2.1 Study Region
The two study counties, Wugong (34°12′-34°26′ N, 108°0′-108°26′ E) and Meixian (33°59′-34°19′ N,107°39′-108°00′ E) in Shaanxi, are located in the western Guanzhong Basin, the south edge of Loess Plateau in China. The region is typical of a warm-temperate sub-humid continental monsoon climate with a mean annual temperature of 13 ℃ and average annual precipitation of 630 mm, with 60-80% occurring between July and September.
The study area has a more than 2,000 years-long agricultural history and is a very intensive agricultural area in China now. The main crops in Wugong and Meixian counties before 1992 were winter wheat and summer maize, with two harvests each year (Fig. 1). Kiwifruit production developed rapidly in Meixian County since 1995. In 2017, the kiwi orchard area increased to 15688 ha, accounting for 64 % of the total cropland area. The dominant soil textures include loam, sandy loam, and loamy sand (Gao et al. , 2021).
Winter wheat and summer maize are still the main crops in Wugong. It provided an ideal case to study the effects of LUCs on nutrient surpluses and accumulations in soils at the county scale. The total arable land of Wugong County is 31369 ha. The topography of Wugong County consists of three types: the frontal zone of the piedmont alluvial fan, the loess platform and valley alluvial terraces. There are six soil types in the county: cinnamon soil, loess soil, flavor-aquic soil, swamp soil, paddy soil, and silt soil. High-quality soils such as loess soil, account for 94.3%.