Key Laboratory of Ecosystem Network Observation and Modeling, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China.
College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100190, China.
Environ Sci Technol. 2022 Jul 5;56(13):9335-9345. doi: 10.1021/acs.est.1c06289. Epub 2022 Jun 22.
Excess nitrate (NO) loading in terrestrial and aquatic ecosystems can result in critical environmental and health issues. NO-rich groundwater has been recorded in the Guanzhong Plain in the Yellow River Basin of China for over 1000 years. To assess the sources and fate of NO in the vadose zone and groundwater, numerous samples were collected via borehole drilling and field surveys, followed by analysis and stable NO isotope quantification. The results demonstrated that the NO concentration in 38% of the groundwater samples exceeded the limit set by the World Health Organization. The total NO stock in the 0-10 m soil profile of the orchards was 3.7 times higher than that of the croplands, suggesting that the cropland-to-orchard transition aggravated NO accumulation in the deep vadose zone. Based on a Bayesian mixing model applied to stable NO isotopes (δN and δO), NO accumulation in the vadose zone was predominantly from manure and sewage N (MN, 27-54%), soil N (SN, 0-64%), and chemical N fertilizer (FN, 4-46%). MN was, by far, the greatest contributor to groundwater NO (58-82%). The results also indicated that groundwater NO was mainly associated with the soil and hydrogeochemical characteristics, whereas no relationship with modern agricultural activities was observed, likely due to the time delay in the thick vadose zone. The estimated residence time of NO in the vadose zone varied from decades to centuries; however, NO might reach the aquifer in the near future in areas with recent FN loading, especially those under cropland-to-orchard transition or where the vadose zone is relatively thin. This study suggests that future agricultural land-use transitions from croplands to orchards should be promoted with caution in areas with shallow vadose zones and coarse soil texture.
过量的硝酸盐（NO）在陆地和水生生态系统中的积累会导致严重的环境和健康问题。在中国黄河流域的关中平原，富含硝酸盐的地下水已经存在了 1000 多年。为了评估包气带和地下水中硝酸盐的来源和归宿，研究人员通过钻孔和现场调查采集了大量样本，并进行了分析和稳定的硝酸盐同位素定量。结果表明，38%的地下水样本中的硝酸盐浓度超过了世界卫生组织规定的限值。果园 0-10 米土壤剖面中硝酸盐的总储量比农田高 3.7 倍，这表明从农田向果园的转变加剧了深层包气带中硝酸盐的积累。基于稳定的硝酸盐同位素（δN 和 δO）贝叶斯混合模型，包气带中硝酸盐的积累主要来自粪肥和污水 N（MN，27-54%）、土壤 N（SN，0-64%）和化学氮肥（FN，4-46%）。到目前为止，MN 是地下水硝酸盐（58-82%）的最大贡献者。研究结果还表明，地下水硝酸盐主要与土壤和水文地球化学特征有关，而与现代农业活动无关，这可能是由于厚包气带的时间延迟。估计硝酸盐在包气带中的停留时间从几十年到几百年不等；然而，在最近 FN 负荷较高的地区，特别是在农田向果园转变或包气带相对较薄的地区，NO 可能在不久的将来到达含水层。本研究表明，在浅层包气带和粗质地土壤地区，未来从农田向果园的农业土地利用转变应谨慎进行。
