Understanding the behavior of nitrate in the unsaturated zone is critical for predicting groundwater nitrate pollution. Although this issue has been intensively investigated for shallow soil profiles, it remains poorly understood in deep soils. We collected soil samples up to 20 m depth under farmland and apple orchards with trees that were 16 and 22 years old (A16 and A22). We then determined soil-water content, stable isotopes (δ²H and δ¹⁸O) and nitrate content under two land use types. The stable isotopes of nitrate under two apple orchards were measured to a depth of 16 m. The mean nitrate contents in the shallow soils (0–6 m) under apple orchards (78.8 ± 87.8 mg kg⁻¹ for A16, 180 ± 155 mg kg⁻¹ for A22) were significantly higher than that of farmland (2.40 ± 1.56 mg kg⁻¹) (p < 0.01), whereas the nitrate contents in the deep soils (6–20 m) were almost similar between apple orchards and farmland. Ammonium or urea-based nitrogen (N) fertilizers (30–61%) were the predominant contributors to nitrate in the shallow soil layers, followed by soil organic N and manure & sewage, whereas the soil organic N (mean 34%) was the primary source of nitrate in deep soils. Isotopic data revealed that the N transformation was controlled by nitrification, and denitrification was not apparent throughout the soil profiles. The downward transport of nitrate lagged behind soil-water. Nitrate was leached by piston flow in the vadose zone, and the nitrate from N application since the 1980 s has only infiltrated to 6 m depth. The difficulty in recharging deep soils mitigates the transport of nitrate to lower depths. The combination of water-nitrate stable isotopes provides new insights for identifying nitrate sources, transformation, and transport in the thick unsaturated zone.

了解非饱和区硝酸盐的行为对于预测地下水硝酸盐污染至关重要。尽管这个问题已经针对浅层土壤进行了深入研究，但对于深层土壤仍然知之甚少。我们在农田和苹果园下采集了 20 m 深处的土壤样本，这些土壤样本的树龄分别为 16 和 22 年（A16 和 A22）。然后，我们测定了两种土地利用类型下的土壤水分含量、稳定同位素（δ ² H 和 δ ¹⁸ O）和硝酸盐含量。对两个苹果园地下16 m深度的硝酸盐稳定同位素进行了测量。苹果园下浅层土壤（0~6 m）硝酸盐平均含量（A16为78.8±87.8 mg kg ^-1，A22为180±155 mg kg ^-1）显着高于农田（2.40±1.56 mg）。 kg ⁻¹ ) (p < 0.01)，而苹果园和农田深层土壤（6-20 m）的硝酸盐含量几乎相似。铵基或尿素基氮 (N) 肥料 (30-61%) 是浅土层硝酸盐的主要贡献者，其次是土壤有机氮和粪肥和污水，而土壤有机氮（平均 34%）是浅层土壤中硝酸盐的主要贡献者。深层土壤中硝酸盐的主要来源。同位素数据显示，氮转化受硝化作用控制，反硝化作用在整个土壤剖面中并不明显。硝酸盐的向下输送落后于土壤水。硝酸盐在渗流区通过活塞流淋滤，自 1980 年代以来施氮产生的硝酸盐仅渗透至 6 m 深度。深层土壤补给的困难减少了硝酸盐向较低深度的输送。水-硝酸盐稳定同位素的组合为识别厚非饱和带中硝酸盐的来源、转化和传输提供了新的见解。