Inter-annual variation in P speciation and availability in the drought-rewetting cycle in paddy soils

Abstract

Successive application of phosphorus (P) fertilizer under different cropping systems could result in remarkable changes in soil P fractions, accompanied by inducing soil P accumulation and increasing eutrophication risks. However, relatively little attention has been paid to annual changes in soil P fractions and P losses via runoff in drought-rewetting paddy fields. In the present study, we explored inter-annual variations in soil P speciation, availability and runoff P in a five-year field trial in a winter wheat and summer rice rotations system, and in different rice-growth stages. Four P fertilization treatments were applied, including P fertilization in both farming seasons (wheat and rice) (PR + W), application of P fertilizer only in rice-growing seasons (PR), application of P fertilizer only in wheat-growing seasons (PW), and no P fertilization (Pzero). Our results showed that P fertilization treatments (PW, PR, and PR + W) significantly altered the proportions of inorganic P (Pi) fractions, while no P fertilization in the wheat-growing seasons (Pzero, PR) significantly decreased NaHCO3-Pi contents in the wheat-growing seasons and decreased NaHCO3-Pi and NaOH-Pi concentrations in the rice-growing seasons (p < 0.05). Application of P fertilizer in the wheat-growing seasons treatments (PW, PR+W) maintained the stability of soil P fractions both in the wheat- and rice-growing seasons. Moreover, compared to the PR+W treatment, the PW treatment reduced total P (TP) losses from runoff. For inter-annual drought-rewetting variation in P speciation and availability, there was a significant correlation between Olsen-P and NaHCO3-Pi and NaOH-Pi contents in the wheat-growing seasons, and Olsen-P was significantly correlated with NaHCO3-Pi, NaOH-Pi, and HCl-P in the rice-growing seasons. Furthermore, we focused on the change in detailed rice-growth stages, and found aeration significantly increased NaHCO3-Pi, NaOH-Pi, NaOH-Po and HCl-P contents, and there was a significant correlation between soil Olsen-P and the P fractions in the A-aeration growth stage in rice, which could be attributed to the short-term drought-rewetting cycle facilitating the release of soil P. The present study revealed changes in soil P speciation and availability in a drought-rewetting cycle in paddy soils, and indicated the optimized agricultural P management strategies had the potential to reduce environmental risks.