）污染稻田是我国面临的重大技术难题，石灰价格经济、操作简便，是应对该问题的有效途径。然而石灰对污染稻田的调控过程周期长，影响因素繁杂，常规监测难以准确量化其在稻田长期施用的正负效应，这成为制约该措施推广应用的主要瓶颈，也是当前亟待解决的关键问题。本项目拟以南方典型Cd污染稻田为对象，通过实验室模拟和“空间换时间”的区域调查，研究长期石灰施用对土壤pH、渗透性、Cd固液分配系数及水稻Cd吸收的影响及其交互作用，解析石灰对稻田系统Cd迁移归趋的影响机制；结合蒙特卡罗模拟和不确定性分析方法，构建稻田Cd累积和迁移过程预测模型，模拟分析不同石灰应用模式下稻田Cd迁移变化趋势，从土壤-水-植物方面多角度揭示石灰调控稻田Cd污染风险的作用机制；统筹考虑区域农业生产和土壤特征，应用权衡分析手段，评估长期石灰施用的可持续性和潜在风险并进行策略优化，为我国稻田Cd污染防治提供科学依据。

我国南方稻田镉（Cd）污染形势严峻，严重威胁农产品安全和人体健康。石灰来源广、价格经济、操作简单，是降低土壤Cd活性和抑制农作物Cd累积的有效手段。然而受材料种类、施用时间和土壤条件等因素影响，石灰在污染稻田的应用效果存在很大的不确定性，这成为制约该修复措施标准化和大面积推广的主要瓶颈。本研究针对这一核心科学问题，以南方典型Cd污染稻田为研究对象，以模型模拟为核心，耦合实验室模拟、区域观测和多情景模拟结果，多尺度多方法多角度揭示石灰修复污染稻田的内在作用机制，量化不同石灰应用模式在污染稻田长期施用的可持续性和潜在风险，形成区域污染稻田石灰应用优化策略。研究发现：（i）中等用量石灰可以显著提升土壤pH，随着可变电荷的增加，土壤对Cd2+的吸附强度超过了Ca2+竞争吸附的影响，进而降低稻米Cd富集；（ii）高用量石灰可以显著提升土壤pH，降低土壤Cd有效态含量，但加剧了区域土壤Mn流失，反而促进了稻米Cd吸收；（iii）随机模型模拟显示区域稻田土壤Cd累积趋势短时间内难以逆转，石灰对于稻田土壤Cd累积的影响较小（iv）秸秆移除措施有助于降低土壤Cd累积量；（v）场景模拟结果显示“石灰施用-秸秆去除-灌溉水清洁”的稻田镉污染综合调控措施连续施用30年可确保区域50%的稻田土壤镉含量下降到国家安全标准范围内；（vi）权衡模型模拟显示土壤pH和无定型Mn变化存在动态交互关系，6.0和132 mg kg-1可分别作为区域土壤pH和无定型Mn的调控标准；（vii）区域大田示范结果表明2.25-4 t ha-1石灰和0.45-0.75 t ha-1锰肥的复合施用可降低57.3%的稻米镉含量且没有影响稻米产量。本研究为我国农田重金属污染防治和风险管控工作提供了科学支撑。