Variation of soil phosphorus under long-term fertilization in red paddy soil

[Objectives]The study was aimed to clarify the change of the total and available phosphorus contents in arable layer of red paddy soil under long-term fertilization practices,to find relationship between the available phosphorus content and the accumulation or loss of phosphorus,and find out the most effective fertilization method of improving yield and soil fertility.[Methods]The experiment was in a double rice cropping system and composed of different fertilization treatments:nitrogen and potassium(NK),NPK fertilizer(NPK),two times NPK fertilizer(NPK2),NPK fertilizer with milk vetch in early rice and with pig manure in late rice(NPKM),and no fertilizer(CK).Irrigation and pesticide were applied following the local management practices,and rice cultivars were replaced periodically.The available and total phosphorus contents in top layer soils of the five fertilization treatments from 1981 to 2012 were analyzed.The relationship between incremental available phosphorus and the surplus or loss phosphorus amount was calculated.[Results]The annual average P deficiency of CK is 22.7kg/hm2,while the available phosphorus content maintains balance at a low level after 32 years.The annual P deficiency of the NK treatment is 27.9 kg/hm2,which is significantly higher than that of CK,and the total phosphorus content is reduced slowly by 8.6%after 32 years,while the available phosphorus is not changed.The average annual P surplus of the NPK,NPK2 and NPKM treatments are 33.3,39.0 and 41.0 kg/hm2,respectively,the total phosphorus contents are increased by 32.1%,89.4%and 165.1%,respectively,and the available phosphorus contents are increased by 2.2,6.9 and 15.3 times,respectively(the annual increase rates are 0.30,1.18 and 1.79 mg/kg,respectively).The increases in the available phosphorus contents are significantly related to the accumulation or loss of phosphorus.Inorganic phosphate cooperated with organic phosphate can improve the soil total phosphorus content as well as the available phosphorus.The rice yields are increased with the increases of the accumulated phosphorus inputs and available phosphorus,and the increase is quick in the early years and slow down in the later years.The yields are increased slowly when the available phosphorus is higher than 20 mg/kg.[Conclusions]In the red paddy soil,the long-term fertilization without phosphate incorporation slowly reduces the soil total phosphorus content,and the available phosphorus content is maintained at lower level.In contrast,the long-term fertilization with phosphate incorporation increases both the soil total and available phosphorus contents and enhances phosphorus efficiencies,and the fastest increase is in the treatments of inorganic phosphate cooperated with organic phosphate.The rice production is increased slowly when the available phosphorus content is higher than 20 mg/kg.The cooperated application of NPK and organic materials is an effective measure in increasing the soil phosphorus capacity and phosphorus availability of arable layer.