Nine-year organic fertilization enhances poorly crystalline iron hydroxide formation and phosphorus availability in a temperate rice–wheat cropping system

Abstract

Phosphorus (P) is typically associated with iron (Fe) hydroxides in paddy soils. Our study investigated the impact of different fertilization treatments on the availability of P regulated by Fe redox transformation in a rice (Oryza sativa L.)–wheat (Triticum aestivum L.) cropping system in the mid-low reaches of the Yangtze River. Five fertilization treatments were examined: control (CK), chemical fertilizer (CF), 35% CF (P) plus pig manure compost (CFM), 100% CF (P) plus straw (CFS), and 35% CF (P) plus pig manure compost and straw (CFMS). Nine-year rice–wheat rotations increased the oxalate-extractable poorly crystalline Fe hydroxides (i.e., Feo) by 33%–87% compared with the initial soil, while fertilization further accelerated this process. Compost application increased the proportion of labile P by 75%–108% and decreased the proportion of non-labile P by 14%–22% compared with the single chemical fertilization treatment. Furthermore, organic fertilization increased the mass proportions of macroaggregates and macroaggregate-associated labile P. The Feo was positively correlated with the content of labile P but negatively correlated with the proportion of non-labile P. Moreover, the reductive dissolution of Fe hydroxides was accompanied by the transformation of P from NaOH-extractable to NaHCO₃- and H₂O-extractable phases. These results indicate that seasonal alternation of drying and wetting can progressively drive the redox transformation of Fe hydroxides and promote the formation of Feo, thereby affecting the availability of P. Therefore, we suggested that P fertilizer should be reduced in the rice season due to the reduction of Fe hydroxides, particularly in the compost-amended soils in the temperate rice–wheat cropping system.