高铁高锰地下水灌溉对稻田氮素迁移转化的影响及机理

The Sanjiang Plain is one of the key commercial grain production bases in China. The uplands in Sanjiang Plain are changing into paddy fields on a large scale. Consequently, this change is resulting in the aggravation of agricultural non-point source pollution, which threatens the water quality of the boundary lakes and rivers. The paddy field is mainly irrigated by the ground water with relatively high concentrations of Fe and Mn, that probably exerts both an adverse influence on the utilization rate of N fertilizer and accelerate of N loss due to the coupling relationship between Fe, Mn and N in the biogeochemical process in paddy field with complicated oxidation-reduction environment, as they are all element with changable valence state. The current study will be conducted based on both the plot and simulated experiments using 15N isotopic tracer technique, at the national ecosystem research network station. The study aims to reveal the effects of exogenous input of Fe and Mn on the N fertilizer utilization and non-point source pollution load, and to reveal the key process related to micro-environment of rice rhizosphere, soil microorganism community structure, and nitrification-denitrification process as that react to high concentrations of Fe and Mn. This research is expected to reveal the influences of ground water irrigation with high concentrations of Fe and Mn on utilization of N fertilizer and on non-point source N output load and forms, and to clarify the mechanism of the stress of Fe and Mn on N utilization and loss. With this required information, we hope to propose optimal field practices to relieve the stress of Fe and Mn. Moreover, the achievements of the current study can provide theoretical foundation and technological guide for irrigation management and non-point source pollution control of paddy field in the Sanjiang Plain.

三江平原是我国重要的优质商品粮生产基地，大规模水田化使农业面源污染加剧，存在影响界江界湖水质安全的风险。三江平原稻田灌溉以抽取地下水为主，该区域地下水中可溶性铁、锰含量较高，铁、锰、氮均为变价元素，而稻田是一个复杂的氧化还原系统，铁、锰、氮在稻田中存在复杂的生物地球化学过程，因此，高铁高锰地下水灌溉可能会影响氮素利用率、促进氮流失。本研究依托国家生态系统研究网络台站，采用小区实验与模拟实验相结合的方法，借助15N 同位素示踪技术，研究外源铁锰输入对氮素利用及氮输出的影响，揭示水稻根际微环境、土壤微生物群落结构及硝化反硝化过程对铁锰胁迫的响应机制。项目预期将确定采用高铁高锰地下水灌溉后稻田氮素利用率及面源污染氮输出形态和负荷的变化程度，阐明"铁锰氮耦合"对氮素迁移转化影响的机理与关键过程，提出缓解铁锰胁迫的田间调控措施。研究成果将为三江平原稻田灌溉水管理及面源污染防控提供理论依据和技术支撑。

三江平原地下水中可溶性总铁总锰浓度较高，采用含高铁高锰地下水灌溉将影响氮素在水田生态系统中的迁移转化特征。项目研究发现，当施肥量较低时，外源铁锰输入对田面水中氮浓度影响不显著，但当施肥量较高时（如拔节肥），外源铁锰输入将在施肥后短期内（24h）显著影响田面水中各形态氮浓度：外源铁单一胁迫将提高田面水中硝氮和总氮浓度，增加面源污染输出负荷；外源锰单一胁迫则降低硝氮和总氮浓度；当二者联合胁迫时其影响程度及阈值因两种离子浓度变化而异。水稻种植期结束后，在不同浓度铁锰处理下的稻田土壤中铵态氮含量不存在显著差异，但在Fe(II)浓度为25 mg/L条件下，土壤硝态氮含量在不同Mn(II)浓度作用下存在显著性差异（P<0.05），说明土壤中铁锰氮三元素之间存在复杂的耦合作用关系。水稻生长指标如分蘖数和单株产量也在一定程度上受到外源铁锰的影响，但多数处理组但未达到统计学差异；水稻不同器官对铁、锰、氮的吸收量对铁锰单一及联合胁迫的响应因其浓度不同差异较大。外源铁单一作用下，水稻根际土壤微生物群落中各菌落含量及真菌与细菌比没有发生显著变化；而在外源锰作用下，革兰氏阳性菌、革兰氏阴性菌、需氧菌、硫酸盐还原菌和放线菌的含量均呈现中浓度（2.5和5.0mg/L）处理组高于对照和高浓度处理组的变化规律。通过添加适当比例生石灰可提高外源铁锰作用下水田土壤表层水的pH，进而调节田面水中硝氮和总氮浓度，较少面源污染输出风险。 研究成果有助于综合评价水田系统“铁锰氮耦合过程”的农学和环境学意义，有助于深化理解三江平原土壤氮素生物地球化学过程。