Effects Of Long-term Fertilization And Soil Management On Phosphorus Fraction's Distribution And Availability On Loess Soil

Application of phosphate fertilizer is key important for the improvement of soil phosphorus level and the sustainable productivity of crops.However,phosphorus resource is limited and non-renewable.The efficient utilization of phosphorus has always been one of the hot spots in scientific research.When phosphate fertilizer was applied,soil P is easily adsorbed and fixed,which reduced the availability and directly affected their contents,forms,and their biological availability.In addition,phosphorus fractions and availability in soil are also affected by soil management practices,such as bare fallow or cropland abandonment.Therefore,it is vital to comprehensively understand phosphorus transformation and bioavailability affected by long-term fertilization and soil management,which serve improving the utilization efficiency of P fertilizer and prolonging the utilization of P resources.In this study,based on long-term field experiments(winter wheat-summer fallow cropping system,winter wheat-summer maize cropping system and different soil management practices)in Guanzhong Plain of Shanxi province started in 1990and laboratory incubation experiments,we investigated 1)dynamics of inorganic and organic phosphorus affected by different fertilization and soil management regimes;2)changes in soil phosphorus fractions and availability under long-term fertilization;3)distribution of phosphorus fractions in water-stable aggregates under long-term fertilization and soil management;4)availability and distribution of phosphorus fractions subjected to diverse acidification degrees following P fertilizer applications under different soil fertility levels.Long-term field experiment under winter wheat-summer fallow cropping system involved seven treatments,i.e.un-amended control(CK),nitrogen(N),N and potassium(NK),P and K(PK),NP,NPK,combination of NPK with organic manure(MNPK).Long-term field experiment under winter wheat-summer maize cropping system involved nine treatments,i.e.CK,N,NK,PK,NP,NPK,combined stalk/straw(S)with NPK(SNPK),combined of low quantities of organic manure(M1)with NPK(M1NPK)and combined of high quantities of organic manure(M2)with NPK(M2NPK).Long-term soil management regimes involved three treatments,i.e.the control under winter wheat-summer maize cropping system(CK),bare fallow(Fallow)and cropland abandonment(Setaside).Incubation experiment selected CK,NPK and M2NPK treatments under winter wheat-summer maize cropping system representing low,medium,and high soil fertility levels,each soil acidified with five pH gradients(pH 8.4,7.9,7.4,6.9 and 6.4),and P application with different rates(0,60,120,240 and 480 mg kg^-1)were added to every pH condition,respectively.The main results and conclusions obtained were as follows:(1)Long-term application of P fertilizer gave significantly higher contents of total P and inorganic P in soil and they showed the increasing trends with experimental duration.The annual increment was 15.5-48.0 mg kg^-1y^-1 and 8.7-29.0 mg kg^-1y^-1,respectively,for total P and inorganic P.Soil organic P showed a linearly increasing trend with the years of fertilization,especially for M1NPK and M2NPK treatments,with annual increments of 7.5mg kg^-1y^-1 and 19.0 mg kg^-1y^-1,respectively.Long-term bare fallow and cropland abandonment had no significant effect on the contents of total P,inorganic P,and organic P,but proportion of organic P to total P increased significantly.(2)Balanced application of N and P fertilizers,such as NP,NPK and MNPK,gave significantly higher wheat yields than CK,with annual yield increases of 205-265 kg ha^-1.Application of P fertilizer led to P surplus in soil and improved the P contents extracted by resin in water(Resin-P),Na HCO₃ solution(Na HCO₃-Pi)and diluted HCl(D.HCl-Pi).As compared with the initial values,balanced application of N and P fertilizers and organic manure with NPK considerably enhanced the proportions of Resin-P and Na HCO₃-Pi but reduced the proportions of organic P and concentrated HCl extracted inorganic phosphorus(C.HCl-Pi).The P uptake by winter wheat was observed to be positively and significantly correlated with the contents of Resin-P,Na HCO₃-Pi and C.HCl-Pi in soil.(3)Under winter wheat-summer maize cropping system,Tiessen-Moir fractionation procedure extracted more available phosphorus but underestimate the contents of iron and aluminum bounded phosphorus,and especially underestimated the total organic phosphorus fractionation compared with that derived from Jiang-Gu and ignition method.Dicalcium phosphorus(Ca₂-P)and octa-calcium bounded phosphorus(Ca₈-P)in Jiang-Gu method had the higher direct contributions to P uptake and Olsen-P in soil.Other fractions had indirect effects on P uptake and Olsen-P,among which apatite phosphorus(Ca₁₀-P)had the least effect.Na HCO₃-Pi and D.HCl-Pi in Tiessen-Moir method had more significant direct contributions to P uptake and Olsen-P.Other fractions exhibited indirect impacts on P uptake and Olsen-P,among which Resin-P had the largest indirect effect.Additionally,contributions of inorganic phosphorus fractions in soil to P uptake were obvious both from Jiang-Gu fractionation procedure and Tiessen-Moir method.Jiang-Gu method can better elucidate the quantity and bioavailability of different inorganic P fractions.(4)Long-term application of synthetic P fertilizer alone or combined with manure markedly improved the contents of inorganic P forms in aggregates and silt+clay fractions,especially application of synthetic fertilizers combined with manure.Additionally,long-term application of synthetic P fertilizer alone showed significantly higher Na OH solution extracted organic P(Na OH-Po)in macroaggregates(>2 mm),the aggregates of 2-0.25 mm and silt+caly fractions(<0.053 mm).But combination of synthetic fertilizers and manure increased the organic P extracted by Na HCO₃solution(Na HCO₃-Po)in the aggregates of0.053-2 mm and Na OH-Po in the aggregates of>2 mm and 0.25-0.053 mm.Long-term application of synthetic fertilizers alone or combined with manure considerably enhanced the proportions of Na OH-Po and concentrated HCl extracted organic phosphorus(C.HCl-Po)to total P in macroaggregates but decreased the proportions of different P fractions in the other aggregates and silt-clay fractions,especially application of synthetic fertilizers combined with manure.There was a significant positive relationship between contents of soil organic carbon and phosphorus fractions of Resin-P,Na HCO₃-Pi,Na HCO₃-Po,Na OH-Pi,D.HCl-Pi,C.HCl-Pi,the residual P fraction(Residual-P)in aggregates and silt+clay fractions.Therefore,long-term fertilization affected P fractions and availability in aggregates significantly,which was closely related to soil organic carbon and the distribution of aggregates.(5)Compared with cropping without fertilizer,cropland abandonment improved the contents of Resin-P in microaggregates(0.25-0.053 mm),Na OH-Po in macroaggregates(>2mm)and Na HCO₃-Pi&-Po,Na OH-Pi in aggregates.Cropland abandonment also increased the allocation of all the P fractions in microaggregates and reduced the allocation proportions of Resin-P,Na OH-Po and D.HCl-Pi to total P in the aggregates of 2-0.25 mm.However,long-term bare fallow only increased the contents of Na HCO₃-Pi in macroaggregates,Na OH-Pi in silt+clay fractions and decreased the contents of C.HCl-Pi and D.HCl-Pi in macroaggregates.Long-term bare fallow increased the allocation of P fractions in the aggregates of 2-0.25 and<0.053 mm,whereas reduced the allocation of P fractions in aggregates of>2 mm.There was also a significant positive relationship between contents of soil organic carbon and phosphorus fractions of Resin-P,Na HCO₃-Pi,Na OH-Pi,Residual-P,Na HCO₃-Po and Na OH-Po in aggregates and silt+clay fractions.Therefore,long-term different soil management practices affected soil organic carbon,thus changed the transformation and availability of phosphorus forms in aggregates.(6)In different fertility soils,Olsen-P increased linearly with increase of P fertilizer application rates under diverse acidification degrees.And the largest increase was observed in soil with low fertility level.Additionally,in different fertility soils with various pH values,content of Ca₂-P and Ca₈-P both increased linearly with increase of P fertilizer application rates.The highest increment of Ca₂-P was under soil with low fertility level,and the highest increment of Ca₈-P was under soil with high fertility.Contents of aluminum bounded phosphorus(Al-P)and iron bonded phosphorus(Fe-P)in soil at each pH gradient were not affected at medium and high fertility soils,while contents of Al-P and Fe-P raised with the increase of P fertilizer application rate significantly at low fertility soils.The contents of occluded phosphorus(O-P)and Ca₁₀-P at each pH gradient were not affected by P applications.Besides,the proportions of Ca₂-P and Ca₈-P to total P markedly enhanced,while the proportions of Ca₁₀-P to total P decreased with the increase of P fertilizer application rate at low fertility soils.In medium fertility soils,P fertilizer application only increased the proportion of Ca₂-P to total P.In high fertility soils,P fertilizer application rates had no effect on the proportions of P fractions to total P.The contents of Fe-P enhanced,while the proportions of it decreased with the increase of pH value.Acidification had no significant effect on the contents and proportion of soil inorganic phosphorus after the application of phosphorus fertilizer in different fertility soils.In summary,long-term application of P fertilizer gave significantly higher contents of total P and inorganic P in soil and they showed the increasing trends with experimental duration.Soil organic P also showed a linearly increasing trend with the years of fertilization,especially application of synthetic P fertilizer combined with manure.Application of P fertilizer improved the contents of Resin-P,Na HCO₃-Pi and D.HCl-Pi significantly under winter wheat-summer fallow cropping system,among which Resin-P and Na HCO3-Pi played a major role in phosphorus uptake of wheat.Under winter wheat-summer maize cropping system,Ca₂-P and Ca₈-P or Na HCO₃-Pi and D.HCl-Pi played the major roles in crop P uptake and soil Olsen-P.Long-term fertilization significantly affected P fractions and availability in aggregates,which was closely related to soil organic carbon and the distribution of aggregates.In addition,lowing soil pH had little effect on improving the availability of soil inorganic phosphorus after the application of phosphorus fertilizer.Long-term cropland abandonment and bare fallow had no effect on the contents of soil total P,inorganic P and organic P during experimental period.However,cropland abandonment increased the P availability in aggregates.While long-term bare fallow had little effect on improving the availability of soil P fractions in aggregates and silt+clay fractions.