Objectives The adsorption and desorption process influences the behavior of P in soil. The effect of long-term fertilization on the adsorption and desorption of soil P was studied, to provide theoretical basis of rational fertilizer application and improve the P availability of yellow soil.

Methods Conducted between 1995 and 2016 on a yellow soil in Guiyang City, the long-term fertilization experiment consisted of five treatments: blank control (CK), no P fertilizer input (NK), NPK chemical fertilizer (NPK), manure alone (M) and NPK plus manure (MNPK). The soil physico-chemical properties were determined. Isothermal adsorption and desorption experiment was conducted to determine the maximum P adsorption capacity (Q_m), P adsorption constant (K), P adsorption free energy (|ΔG|), buffering capacity of soil P (MBC), P saturation degree (DPS), P immobilization amount (PI), P desorption rate (DR).

Results Long-term fertilization treatments significantly affected soil physical and chemical properties. M and MNPK treatments significantly reduced soil clay content, increased soil organic matter content and pH, and increased soil total P, Olsen-P and microbial biomass P content. NK and NPK treatments had no significant effect on soil clay content, but decreased soil organic matter and pH. NK treatment reduced soil P content and availability. Langmuir equation was proved well to predict the absorption and desorption of P in soil ( R ²=0.953–0.995; P＜0.01). Compared with CK, Q_m of NK and NPK treatments increased by 39.2% and 40.9%, respectively, while that of M treatment decreased by 20.0%, and there was no significant difference in MNPK treatment, which indicated that chemical fertilizer application could increase the adsorption sites of soil for P, while manures application could reduce the adsorption sites, and the adsorption sites changed little when combined application of manures and chemical fertilizers; K, |ΔG|, MBC and PI were such that MNPK≤ M< CK < NPK < NK, while DPS followed this order–MNPK >M > NPK > CK > NK, average of DR followed this order–MNPK>M>CK>NPK>NK. When the concentration of addition P was less than 10 mg/L, the distribution proportion of P in solid phase and solution phase in NK and NPK treatments was 85.8–100.3, CK treatment was 4.2–28.8, M and MNPK treatments was 2.5–7.7, which indicated that when the added P concentration was low, most of the P in the treatments of long-term application of chemical fertilizer was absorbed by the soil, which was difficult to be used by crops. However, most of the P in the treatments of long-term application of manures remained in the soil solution, which was conducive to the absorption and utilization by crops. Correlation analysis showed that the correlation coefficients of Q_m, K, |ΔG|, MBC and PI with soil clay content, pH and organic matter were greater than those with soil P nutrient, which showed that the influence of clay content, pH and organic matter on the characteristic parameters of soil P adsorption and desorption was greater than that of soil P nutrient.

Conclusions In conclusion, under the long-term fertilization conditions, the effect of fertilization structure on P adsorption and desorption characteristics of yellow soil is greater than that of P application rate. Long-term application of manures could reduce the adsorption of soil P, promote its desorption, and improve its availability in the soil, while long-term application of chemical fertilizer is on the contrary. Long-term combination of NPK fertilizers with manures appear to be a better fertilization mode in yellow soil, because it can integrate the advantages of organic fertilizer and chemical fertilizer. When the P concentration is low, it can decrease the soil adsorption capacity and increase desorption capacity, and improve P availability in soil. When the P concentration is high, it can reduce the desorption rate of soil P and reduce the risk of P loss. However, in this study, the risk of P loss in MNPK treatment is high, because of long-term excess P fertilizer application resulted in the P accumulation.