3.3 Total phosphorus and available phosphorus
change
The soil TP contents range from 0.71 g/kg to 1.00 g/kg,following the order of vegetable field > orchard > cropland
in the soil depths from 0 to 25 cm (Table 2). There are no significant differences between orchard and vegetable
field. However, for the soil depths of 0–25 cm, the TP contents in vegetable field are significantly higher
than that in the cropland. For the deeper soil layers (40–70 cm), the TP is significantly higher in orchard than that in the other two agricultural land use types (p < 0.05). Following the same pattern as observed for the SOC
and TN, the TP contents in each agricultural land use type decrease generally with soil depths increase, in particularly in the soil profile of 0–40 cm (Table 2). Correspondingly, the TP contents in the depths of 25–100 cm
range from 0.51 to 0.70 g/kg in these land use types. The available phosphorus in vegetable field is the
highest (Table 3), corresponding well to the highest P fertilizer inputs and frequent irrigation (Table 1). In the
depths from 0 to 40 cm, the available phosphorus contents in vegetable field range from 22.03 mg/kg to 66.10
mg/kg which is about 2.5–6.5 times higher than those in the other two agricultural land use types (Table 3). It is
presumed that high available phosphorus contents are prone to potential risk of P loss and non-point source
pollution.