Nitrate accumulation in the soil profile is the main fate of surplus nitrogen after land-use change from cereal cultivation to apple orchards on the Loess Plateau

Quantifying the nitrogen (N) surplus (the difference between N inputs and outputs) of an agricultural system is crucial for improving N use efficiency and reducing its loss. The fates of surplus N during land-use change from cereals to apple orchards from 1990 to 2017 in Luochuan County on the Loess Plateau, were quantified. The ratio of apple orchard area to total agricultural land in the county increased from 18% in 1990 to 73% in 2017. As cereal lands were converted to apple orchards, the annual N input and N surplus of the total cereal-orchard system increased significantly from 161 kg N ha-1 and 58 kg N ha-1 in 1990 to 543 kg N ha-1 and 477 kg N ha-1 in 2017, respectively. The average nitrate accumulation in the 0-2 m and 0-6 m soil profiles reached 2635 kg N ha-1 and 5611 kg N ha-1, respectively, in 2017. Approximately 67% of the total N surpluses from 1990 to 2017 accumulated in the soil profile in the form of nitrate. This result highlights the role of nitrate accumulation in the vadose zone as the main fate of surplus N in the intensive agricultural landscape, which should be considered an important component of the soil N budget to optimize production and environmental protection.