地下水位升降对菜地可溶性氮素累积与转化的影响研究

The groundwater level rises sharply with the concentrated rainfall in summer in the North China Plain, and declines after the rainy season. Groundwater fluctuation has significant effects on the transport and transformation of nitrogen along soil profile, especially in vegetable fields with high inputs of fertilizer and organic manure. Large amounts of nitrate and soluble organic matters can be leached to the saturated layer in the deeper soil by the rainfall, which promotes the occurrence of denitrification and reduces the risk of nitrate pollution in groundwater. The project intends to monitor the spatial and temporal dynamic characteristics of DTN, NH4+-N, NO3--N, DON, DOC and soil moisture in soil profile, leaching solution and groundwater, under the control of groundwater fluctuations and fertilization treatment both in situ test and outdoor simulated soil column. The abundance of functional genes for denitrification and the denitrification rates were quantitatively analyzed using Robotized Incubation System and qPCR technology, respectively. The purpose is to identify the characteristics of the accumulation and leaching of soil soluble nitrogen under fluctuating groundwater levels, clarify the effects of soil moisture and DOC on nitrogen transformation and transport in the profile, and unfold the mechanism for nitrate nitrogen removal by denitrification. These results will provide theoretical basis for water, carbon and nitrogen management in vegetable fields and methods for controlling non-point source pollution in farmland.

华北平原夏季集中降雨导致浅层地下水位急剧上升，雨季后又显著下降。地下水位升降过程对剖面土壤氮的运移和转化产生显著影响，特别是在有机肥和化肥用量均很高的露地菜田。大量硝酸盐和可溶性有机物淋洗至水位升高后的土壤水分饱和区后，促进反硝化作用并降低地下水中硝酸盐污染的风险。本项目拟采用甘蓝-大白菜田间原位试验和室外模拟土柱试验相结合的方法，研究不同施肥处理和地下水位升降幅度下，土壤剖面、淋溶液以及地下水中可溶性氮素、DOC和土壤水分时空动态特征；利用qPCR技术和自动培养监测系统，定量分析剖面下部土壤水分饱和区反硝化功能基因的丰度及表达和反硝化速率。目的是摸清地下水位升降下可溶性氮素在剖面累积和淋失规律，明确反硝化作用对硝态氮的去除效果及机理，阐明土壤水分、DOC对剖面氮素运移和转化的影响，揭示地下水升降对菜地地下水污染的影响机制。该研究结果为加强菜地水碳氮管理，控制农田面源污染提供理论依据。

以地下水埋深较浅的甘蓝白菜露地菜田为研究对象，通过田间试验及土柱模拟研究了地下水位升降对剖面土壤可溶性氮素淋失、累积、转化等动态变化及关键影响因子。明确了常规施肥模式下，全年可溶性氮素淋失量占氮投入量的27.6%，夏季休闲期占全年氮素淋失量的28.97%。地下水埋深上升过程中可溶性氮淋失以硝态氮为主，其次是铵态氮和可溶性有机氮；水位波动对硝态氮和铵态氮的淋失、剖面累积的效应不同；上层水位波动带（近根区）是各形态可溶性氮素发生转化的关键区域，对硝态氮去除效应显著。根据可溶性氮素淋失过程中土壤剖面DOC和反硝化功能基因的变化，利用随机森林模型明确DOC显著影响硝态氮淋失、反硝化基因nirS和nirK显著影响铵态氮。研发模拟地下水位波动对土壤氮素损失影响研究的土柱及监测方法，从水位上升幅度、高水位停留时间以及比较水位上升和下降等波动过程分析可溶性氮素淋失的影响效应，明确水位波动、高水位停留时间显著促进可溶性氮素淋失；水位下降相对于水位上升过程，剖面淋溶液可溶性氮素淋失风险更大。开展填闲作物对地下水位消长引起氮素淋失的阻控效果研究，从填闲作物氮素带走量、剖面无机氮和DOC含量、反硝化功能基因等因素明确其阻控机制。本研究对定量评价农田氮素淋失、提高我国农田面源污染氮淋溶估算与预测水平提供数据支持，并将对减少土壤残留氮素淋失、降低地下水污染风险和有效进行农业面源污染防控提供理论依据。