Response of wheat yield and soil phosphorus fractions to long-term fertilization under rainfed winter wheat–summer fallow cropping system

Objectives Selection and optimization of fertilizer managements are very important for the improvement of phosphorus resource use efficiency. In this paper, the content of inorganic and organic phosphorus fractions of soils after being subjected to diverse long-term fertilization was determined, which will provide theoretical base for the choose of P management under a specific cropping system in loess soil.

Methods The research was based on a 20-years’ fertilization experiment under rain-fed winter wheat–summer fallow cropping system (since 1990) on a loess soil. The long-term experiment was composed of seven treatments as no fertilizer control (CK), urea (N), urea and potassium sulfate (NK), super calcium phosphate and potassium sulfate (PK), NP, NPK, combination of NPK with organic manure (MNPK). The soil samples were collected in the top 0−20 cm layer. By employing the P fractionation method proposed by Hedley and lately modified by Tiessen and Moir, we investigated the inorganic and organic phosphorus fractions. We also analyzed the relationship between the above-ground phosphorus uptake of winter wheat and the contents of soil phosphorus fractions, and calculated the soil P balances of the corresponding treatments.

Results The application of NP, NPK and MNPK brought significantly higher wheat yields than those of CK, N, NK and PK did, with annual yield increases of 205−265 kg/hm², and the maximum grain yield was observed on MNPK plot. NP, NPK and MNPK fertilization also significantly enhanced the P uptake in both grain and straw as compared to CK. Compared with CK, soils in treatments of PK, NP, NPK and MNPK showed substantially higher inorganic phosphorus contents extracted by anionic exchange resin (Resin-P), NaHCO₃ solution (NaHCO₃-Pi), NaOH solution (NaOH-Pi), concentrated HCl (C.HCl-Pi) and the residual P fraction (Residual-P), while decreased the content of organic phosphorus extracted by NaHCO₃ and NaOH (NaHCO₃-Po, NaOH-Po). Phosphorus application showed no effects on the contents of diluted HCl extracted inorganic phosphorus (D.HCl-Pi) and concentrated HCl extracted organic phosphorus (C.HCl-Po), while MNPK significantly increased the contents of D.HCl-Pi and C.HCl-Po. Although there were no clear changes in the contents of Resin-P, NaHCO₃-Pi, NaOH-Pi, C.HCl-Pi and D.HCl-Pi, the N and NK treatments significantly enhanced the contents of Residual-P and C.HCl-Po over CK treatment and decreased the contents of NaHCO₃-Po and NaOH-Po. Compared with the initial soil sample, MNPK considerably enhanced the proportions of labile inorganic P to total P but decreased those of the Residual-P and organic P. The above-ground P uptake was positively and significantly correlated with the contents of Resin-P, NaHCO₃-Pi and C.HCl-Pi in plough layer soils. The contents of D.HCl-Pi, Resin-P and NaHCO₃-Pi increased with the increasing P balance.

Conclusions All the combined application methods of NP, NPK and MNPK have not only generated desired grain yield but also maintained high proportion of available phosphorus pool in plough layer soils under the tested soil condition.