Partial substitution of chemical nitrogen fertilizer with organic manure is more feasible than full substitution for soil phosphorus risk management

The substitution of mineral fertilizer with organic fertilizer can help improve soil fertility and reduce environmental emissions, but the extent to which organic fertilizer should replace chemical nitrogen (N) fertilizer with respect to phosphorus (P) risk management remains unclear. In this study, we aimed to investigate the effects of organic fertilizer substitution on soil P and crop yield through a five-year field experiment. Four treatments were arranged, including chemical fertilizer N but no P (C₁), chemical fertilizer (C₂), 25 % organic fertilizer substitution based on N addition (M₁), 50 % organic fertilizer substitution (M₂) and 100 % organic fertilizer substitution (M₃). The results indicated that the overall yield of the three organic fertilizer substitution treatments was lower than that of the C₂ treatment, but only M₃ and C₂ showed significant differences. Compared with the C₁ treatment, the C₂, M₁, and M₂ treatments did not increase TP, Olsen-P or labile-P, while their contents were high in the upper soil of M₃. Path analysis between different P fractions and Olsen-P revealed that resin-P, NaHCO₃-P_i, and NaOH-P_i had the greatest direct effect on Olsen-P, but NaHCO₃-P_o and NaOH-P_o might transform into Olsen-P through NaHCO₃-P_i and NaOH-P_i. Our results suggested that partial substitution of chemical fertilizers not only ensured yield but also did not increase the risk of P leaching, making it more feasible than full substitution with organic fertilizers. The substitution of organic fertilizer would lead to more conversion of organic P into inorganic P to maintain the soil Olsen-P level. These results contribute to a better understanding of the influence of the substitution of organic fertilizer on the P pool and thus clarify the appropriate proportion of substitution.