Vertical Distribution Patterns of Nitrogen and Phosphorus in Soil Solution: Insights from a Wetland Trial Site in the Li River Basin

Agricultural activities contribute significantly to the pollution of groundwater through the incomplete absorption of nitrogen (N) and phosphorus (P). Understanding the vertical migration patterns of N and P in soil solution is crucial for controlling groundwater quality. This study is based on monitoring data of soil solution nitrogen and phosphorus at different depths (30 cm, 60 cm, 100 cm) in the Huixian Wetland Experimental Area in the Li River Basin from March to December 2021. The vertical distribution patterns of nitrogen and phosphorus in soil solution in the study area are elucidated from three aspects: seasonal variations, karst types, and land use. The results indicate that the following: (1) NO₃⁻-N is the predominant form of nitrogen, generally decreasing with increasing soil depth, while NH₄⁺-N concentrations show slight increases and TP concentrations remain relatively stable. Overall, NO₃⁻-N and TN concentrations tend to accumulate at 30 cm and 60 cm depths during both irrigation and non-irrigation seasons, with no distinct distribution patterns observed for NH₄⁺-N and TP. (2) During the irrigation season, the migration distance of NO₃⁻-N in non-karst landforms is mainly at 60 cm, while in peak forest plains, it is mainly at 100 cm, with no clear trend observed in NO₃⁻-N concentrations in peak cluster depressions. In non-irrigation seasons, the distribution of NO₃⁻-N content in non-karst landforms and peak cluster depressions is mainly 30 cm > 60 cm > 100 cm. The downward migration distance of NH₄⁺-N generally follows the order of peak cluster depressions > peak forest plains > non-karst landforms. (3) During the irrigation season, NO₃⁻-N concentrations in paddy fields remain relatively high at 100 cm, while in drylands, NO₃⁻-N concentrations generally follow the pattern of 30 cm > 60 cm > 100 cm but may exhibit anomalous increases or decreases at 60 cm and 100 cm depths during heavy rainfall.