Objective This study investigated the variation of soil P availability when the field become upland from paddy field, and the major factors causing the variation. The research will provide a basis for academic phosphorus fertilization in the uplands.

Methods Soil samples were collected in the paddy fields developed from parent materials of limestone, quaternary laterite and sand shale, and in the neighboring uplands derived from paddy fields, respectively. The total, available, and water-soluble P content, and the contents of inorganic P fractions (Al-P, Fe-P, Ca-P, and O-P) were analyzed. Principal component analysis (PCA) and random forest analysis method were used to evaluate the contribution of soil properties to the change of P availability, and to screen the main factors causing the variation.

Results After transferred from paddy field to upland, the available P content in the quaternary laterite-developed soil was not changed significantly, neither the tested inorganic P fractions. While the available P contents in limestone- and sand shale-developed soils were increased by 20.48 and 17.60 mg/kg (P<0.05), and soil phosphorus activation coefficients increased by 2.67 and 2.22 percent points, the Al-P increased by 46.07 and 51.28 mg/kg, Fe-P in limestone-developed upland was increased by 62.11 mg/kg. The water soluble P contents in all the three uplands were not exceeding the P risk threshold. According to the correlation analysis result, the P activation coefficients in the derived uplands were positively correlated with Fe-P (P<0.05) and Al-P (P<0.01). PCA and random forest analysis showed that Fe-P and Al-P were the main factors driving the change of availability of soil phosphorous.

Conclusions In the hilly area of southern Hunan Province, transferring paddy fields to uplands has caused increase of the phosphorus availability in limestone- and sand shale-developed red soils, due to the enhanced proportion of Fe-P and Al-P in total P, not in the quaternary laterite-developed red soil. The water soluble P contents in all the transferred uplands are lower than the P risk threshold, showing low risk of soil P leaching.