Effects of flooding on transformation of inorganic phosphorus fraction in calcareous soils

Objectives When applied to soil, phosphorus (P) is easily converted to Fe-P and O-P, which might be affected by the redox process of iron oxide. Here, we studied the valent state and form of iron under flooding condition, and its relationship with the transformation of inorganic P fractions.

Methods A slurry incubation experiment was employed to simulate flooding condition in the laboratory, using calcareous soil receiving 0 and 78.6 kg/hm² of P for 6 years (P0, P180). Soil samples were loaded into silling vials, sealed and incubated for 40 days at (30 ± 1)℃ under illumination and dark conditions. Available P and inorganic P fractions were measured before and after incubation. Soil Fe (Ⅱ) content was monitored regularly during the incubation process. The relationship between inorganic P fractions and the iron redox process was discussed.

Results Soil available P increased dramatically as a result of P fertilization. Soil available P in P0 and P180 treatments were (7.65 ± 1.65) mg/kg and (33.5 ± 2.01) mg/kg. The applied P existed mainly as Ca₁₀-P, Ca₈-P, Al-P, and Fe-P fraction, and less than 1% existed as Ca₂-P. Flooding incubation in the dark increased soil available P by 8.44 mg/kg and 2.95 mg/kg in P0 and P180 treatments. After flooding incubation, the Ca₈-P content decreased while Fe-P, O-P and Al-P increased in P180 treatment. Under illumination and dark condition, Ca₈-P decreased by 106.8 and 156.2 mg/kg, Fe-P increased by 23.4 and 47.0 mg/kg, O-P increased by 64.1 and 92.9 mg/kg, and Al-P increased by 33.8 and 34.7 mg/kg, respectively. The observed decrease in Ca₈-P under dark conditions was higher than under illumination, while Fe-P and O-P recorded a higher increase in the dark condition than under illumination. We found a significant negative correlation ( P < 0.05) between the amount and maximum velocity of Fe (Ⅱ) reduction and the variation of Ca₈-P content, and a significant negative correlation with the increased amount of Fe-P and O-P contents.

Conclusions Under flooding conditions, Fe (Ⅲ) is reduced to Fe (Ⅱ), leading to the formation of Fe (Ⅱ) and Fe (Ⅲ) mixtures. Consequently, the mixtures have a larger specific surface area and more P adsorption sites, leading to the formation of O-P, Fe-P, and Al-P fractions. Illumination decreases Fe (Ⅲ) reduction, which may be responsible for the lower transformation rate of Ca₈-P fraction to O-P, Fe-P, and Al-P fractions.