Long-term high nitrogen surplus in intensive apple-planting regions results in huge legacy nitrogen in the vadose zone: Potential or real risk to the groundwater?

Abstract

Understanding the fates of nitrogen (N) fertilizers applied to soil at the different scales is crucial to increase N use efficiency and reduce N loss to the environment. However, most of studies focus on the field scale. It is a major challenge to study the long-term N fates in intensive agricultural systems at large scale due to high uncertainty. We studied the impact of land-use changes from cereals to apple orchards at a county scale over nearly four decades on N surplus, nitrate accumulation in deep soil profiles, and groundwater quality. The average N surplus in croplands of Liquan County increased from 53.4 kg N ha-1 yr-1 in 1981–411.9 kg N ha-1 yr-1 in 2019. The average of nitrate accumulation in the 0–8 m soil profiles of apple orchards in the northern, middle, and southern regions of the county (rainfed (farming that relies on natural rainfall rather than irrigation), transition, and irrigated) were 6075, 11554, and 16723 kg N ha-1, respectively, which were higher than that of cereal lands (443 kg N ha-1). Approximately 53.7 % of the total N surplus in croplands at the county was accumulated in the 0–8 m soil profile in the form of nitrate. The nitrate concentrations of 52.0 % of groundwater samples exceeded the World Health Organization (WHO) standard (11.3 mg N L-1), the samples were mainly located in the southern region of the county, where the vadose zone ranged from 10 to 20 m depth, and there were no groundwater samples that exceeded the standard in the rainfed region. This study highlighted that the large nitrate storage in the vadose zone was the main fate of surplus N over the past 40 years in intensive agricultural systems and has caused severe nitrate pollution of groundwater in irrigated regions with shallow vadose zones.