洱海近岸农田土壤氮形态转化对浅层地下水升降的响应

Soil in the nearshore farmland of Erhai Lake is the important intermediate place for nitrogen and other nutrients exchange between lake and land. The shallow groundwater fluctuation affects the nitrogen transformations of soil in the nearshore farmland. The transformation of nitrogen in soil changes the migration and accumulation of nitrogen in farmland soils, which will be serious threats to the water quality of Erhai Lake and groundwater of its surroundings. In this project, we will focus on the nearshore vegetable soil in Erhai Lake to investigate the changes of soil biochemical factors and nitrogen forms in the process of shallow groundwater fluctuation by the simulated tests and the situ monitoring of shallow groundwater fluctuation. Dynamic changes and distribution of soil nitrogen forms in the process of shallow groundwater fluctuation will be studied. The change characteristics of physicochemical parameters and microbial behaviors related to soil nitrogen transformation will be analyzed. The relationships between soil biochemical factors and nitrogen transformation will be unraveled. The action mechanism of the shallow groundwater fluctuation on soil nitrogen transformation will be revealed. The results will provide strong date support for the establishment of soil nitrogen transport model with shallow groundwater movement, and also provide solid theoretical basis for the accurate prediction of the water pollution caused by the nitrogen of farmland soil for Erhai Lake and its surroundings.

洱海近岸农田土壤是水陆间氮等营养物交换的重要中间场所，近岸农田浅层地下水的变化影响着土壤氮形态的转化，从而影响着氮素在农田土壤中的聚集和迁移，严重威胁着洱海周边地下水及洱海的水质安全。本项目拟以洱海近岸菜地土壤为研究对象，通过浅层地下水升降的室内模拟试验和野外监测试验，监测水位升降过程中土壤生化环境和氮形态的变化，研究土壤氮形态在浅层地下水升降过程中的动态变化及分布规律，分析浅层地下水升降过程中与土壤氮转化相关的理化指标及微生物行为的变化特征，阐明生化环境变化与土壤氮形态转化的关系，揭示浅层地下水升降对土壤氮转化的作用机理。本项研究可为农田土壤氮随浅层地下水迁移模型的建立提供强有力的数据支撑，而且也为农田土壤氮对洱海周边地下水及洱海水污染的准确预测提供一定的理论基础。

浅层地下水位波动影响着土壤剖面中含水率和氧化还原环境，显著改变了土壤剖面中的氮形态，加速了剖面土壤氮向浅层地下水中迁移。本项目通过原位监测和室内模拟试验相结合的方法，研究了浅层地下水位波动下农田土壤剖面和浅层地下水中氮形态的变化特征，揭示了剖面土壤生化环境对氮形态转化的影响机理，定量化确定了浅层地下水位升降下土壤剖面氮流失量。结果表明：水位波动下整个土壤剖面TDN和NO3—N表现出释放的趋势，而NH4+-N呈现出累积的趋势，DON在A层表现出释放趋势，C层和D层呈现出累积趋势。淹水初期的前12d，土壤剖面出水中TN、ON和NO3--N浓度迅速下降，而NH4+-N浓度快速上升，是土壤氮素流失的关键期。干湿交替显著增强了细菌丰度的变化，三组（NH4+-N、ω和NO3—N、pH、DON）和两组（ω和pH、NH4+-N、NO3—N和DON）因素分别显著影响着土壤剖面由干变湿和由湿变干下丰度前15的细菌门变化。浅层地下水位波动下A层和B层的氮转化功能基因丰度显著大于C层和D层，前期淹水降低了土壤剖面硝化功能基因amoA绝对丰度，增加了反硝化功能基因nir和nosZ丰度。两个关键的功能基因组nosZ/narG和nosZ/(nirK+nirS)直接驱动着NH4+-N和NO3—N的转化。持续淹水条件下1.7%的0-100 cm土壤氮库储量被流失，干湿交替是持续淹水条件土壤TN损失量的2.72倍，基于土壤氮流失率和土壤氮库储量，浅层地下水位波动下洱海湖周农田0-100 cm土壤氮损失强度为332.0（大田作物）和556.0（露地蔬菜）kg/hm2，2016年总的土壤氮素流失量为3506 Mg。若雨季蔬菜种植区出现短期的浅层地下水位波动，造成的总氮流失量将增加45%，若蔬菜地改种大田作物，总氮流失量将减少16%。因此，种植结构调整和浅层地下水位的合理管控是减少农田土壤氮素流失的关键。