Abstract: The aim of this study was to determine the effects of straw return and chemical fertilizer application on the inorganic phosphorus content in lime concretion black soil. Therefore, we conducted a multi-year field experiment with various straw return and fertilization treatments, and analyzed the profiles and accumulation characteristics of different forms of inorganic phosphorus in soil. The field experiment had six treatments: conventional fertilizer application+straw total return (HN₁); 50% of conventional fertilization+straw total return (HN₄); no fertilizer+straw total return (HN₀); conventional fertilization+straw removal (N₁); 50% of conventional fertilization+straw removal (N₄); and no fertilizer+straw removal (N₀). The results showed that under the same level of chemical fertilizer application, straw return significantly increased the contents of soil Ca₂-P and Fe-P by up to 219.05% and 51.35%, respectively, compared with those in the straw removal treatments. The proportions of Ca₂-P and Fe-P out of total inorganic phosphorus also increased significantly with straw return. In general, the Ca₈-P and Al-P contents decreased to some extent while Ca₁₀-P decreased significantly, and the total amount of inorganic phosphorus increased when straw was returned. The activation of inorganic phosphorus in soil with straw return and chemical fertilizer application decreased with decreasing amounts of fertilizer and increasing soil depth. The application of chemical fertilizers without straw return significantly reduced the contents of Ca₂-P, Ca₈-P, Al-P, and Fe-P in the soil. Compared with straw removal alone, straw removal+chemical fertilizer promoted the conversion and decomposition of potential phosphorus sources such as Ca₁₀-P. The main forms of inorganic P in soil were Ca-P (Ca₂-P, Ca₈-P, and Ca₁₀-P) and Fe-P. Under conventional fertilization conditions, straw return resulted in a rapid increase in Olsen-P content in soil. Under 50% conventional fertilization, straw return increased the Olsen-P content in surface soil. Returning straw to soil without fertilizer application did not significantly increase the Olsen-P content in soil. The correlations between soil Olsen-P and soil Ca₂-P, Ca₈-P, Al-P, and Fe-P reached significant or extremely significant levels, indicating that these four forms of inorganic P were closely related to soil Olsen-P, and may be important sources of available P in the soil. Therefore, the no-fertilizer and straw returning treatment could accelerate the depletion of inorganic P in soil. Together, the results showed that the rational application of straw and chemical fertilizers can improve the soil P supply by converting low-activity inorganic P into high-activity forms of inorganic P such as Ca₂-P and Fe-P.

Key words: rice-wheat rotation, straw returning, lime concretion black soil, inorganic phosphorus, profile distribution