Effects of layered soil on the accumulation and leaching of nitrate-nitrogen in shallow groundwater regions

Excessive nitrogen application in agriculture causes the accumulation of nitrate-nitrogen (NO₃⁻-N ) in the vadose zone and intensifies nitrate pollution in groundwater. Heterogeneous layered soil is relatively common in nature and plays an important role in controlling pollutants entering groundwater from the surface. Heterogeneous layered soil exists in the low plain area of North China which is sensitive to groundwater pollution owing to its shallow groundwater burial and short nitrate leaching path. Thus, it is important to clarify the influence of the heterogeneous layered soil structure on the NO₃⁻-N migration process to prevent nitrate pollution in groundwater. In this study, four typical soil profiles, heterogeneous and relatively homogeneous, and two land use types were selected in Nanpi County, Hebei Province. The four typical soil profiles included three heterogeneous layered soils (P1, P2, P3), one relatively homogeneous profile (P4), and two land uses, which were unfertilized grassland with multiple 30 cm thin clay soil interlayers (P1), fertilizing farmland with multiple 30 cm thin clay soil interlayers (P2), fertilizing farmland with 140 cm thick clay soil interlayers (P3), and fertilizing farmland with relatively homogeneous silty loam (P4). The effect of layered soil on the accumulation and leaching of NO₃⁻-N was studied by analyzing the relationship between the physical and chemical properties of different layered soil profiles and NO₃⁻-N content in soil profiles and groundwater. The results showed that the vertical distribution of NO₃⁻-N was affected by the depth and thickness of the clay loam soil layer. The NO₃⁻-N contents in the three heterogeneous layered soil profiles were higher than that in the homogeneous profile with silt loam. In the three heterogeneous layered soil profiles, P3 with a 140-cm clay soil interlayer, its’ peak content of NO₃⁻-N (238 mg·L^–1) and the accumulation layer thickness (100–250 cm) were the highest. In the rainy season of 2018 (from August to September), the leaching amounts of NO₃⁻-N in the heterogeneous profiles were P3 (319.2 kg·hm^–2) < P1 (383.9 kg·hm^–2) < P2 (554.7 kg·hm^–2), which indicated that the control effect of the layered soil profile with a thick clay loam interlayer on NO₃⁻-N leaching was significantly better than that with multiple thin clay loam interlayers (P<0.05). NO₃⁻-N in shallow groundwater was affected by the soil deposition structure of the aquifer. The over-limit ratio and average increasing rate under a heterogeneous deposition profile with clay loam interlayers (P2, 93% and 2.14 mg·L^–1·d^–1) were significantly higher than those of the homogeneous deposition profile with silt loam (P4, 21% and 0.53 mg·L^–1·d^–1). This study verified that layered soil profiles have a blocking effect on soil water and NO₃⁻-N migration; and thicker the clay loam interlayer, the stronger is the blocking effect of soil water and NO₃⁻-N migration. The interaction of soil water and solution between the vadose zone and groundwater is frequent in shallow groundwater regions; thus, the NO₃⁻-N concentration in groundwater is controlled by the structure of the layered soil in the vadose zone and the depth of groundwater. These results provide a scientific basis for the prevention and control of nitrate pollution in shallow groundwater regions.