Land use change (LUC) is a key factor affecting nutrient budgets in agricultural soils. Comparing the long-term trajectories of nutrient budgets and changes in soils at a regional scale with contrasting LUC is critical to optimizing nutrient management and minimizing adverse environmental effects. We investigated the nutrient budgets and changes in soils in two counties in the south Loess Plateau, China, with contrasting LUCs from 1992 to 2017. Wugong County has cereals as the main crop, whereas Meixian County has the main cereal crops changed to kiwi orchards. We found that nitrogen (N), phosphorus (P), and potassium (K) inputs in two counties increased rapidly, and the nutrient outputs by crop harvests remained relatively stable. This resulted in increasing nutrient surpluses in the soils of two counties. Nutrient surplus in the orchard-dominated county was higher than that of the cereal-dominated county, and nutrient use efficiencies were contrasting. High N surplus in the orchard-dominated county resulted in high nitrate-N accumulation in deeper soil profiles of orchards. High P and K surpluses in the two counties significantly increased available P and K in 0-20 cm depth. Soil available P and K in the orchard-dominated county were significantly higher than in the cereal-dominated county, which was also significantly higher than the threshold values of available P and K contents in soils. We conclude that comprehensive measures should be taken to control nutrient surpluses, which will help to balance nutrient inputs and outputs and minimize nutrient losses in intensive horticultural crop systems.

Since 1990s, dramatic land-use changes have occurred across mainland China. Large areas of cereal lands have been converted into horticultural crops because of lucrative economic benefits. Fruits and vegetables together in China have consumed more than 30% of N fertilizers. Therefore, understanding the long-term effects of land use change from cereals to orchard on N budgets in croplands at the county scale over long-term is very important for managing N in agricultural systems in China. We selected three Counties (Luochuan (LC), Mixian (MX), and Wugong (WG)) on the Loess Plateau with different land-use changes since 1990s in Shaanxi to compare the changes of N budgets in croplands at the county scales. The main crops in the three Counties were cereals (wheat and corn) before 1990s. After 1990s, the land uses in LC and MX changed dramatically; and LC and MX become the main apple and kiwifruit production county in China, respectively. The main crops in WG are still wheat and corn. It provides an ideal reference to compare the effects of land use changes on N budgets in croplands at the county scale. The N inputs and outputs, N surplus and N use efficiency (NUE, computed as N in harvested crop products divided by N inputs) at the three Counties from 1990 to 2017 were quantified. The annual N inputs and N surplus of the three Counties since 1990s were increased, and NUE decreased significantly. Compared to WG, the annual N input and N surplus of cropland of LC and MX were very high, and NUE was very low. For example, NUE in LC decreased from 64% in 1990 to 12% in 2017; and NUE in WG decreased from 55% in 1992 to 38% in 2017. To understand the fate of surplus N in cropland of LC, we also collected soil profile samples (0-6 m) from cereal lands and apple orchards in different sites of the county. The average nitrate accumulation in 0–6 m soil profiles reached 5611 kg N/ha in 2017. Approximately 67% of the total N surpluses in LC from 1990 to 2017 was accumulated in soil profile as nitrate. Land use change from cereals to orchard result in high surplus N in croplands at the county scale. The nitrate accumulation in the vadose zone is 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.