Phosphorus adsorption and desorption characteristics as affected by long-term phosphorus application in black soil

Objectives Comparison of P adsorption-desorption characteristics in long-term soil treated with and without P fertilizer can provide a theoretical basis for P management in black soils.

Methods The long-term field trial on black soil started in 1990 in Gongzhuling City, Jilin Province. The four treatments were no fertilizer (CK treatment), nitrogen (N) and potassium (K) fertilizers (NK treatment), NPK fertilizers (NPK treatment), and NPK + manure (NPK+M treatment). We analyzed soil physicochemical properties. Soil P adsorption-desorption characteristic parameters maximum adsorption capacity (Q_m), adsorption constant (K_Q), maximum soil P buffering capacity (MBC), degree of P sorption saturation (DPS), maximum desorption capacity (D_m), and desorption ratio (D_r) in different soil layers were determined using isothermal adsorption and desorption experiment.

Results The adsorbed and desorbed P increased initially and stabilized with increasing P concentration in the equilibrium solution. Compared to the untreated soil, the adsorbed P decreased, and desorped P increased in the soil treated with P fertilizers. Compared to the untreated soil, Q_m and MBC decreased by 4.94%−63.46% and 15.90%−75.18%, while D_r increased by 8.52%−474.0% in the 0−60 cm soil profile. Notably, the most significant changes were recorded in the NPK+M treatment. The long-term P input increased total P and soil organic matter (SOM) by 34.40%−145.5% and 12.77%−50.07%, decreased the free iron and aluminum oxide (Fe_d+Al_d) by 5.14%−11.35% in different soil layers compared to the soil without P fertilizer. SOM, Fe_d+Al_d, and total P in the soil without P fertilizer (P<0.05), as well as Fe_d+Al_d, organic-bound iron and alumina oxide (Fe_p+Al_p), pH, and SOM in the soil with P fertilizers were the main factors influencing the differences in the characteristic parameters of P adsorption and desorption. The main factors explained 77.59% and 90.62% of the total variation in the soil without P and with P fertilizers, respectively. The correlation between Olsen-P and DPS showed that the DPS threshold value for environmental purposes was about 8%. The DPS_M-P (calculated by Mehlich-3 extractable P, Fe, and Al) and DPS_O-P (calculated by Olsen-P and Q_m) ranged from 7.77% to 25.96% and 17.24% to 24.75%, respectively, in 0−60 cm soil profile in NPK+M treatment, which far exceeded the threshold, increasing the risk of P loss.

Conclusions Our research suggests that long-term exogenous P input decreases adsorbed P and increases desorbed P, respectively. SOM and Fe_d+Al_d are the main drivers of P adsorption and desorption characteristics in soil without P fertilizer. In the soil treated with combined P fertilizer, especially NPK fertilizers and manures, Fe_d+Al_d, Fe_p+Al_p, SOM, and pH were the main factors affecting the P adsorption and desorption characteristics.