Abstract
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.
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