Study of nitrogen transport and transformation mechanisms in structural profiles of different air envelopes

In this study, we investigated the nitrogen transport blocking effect of air inclusion zones containing silty clay interlayers in the alluvial plains of the middle and lower reaches of the Yellow River by means of soil column experiments. The three types of air inclusion zones were drenched by setting ammonium and nitrate nitrogen solutions at a concentration of 50 mg/L each. Three unsaturated zone configurations (silt sand, silt sand-fine sand, and silt sand-silty clay-fine sand) were tested to analyze ammonium and nitrate nitrogen transport under leaching conditions. Chloride tracer tests revealed distinct transport rates: fine sand (4–6 hours), silt sand (198 hours), and silty clay showing the slowest movement. These results confirmed the Yoon-Nelson model's reliability in assessing pore uniformity (R² > 0.97), All three soils have good pore space.Ammonium nitrogen was primarily adsorbed in surface layers (at 0 ~ 12.5cm). Clay interlayers with high particle density and small pores increased solution retention by 15%, achieving a maximum adsorption capacity of 48.4 mg/L (at 0 ~ 12.5cm) and effectively delaying ammonium infiltration. Nitrate nitrogen migrated rapidly due to weak adsorption, reaching equilibrium (≈ 40 mg/L) in all columns within 18 days. It is possible that air-packed zones containing chalky clay interlayers promote denitrification by prolonging the retention time. The layered structure with clay interlayers proved most effective at retaining ammonium nitrogen, while nitrate nitrogen remains a persistent groundwater contamination risk due to its high mobility. These findings provide critical insights for developing targeted nitrogen pollution control strategies in the region.