Effects of rainfall and fertilization on soil nitrate nitrogen migration distribution around water−replenishing rivers

Soil nitrate-nitrogen migration around recharge rivers is affected by multiple factors, such as the hydrological condition of the rivers, rainfall, and fertilization. The study of soil nitrate-nitrogen distribution and leaching characteristics under the joint action of multiple factors can lay the foundation for an in-depth understanding of the soil nitrogen cycling process. In this study, we used the soils of a farmland and agroforestry composite area (AF) around the Baiyangdian River on the North China Plain as the research focus. The farmland area was set up with fertilization treatments such as no fertilization (CK), optimized fertilization (YH), and habitual fertilization (XG); we then comprehensively analyzed the distribution of the soil nitrate-nitrogen profile and leaching characteristics under the effect of different types of rainfall and ecological recharge, among other factors. The results showed that nitrogen application, rainfall type, and ecological water recharge were the key factors influencing nitrate-nitrogen migration around the recharged river. The amount of applied nitrogen determined the total amount of leached soil nitrate-nitrogen (P<0.01). With the increase in nitrogen application, the nitrate-nitrogen content and nitrate-nitrogen loss within the soil profile increased significantly. The total nitrate-nitrogen loss of YH treatment decreased by 40.6% compared with XG treatment, and the total nitrate-nitrogen loss in the AF treatment decreased by 40.1%. Rainfall and ecological recharge altered the migration of nitrate and nitrogen in the soil profile. Rainfall mainly affected the migration of nitrate-nitrogen from the surface to the middle and lower soil layers, and ecological recharge mainly affected the migration of nitrate-nitrogen from the middle and lower layers of the soil to the lower layers. In addition, the effects of different rainfall types on the migration of soil nitrate-nitrogen varied. The migration rate and amount of nitrate-nitrogen in the lower layer of the soil differed between two rainfall intensities. The peak of nitrate-nitrogen accumulation was in the 40–60 cm soil layer during both regular and heavy rainfall. Regular rainfall (3 d cumulative rainfall of 65.2 mm) resulted in an increase of 12.4 kg·hm⁻² (9.7%) in nitrate-nitrogen storage in the 40−60 cm soil layer with XG treatment (compared with the 0−40 cm layer), while heavy rainfall (5 h cumulative rainfall of 49.2 mm) resulted in an increase of 16.3 kg·hm⁻² (16.7%) in nitrate-nitrogen storage in the 40−60 cm soil layer with XG treatment. Ecological rehydration resulted in a reduction of nitrate-nitrogen storage by 42.7 kg·hm⁻² and 39.6 kg·hm⁻² in the 100−140 cm soil layer with the YH and XG treatments, respectively. These findings have important implications for managing soil nitrogen pollution in the context of ecological recharge and extreme rainfall.