Soil phosphorus accumulation and phosphorus fraction distribution in a paddy field under long-term cattle manure application regimes at different frequencies

Efficient phosphorus (P) management in paddy fields is crucial for sustainable crop production and effective use of limited P resources. Although cattle manure supplies P and organic matter, long-term application may promote soil P accumulation and alter P fraction distribution. This study evaluated temporal changes in soil P fractions, available P, and brown rice yield in a rice (Oryza sativa L.) paddy field established in 2005 under long-term cattle manure application regimes at different frequencies. The experiment included annual (M1), biennial (M2), triennial (M3), and 6-year interval (M6) cattle manure application regimes, with chemical fertilizer (CF) and no-fertilizer control (CL) treatments. Soil P fractions were determined using the Sekiya sequential extraction method as acetate-extractable P (AA-P), ammonium fluoride-extractable P (NH4F-P), and sodium hydroxide-extractable P (NaOH-P). Available P was evaluated using the Bray II and Truog methods, and the relationships between soil total carbon (C) and P fractions were examined using linear models adjusted for year and treatment. Soil total P showed significant positive temporal trends in M1 and M2, whereas M3 and M6 showed no significant trend. In M1, M2, and M3, NH4F-P increased, whereas AA-P and NaOH-P did not show consistent increases. AA-P was the dominant fraction among the sequentially extracted inorganic P fractions in the analyzed cattle manure sample. Soil total C was positively associated with total P, AA-P, and NH4F-P. Available P did not increase consistently. Mean annual relative yields were numerically higher under annual and biennial application, whereas triennial and 6-year application maintained yields comparable to CF. These results suggest that triennial or 6-year application, at rates comparable to those tested, can reduce cumulative P inputs while maintaining yields comparable to CF, although continued monitoring is needed to clarify the long-term significance of the NH4F-P increase under M3.