Effect of Different Proportions of Mineral Nitrogen Fertilizer Substitutions with Organic Manure on Soil Phosphorus Fractions Distribution in an Entisol Profile

The partial substitution of mineral nitrogen (N) fertilizer with organic manure can mitigate the negative effect of N on the environment. However, little is known about whether the introduction of additional phosphorus (P) with different proportions of organic manure application promotes P migration in soil profile, which is closely related to P non-point source pollution. Five field fertilization experiments mineral N, P, potassium (K) fertilizers only (NPK) and 30%, 50%, 70%, 100% of mineral fertilizer N substituted with organic manure (PM30, PM50, PM70, PM100), were conducted to evaluate the effect of different proportions of organic manure substitution for mineral N fertilizer on distribution of P fractions in soil profile. P mainly accumulated in the soil in the form of inorganic P, and the total P (TP) and total inorganic P (TIP) showed increasing trend with increasing soil depth under organic manure substitution treatments. Detailed P fractionation data showed that compared with NPK treatment, labile inorganic P fractions mainly accumulated in surface soil under organic manure substitution treatments, while stable inorganic P fractions migrated vertically. Moreover, the high proportion of organic manure substitution treatment had a higher labile and stable inorganic P contents in the surface soil and the deep soil relative to the low proportion of organic substitution, respectively. The high proportion of organic manure substitution treatment would lead to a higher risk of P loss by surface runoff and leaching. Therefore, when substituting mineral N fertilizer with organic manure, P should also be considered, and the substitution proportion of organic manure needed to be optimized from the perspective of reducing the risk of P loss.