2.3.2 Calculation of nitrate accumulation in soil profile
The nitrate accumulation in the soil profit was calculated as follow:
Soil nitrate accumulation (kg N ha-1) = (BD × t × c) / 10 (6)
Where
BD soil bulk density (g cm-3);
t thickness of the soil layer (cm);
c nitrate content in the soil (mg N kg-1);
10 conversion coefficient.
The soil bulk density (BD) of the top 60 cm was obtained by Lu et al., (2016) in the same study region; the soil bulk density of the 0-20, 20-40, and 40-60 cm soil layers in the kiwi fruit orchard and cereal fields were 1.28, 1.37, and 1.38 and 1.37, 1.45, and 1.47 g cm-3, respectively. The BD of the deep soil layers below 60 cm were the same as those of 40-60 cm depth because of small variations in deeper layers (Gao et al., 2021).
The total nitrate storage within soil depths of 0-2 m and 0-5 m over the entire cereal-orchard system in Meixian and Wugong counties was calculated. The cumulative amount of nitrate in the county was calculated by multiplying the cumulative amount of nitrate by the cultivated area of different crops. The nitrate accumulation is the average value of soil nitrate accumulation within a depth of 0-2 m or 0-5 m (kg N ha-1). The area of kiwi fruit orchards of different ages was calculated by cereals converted to kiwi orchards each year from 1992 to 2017 in Meixian County. The cumulative amount of nitrate in the county in 1992 was calculated based on 2017.