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 (P-i) fractions, three organic P (P-o) fractions, and several soil parameters to investigate how land use conversion affects P-i and P. fractions in a pen-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 P-i (SL-P-i), aluminum-bound P-i (Al-P-i), calcium-bound P-i (Ca-P-i), and iron-bound P-i (Fe-P-i), respectively, after conversion from rice paddies to vegetable fields. Most of the increase in P-i was in the form of Fe-P-i, which increased from 8% of total P (TP) on paddy soil to 31% on the soil with > 20-year vegetable cultivation, followed by Al-P-i, which increased from 2% to 19% of TP. For P-o 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-P-i fraction, followed by Ca-P-i and Al-P-i 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.