Increment of soil phosphorus pool and activation coefficient through long-term combination of NPK fertilizers with manures in black soil

Objectives Long-term fertilization affects soil phosphorus (P) fraction and availability. The change of soil P and P fractions contents and the availability of soil P under long-term different fertilization were investigated, which will offer scientific support for P management in the black soil.

Methods The used long-term fertilization experiment was started in 1990 on a black soil in Gongzhuling City, Jilin Province. The cropping system was continues mono maize. The selected four treatments included no fertilizer (CK), applying urea and potash sulfate (NK), applying urea, super-calcium phosphate and potash sulfate (NPK), and NPK plus pig manure (NPKM). For the three chemical fertilizer treatments, the corresponding inputs were N 165 kg/hm², P 36 kg/hm² and K 68 kg/hm². The pig manure input was 21 t/hm², and the N−P−K input were 165−75−145 kg/hm². Soil samples were collected for every five years (1990, 1995, 2000, 2005 and 2010). Using the Hedley sequential extraction method, the total P and Olsen-P and P fraction contents were measured, and the soil P activation coefficient(PAC) was calculated.

Results With the extension of the experimental years, the soil total P and Olsen-P contents and PAC in treatments of CK and NK displayed downward trends, while those in NPK and NPKM showed upward trends. Compared with the beginning samples, the soil total P and Olsen-P contents and PAC in NPKM treatment were respectively increased by 88.47%, 12.98 and 6.42 times; and those were decreased by 15.12% and 32.67%, 33.47% in CK and by 12.57%, 52.49% and 2.55% in NK treatment; and the Olsen-P contents and PAC were respectively increased by 1.65 and 2.67 times, and the total P contents were not significantly increased in NPK. The inorganic P was increased significantly in NPK and NPKM treatments, while the organic P was not. The inorganic P was the main fraction of soil P in black soil, accounting for 80.53%−90.43% of the total P. Mid-liable P was the main fraction in the P pool of black soil, and accounted for 50.37%−55.06% of the total P, while labile P fraction accounted for 7.61%−19.02%. The ratio of labile P fraction in total P, especially in inorganic P, was increased significantly in NPK and NPKM treatments. Resin-P, NaHCO₃-Pi, NaOH-Pi, Conc. HCl-Pi, Residual-P contents were decreased in CK and NK treatments, Resin-P, NaHCO₃-Pi, NaOH-Pi, Dil.HCl-Pi contents were increased and Conc. HCl-Pi, Residual-P contents were decreased in NPK treatment, all inorganic P fractions were increased in NPKM treatment. The variation of Olsen-P was significantly correlated with the contents of Resin-P, NaHCO₃-Pi, NaOH-Pi, Conc. HCl-Po, and the coefficient was 0.972, 0.665, 0.860 and 0.605 in turn.

Conclusions Long-term combined application of NPK fertilizers plus manure not only has caused the significant increase of total P, Olsen-P contents, but also increased the ratio of labile P to inorganic P, and increased the availability of soil P. Therefore, the combined application of NPK and manure is recommended for the effective improvement of P fertility in black soil.