Objective  The transformation of soil phosphorus is closely related to exogenous organic matter inputs. An indoor soil culture experiment was conducted to study the effectiveness and fractionation of phosphorus in response to the application of co-compost of straw with different iron oxides in two calcareous soils (lime concretion black soil and fluvo-aquic soil), which will provide a scientific basis for the rational agricultural utilization of iron-contained decomposed straw.

  Method  There were five treatments: blank control (CK), decomposed straw (S), decomposed straw + ferrihydrite (SF), decomposed straw + goethite (SG) and decomposed straw + hematite (SH). The transformations of different phosphorus fractions and their relationship with soil physicochemical properties were analyzed to evaluate the effects of the application of the decomposed straw with different iron compounds on phosphorus effectiveness in lime concretion black soil and fluvo-aquic soil.

  Result  The results showed that ① the application of decomposed straw could significantly increase the available phosphorus contents in two soils (P < 0.05). After 60 days of incubation, the available phosphorus content increased by 10.2% - 14.8% and 24.7% - 35.0% respectively in lime concretion black soil and fluvo-aquic soil, and the enhancement effect of phosphorus effectiveness by the four treatments of decomposed straw application in lime concretion black soil and fluvo-aquic soil were in order of S > SG > SH > SF and SF > S > SG > SH, respectively. ② The relative contents of active and moderately stable phosphorus pools increased while the relative contents of stable phosphorus pool decreased in lime concretion black soil with the application of decomposed straw. After 60 days of incubation, the accumulation of active phosphorus in SG treatment was significantly higher than those in other treatments (P < 0.05), which increased by 19.8% compared with CK. In fluvo-aquic soil, the relative content of the active phosphorus pool increased, while the relative contents of moderately stable phosphorus (except SG treatment) and stable phosphorus pools (except S and SH treatments) decreased. After 60 days of incubation, the accumulation of active phosphorus in SF treatment was significantly higher than those in other treatments (P < 0.05), which increased by 11.4% compared with CK. ③ Correlation and redundancy analyses showed that the available phosphorus contents in two soils were significantly positively correlated with soil organic matter content, and the transformation of phosphorus fractions was significantly influenced by soil pH in lime concretion black soil, and by soil alkaline phosphatase activity in fluvo-aquic soil.

  Conclusion  The application of decomposed straw significantly enhanced the phosphorus effectiveness, which mainly promote the conversion of inactive phosphorus to active phosphorus in two soils. And the application of decomposed straw with goethite was more effective in lime concretion black soil, while the application of decomposed straw with ferrihydrite was more effective in fluvo-aquic soil.