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.