Nitrate dynamics in deep soils of the Loess Plateau: Impact of different land use types

• Deep soil nitrate isotopes in Loess Tableland land uses were obtained. • Reveals nitrate accumulation in deep soils driven by historical land use changes. • Indicator-based attribution analysis of land use impacts on groundwater recharge. The nitrogen cycling characteristics of soil in the Loess Plateau are critical for regional agriculture and environmental protection. This study investigates deep soil water nitrate distribution and dynamics under different land use types using δ 15 N-NO 3 – , δ 18 O-NO 3 – , δ 18 O-H 2 O, δ 2 H-H 2 O, and hydrochemical ions (F – , Cl – , NO 3 – , and SO 4 2– ). The results indicate that agricultural lands, such as cornfields and apple orchards, exhibit higher nitrate concentrations in shallow soils (cornfields: median ∼ 50 mg/L, extremes > 80 mg/L) due to fertilization and irrigation. However, deeper soil layers did not show significant nitrate enrichment despite prolonged cultivation. In contrast, grasslands and woodlands maintain lower and stable nitrate levels (grasslands: 10–15 mg/L, woodlands: ∼5 mg/L) due to natural nitrogen fixation. Nitrate sources differ by land use. Apple orchards and shrublands are influenced by NH 4 + -fertilizers and soil nitrogen, while cornfields utilize both NH 4 + - and NO 3 – -fertilizers. Land-use changes from agriculture to woodland increased deep soil nitrate (woodlands by 22.7 % and shrublands by 83 %), suggesting heightened leaching and groundwater pollution risks. Soil moisture also impacts nitrate accumulation. Shrublands exhibit the highest nitrate accumulation (27.24 kg N·ha −1 ·cm −1 ), followed by cornfields (0.74 ± 0.2 kg N·ha −1 ·cm −1 ), while grasslands show the lowest accumulation (0.43 ± 0.03 kg N·ha −1 ·cm −1 ). To mitigate groundwater nitrate pollution, rational fertilization, conservation tillage, and efficient irrigation are recommended for agricultural lands. For natural lands, maintaining vegetation and soil management practices is essential. Tailored strategies are crucial to optimize soil health and water quality.