Nitrate accumulation in the soil profile of uplands is an important fate of surplus nitrogen (N) fertilizer in intensive agricultural regions. A comprehensive understanding of the spatial variation in nitrate accumulation characteristics in the deep (>2 m) soil profile and identification of the key driving factors are important for mitigating N loss and associated environmental problems. We sampled the deep soil profiles (to 8–13 m) in four regions (i.e., R1, R2, R3 and R4) from north to south of the apple-planting region on the Loess Plateau (LP), spanning a highly spatial variation of climates, soils, orchard standing ages, and N fertilizer rates. Regression analysis and random forest analysis were used to identify the characteristics of nitrate accumulation in the vadose zones at the regional scale. The results showed that nitrate accumulation characteristics (represented by four indices: peaks of nitrate content, peak depths, nitrate accumulation depths and nitrate accumulation amounts) were significantly positively correlated with the stand ages of orchards. Nitrate stock in the soil profiles (0–8 m) increased from north to south and averaged 4149 and 13,633 kg N ha⁻¹ in the 25-yr-old orchards for R1 and R4, respectively. Nitrate stock in the soil profiles (0–13 m) of 30-yr-old orchards in R4 averaged 19,283 kg N ha⁻¹, ranging from 12,859 to 25,900 kg N ha⁻¹. The nitrate accumulation depths in the orchards in the R1 and R4 regions were deeper due to the high soil sand content and high water input, respectively. It highlights that the water input rate and soil sand content were key controlling factors for peak depth and nitrate accumulation depth, while stand age and N fertilization rate mainly controlled the peak of nitrate content and nitrate accumulation amount. The southern apple-planting region with a shallow vadose zone and high irrigation is suggested to be designated as the nitrate vulnerable zone.

高地土壤剖面中硝酸盐的积累是集约型农业区剩余氮 (N) 肥料的重要归宿。全面了解深层（> 2 m）土壤剖面中硝酸盐积累特征的空间变化并确定关键驱动因素对于减轻氮损失和相关的环境问题非常重要。我们对黄土高原（LP）苹果种植区从北到南四个区域（即R1、R2、R3和R4）的深层土壤剖面（至8-13 m）进行了采样，跨越了高度的空间变化气候、土壤、果园立龄和氮肥用量。回归分析和随机森林分析用于识别区域尺度上渗流带中硝酸盐积累的特征。结果表明，硝酸盐积累特征（以硝酸盐含量峰值、峰值深度、硝酸盐积累深度和硝酸盐积累量四个指标为代表）与果园林龄呈显着正相关。土壤剖面 (0–8 m) 中的硝酸盐储量从北向南增加，平均为 4149 和 13,633 kg N haR1 和 R4 在 25 岁果园中分别为^{-1 。}R4 中 30 岁果园土壤剖面 (0–13 m) 中的硝酸盐储量平均为 19,283 kg N ha ^-1，范围为 12,859 至 25,900 kg N ha ^-1。由于土壤含沙量高，水分输入量大，R1和R4区域果园的硝酸盐积累深度较深。说明水分输入速率和土壤含沙量是峰值深度和硝酸盐积累深度的关键控制因素，而林龄和施氮量主要控制硝酸盐含量峰值和硝酸盐积累量。建议将渗流带浅、灌溉量大的南部苹果种植区划为硝酸盐易受害区。