Abstract :
[en] Application of phosphorus (P) fertilizer is an important agricultural measure to increase soil P supply capacity and crop yields. Long-term P fertilization has greatly increased the soil P level, which also lead to waste of phosphate rock resources and diffuse source of water pollution in China. The activation of existing soil P by crop plants has become a major issue in the sustainable management of soil fertilization. The transformation of P in soil is a complex process depending on physical, chemical, and biological reactions. However, there is a lack of systematic research in the process and factor analysis of the effect of P fertilizer on the soil P availability is needed.
In this study, a 29-year fertilization experiment on a black soil in Gongzhuling City, Jilin Province in China was sampled. Firstly, we analyzed the changes of P forms with multiple methods and soil properties in topsoil on time scale. This part comprehensively evaluated the P forms, quantified contribution rate of P forms to P availability, and identify the effects of fertilization on P forms. Secondly, we analyzed the changes of P forms with modified Hedley sequential method and soil properties on vertical scale. This part identified the vertical distribution of different P forms, the environmental risk of P loss, and the main soil factors affecting the P forms at each soil depth. Finally, we performed batch experiments to investigate the characteristics of P adsorption-desorption and identify the mechanism of soil properties affecting the availability of P without and with P fertilizers. All these results will expound the effect of different fertilizer treatments on P availability. The main conclusions are as follows: The experiment of P forms on the time scale showed that fertilization promoted the accumulation of labile P and mid-labile P fraction in the soil, decreased the proportion of stable P in the soil, especially in the chemical fertilizers (NPK) plus manure (NPKM) treatment. Further, XANES analysis showed that accumulated inorganic P (Pi) were dominated by aluminum phosphate (Al-P) and monobasic calcium phosphate monohydrate (MCP) for NPK treatment, and by Al-P, MCP and tricalcium phosphate (TCP) for NPKM treatment. NaHCO3-Pi and NaOH-Pi were the potential sources of resin-Pi, which decided the P availability. Redundancy analysis (RDA) test showed that soil organic matter (SOM) and organic-bound iron and alumina oxide (Fep+Alp), increased by fertilization, were the main factors influencing the transformation of P fractions. These soil factors increased activity of soil P fractions and reduced the P fixation in soil.
The experiment of P forms on the vertical scale showed that long-term NPKM treatment significantly increased total P content. and Olsen-P concentration far III IV exceeded environmental threshold for soil Olsen-P (50.6 mg kg–1) in the 0–60 cm soil profile. Pi was the main form in soil profile. Under the conditions of P deficiency, labile and stable P were the main reduced P fractions in the soil. The application of P fertilizer mainly promoted the accumulation of labile and mid-labile P. Moreover, resin-extracted P (resin-P), NaHCO3-Pi, NaOH-Pi and dilute HClextracted P (dil.HCl-Pi) would move to deeper soil layer when excess P was accumulated in the topsoil. The distribution and availability of soil P fractions were affected by soil properties in the profile. For topsoil (0-20 cm), the increase of SOM, mehlich-3 extractable iron (M3-Fe), and Alp caused by fertilization, were the main factors influencing the differences in the P fractions. For subsoil (20-60 cm), soil mineral components, i.e., free Fe/Al oxide and CaCO3, were the main factors influencing the P fractions.
In comparison with soil without P fertilizers, the values of maximum adsorption capacity (Qm) and maximum buffering capacity of soil P (MBC) were decreased, while desorption ratio (Dr) and maximum desorption capacity (Dm) increased in the soil with P fertilizers. It indicated that fertilization could reduce the adsorption of soil to P. Long-term P input significantly increased the content of total P and SOM and decreased the Fed+Ald when compared to that in the soil without P fertilizers.
SOM, Fed+Ald and total P in the soil without P fertilizers, as well as Fed+Ald, Fep+Alp, pH and SOM in the soil with P fertilizers, respectively, were the main factors influencing the differences in P adsorption and desorption characteristic parameters.
In conclusion, P forms were the most important factors affecting P activation coefficient (PAC). NaHCO3-Pi and NaOH-Pi were the potential sources of resin-Pi and drove the P availability. Fertilization, especially in the presence of organic amendment, could improve P availability mainly by enhancing the high active P fractions and decreasing the P adsorption capacity of soil. Field experiment results confirmed that the change of SOM and Fe/Al oxides caused by fertilization were the main factors affecting P availability. Moreover, excessive P input would not significantly increase crop yield, but increase the soil available P to levels which exceed the environmental threshold and increase the risk of P loss to environment.
