Effect of Land Use Conversion from Rice Paddies to Vegetable Fields on Soil Phosphorus Fractions

Excess phosphorus (P) from agricultural soils contributes to eutrophication in water bodies. Samples (n = 60) were taken from sites where rice paddies have been converted to vegetable fields for 0, < 10, 10–20, and > 20 years and analyzed for five inorganic P (Pi) fractions, three organic P (Po) fractions, and several soil parameters to investigate how land use conversion affects Pi and Po fractions in a peri-urban area of China with soils characteristic of many agricultural areas of Asia. Significant increases of 33, 281, 293, and 438 mg kg−1 were found for soluble and loosely bound Pi (SL-Pi), aluminum-bound Pi (Al-Pi), calcium-bound Pi (Ca-Pi), and iron-bound Pi (Fe-Pi), respectively, after conversion from rice paddies to vegetable fields. Most of the increase in Pi was in the form of Fe-Pi, which increased from 8% of total P (TP) on paddy soil to 31% on the soil with > 20-year vegetable cultivation, followed by Al-Pi, which increased from 2% to 19% of TP. For Po fractions, there was no significant change in P concentrations. The conversion of land use from paddy fields to high intensity vegetable fields was causing significant changes in soil P fractions. Management practices were causing a buildup of soil P, primarily in the Fe-Pi fraction, followed by Ca-Pi and Al-Pi fractions. If current trends continue, a 30%–70% increase in TP could be expected in the next 20 years. Farmers in the area should reduce P application and use to maximize P uptake.