层状包气带黏土层厚度对硝态氮迁移的影响

层状包气带结构中黏土层对污染物进入地下水具有阻滞作用,黏土层的厚度对硝态氮(NO_3~--N)在包气带迁移中的淋失、累积以及反硝化作用等具有非常重要的影响,而目前关于这方面的研究还不足。该研究通过设置高度为40 cm、砂土与黏土层厚度比分别为3∶1,1∶1,1∶3的"上粗下细"型以及全黏土型的4组填充土柱,采用稳定浓度的定水头淋滤试验,研究黏土层厚度不同的土柱NO_3~--N溶液入渗过程、土壤NO_3~--N淋滤、累积和反硝化特征,进而阐明层状包气带黏土层厚度对NO_3~--N迁移的影响。结果表明:湿润锋运移深度和累积入渗量与入渗时间的关系在溶液穿越砂黏土层界面前后由非线性趋于线性,累积入渗量随黏土层厚度增加而显著减小(P0.05);当土柱内黏土层厚度达到40 cm时,其对NO_3~--N淋滤的阻滞作用明显强于黏土层厚度为10～30 cm的土柱;淋滤试验过程中在砂黏土层界面形成水分滞留层,界面处黏土层中NO_3~--N和NO_2~--N累积量均达到峰值,且随着深度的增加,NO_3~--N和NO_2~--N累积量降低;黏土层厚度差不小于20 cm的土柱内NO_3~--N累积量差异显著(P0.05),而40 cm黏土层的土柱反硝化量(0.15±0.05) g]显著高于黏土层厚度为10～30 cm的土柱(P0.05),说明当黏土层达到一定厚度时(如40 cm),对NO_3~--N的阻滞作用和反硝化作用具有显著影响,对防止NO_3~--N淋失进入地下水产生重要作用。该研究可为层状包气带土壤条件下农田施肥管理与地下水保护提供科学依据。

Effect of the thickness of clay layer in the layered vadose zone on nitrate nitrogen migration

Abstract: The clay layer in the vadose zone with layered structure in soils has an important effect on blocking the pollutants which will enter into the groundwater. At present, there is insufficient knowledge about the effects of the thickness of the clay layer on the migration process of nitrate nitrogen (NO3--N). Nitrate nitrogen is more prone to leaching through the ''up thick and down thin'' ( the up layer of soil with larger particles and the down layer of soil with smaller particles) vadose zone with layered structure, which can cause a higher risk of nitrate nitrogen pollution to groundwater. In this study, we set up four groups soil columns with 40 cm height, the types of four groups soil columns included "up sand and down clay" and full filling with clay. The ratio of thickness (between sand and clay layer) was 3:1, 1:1, and 1:3, and the last type soil column had 40 cm clay. The content of the study in the leaching experiment under the condition of stable water head and stable concentration of nitrate nitrogen with infiltration, which included infiltration process of nitrate nitrogen solution, the characteristics of accumulation and denitrification of nitrate nitrogen in soil columns. In order to clarify the effects of the clay layer thickness on the migration process of nitrate nitrogen in the layered vadose zone, the soil columns with different thickness of clay layer. The results showed that the infiltration process of solution could be divided into two stages, namely, the relationship between the cumulative infiltration capacity with the wetting front transport depth and the infiltration time gradually changed from nonlinear to linear when the wetting front crossing the sand-clay layer interface. Meanwhile, the cumulative infiltration capacity decreased significantly with the increasing in the thickness of clay layer in the soil columns (P<0.05). When the thickness of the clay layer in the soil column reached 40 cm, the effect of blocking on the nitrate leaching of nitrogen was significantly stronger than other soil column which was with the clay layer of 10-30 cm. During the leaching experiment, a water retention layer was formed at the interface of the sand-clay layer, which made the cumulative values of nitrate nitrogen and nitrite nitrogen in the clay layer at the interface were both large, while the accumulation of nitrate nitrogen and nitrous nitrogen decreased with the increasing of depth in clay layer in the same soil column. When the difference of thickness in clay layer was not less than 20 cm during different soil columns, the accumulation of nitrate nitrogen in the soil column was significantly different (P<0.05). The soil column which thickness of clay layer was 40 cm had the largest amount of nitrate nitrogen used in denitrification (0.15±0.05) g] was significantly higher than the soil column with 10-30 cm in the thickness of clay layer (P<0.05), which indicated that when the thickness of clay layer reached a certain degree (such as 40 cm), it not only blocked nitrate nitrogen, but also affected the amount of denitrification. Occurrence had a significant impact which could play an important role in preventing the leaching of nitrate nitrogen into groundwater. This study can provide a reference for the management of farmland fertilization and groundwater protection under soil with layered vadose zone conditions.