4.2 Different fates of the surpluses of N, P and K
The percentages of P and K uptakes by crops in Meixian were significantly lower than that of Wugong (Fig. 6), due to their excessive application in kiwifruit orchards. High P and K surpluses in the two counties from 1992 to 2017 significantly increased available P and K in soil (Fig. 4). The soil available P and K in Meixian County were 7.3 times and 3.4 times higher than that in Wugong County due to the higher P and K surpluses in the intensively horticultural crop region (Fig. 3). This is consistent with the studies of others, i.e., LUC is an important cause for the high soil nutrient levels (Luz et al., 2019; Roobroeck et al., 2021; Kim et al., 2023; Mulimbi et al., 2023). The average available P and K in 0-20 cm soils Meixian in 2017 were 111 mg kg-1 and 509 mg kg-1 (Fig.4), which are much higher than the optimum thresholds of available P and K in soils (Liu et al., 2021; Tian et al., 2021; Kochiieru et al., 2022). The excessive accumulation of P and K in the soil results in a waste of resources and environmental pollution.
Unlike P and K, the fates of surplus-N in the soil is more complex. N uptakes by crops was the main fate of N input in the Wugong; however, for Meixian, surplus-N in soil was significantly higher than crop uptake (Fig. 6). Nitrate accumulation in soil profiles of kiwifruit orchards was higher than that of cereal fields, which also increased with the increase in stand ages of the orchards (Fig. 5). The nitrate contents in deep soil profiles of kiwifruit orchard were high (Fig. 5), however, the roots of kiwifruit vines are shallow (Bai et al., 2022; Dutta et al., 2023; Wu et al., 2023), which will limit the nitrate uptake from the deep soil profiles. The average nitrate accumulation in 0-5 m soil profiles of kiwifruit orchards in stand ages of 11-20 years was as high as 3071 kg N ha-1 (Fig. 5). The nitrate content in the 4.8-5 m layer was as high as 64.8 kg N ha-1, indicating that the surplus N leached into the deeper soil layers or groundwater. Assuming nitrate in the soil profile of 5-10 m is the same as our studies in the region (Gao et al., 2021), the proportion of nitrate in the soil profile of 0-10 m to the surplus N is 73.5 % in Meixian County. This value is higher than the results (40.30%) from (Gu et al., 2015) and 18.5-24.8% from Sun et al., (2020).
Denitrification is usually considered the main process for eliminating nitrate in the deep vadose zone. Contrary to subtropical and tropical regions with strong denitrification in deep soil profiles, low content of soil organic carbon and higher dissolved oxygen that denitrification is limited (Wang et al., 2024), which leads to more nitrate accumulation in kiwifruit production regions such as Meixian County.