Effect of long-term fertilization on phosphorus and zinc fractions in a Mollisol

【Objectives】 Phosphorous fertilizer is often applied in a Mollisol with high fertility for obtaining higher crop yields in Mollisol regions in Northeast China which are important grain production bases in China. It could provide guarantee for security of food and environment by researching the effect of long term fertilization on soil phosphorous (P) and zinc (Zn) fractions.【Methods】 Different P and Zn fractions were tested based on a 13-year long term field experiment in a typical Mollisol region. There were 5 treatments, no fertilizer (CK), chemical fertilizer (NPK) and chemical fertilizer plus different amounts of organic manure (NPK+OM1, NPK+OM2 and NPK+OM3). 【Results】 The content of organic P (T-Po) was not significantly affected under the NPK treatment, while the content of inorganic P (T-Pi) was increased significantly, and the content of Olsen-P is 32.6 mg/kg in CK treatment and was increased significantly in the treatments with combination application of chemical fertilizer and organic manure. P was accumulated in soil at a rate of 16 kg/hm2/year during 13 years of the P fertilizer application, the cumulated P mainly included fractions of Al-P and Fe-P, about 20% of which was Olsen-P. After 13 years of the combination application of P fertilizer and organic manure the contents of T-Po and T-Pi were increased by 47.2-67.9 mg/kg and 253.4-410.6 mg/kg, respectively, and the P accumulated rates in the NPK+OM1, NPK+OM2 and NPK+OM3 treatments were 49.4, 64.2 and 70.4 kg/hm2/year, respectively, 15%-21% of which was Olsen-P including mainly Ca2-P, Ca8-P, Al-P and Fe-P fractions. About 10%-21% P was stored in soil as T-Po fraction, and the proportion was decreased with the increase of the amount of organic manure application. The DTPA-Zn contents were increased significantly in NPK, NPK+OM1, NPK+OM2 and NPK+OM3 treatments, and the contents was as high as 1.96 mg/kg in the CK treatment. The contents of Exch.-Zn and AFeO-Zn in the NPK treatment were increased due to soil acidification resulting from the long term chemical fertilizer application. The combination application of chemical fertilizer and organic manure increased significantly the contents of different Zn fractions with exception of the Res-Zn content, and 55%-88% of Zn stored in soil was AFeO-Zn fraction. The path analysis result showed that the Ca2-P content impacted directly DTPA-Zn content, and the Ca8-P, Al-P and Fe-P contents impacted indirectly the DTPA-Zn content by the Ca2-P content. 【Conclusions】 P was stored in soil as high available T-Pi after the application of P fertilizer, and Zn was stored in soil as mainly AFeO-Zn fraction. The antagonism between P and Zn was not observed in the Mollisol, and the Ca2-P content had the largest impact on the DTPA-Zn content, and a positive relation was found.