Mechanisms Of Difference In Phosphorus Availability And Fertilizer P Use Efficiency In Three Soils Under Long-term Fertilizations

Continuous excessive application of phosphorus(P)fertilizer can cause P accumulation in soils,which will affect the P availability and fertilizer P use efficiency(PUE),waste the finite P rock resources and increase the risk of P loss.Based on three long-term(since 1990)experimental sites,soil samples were collected from five different fertilization treatments:control(CK);inorganic nitrogen(N)and potassium(K)(NK);inorganic N,P and K(NPK);NPK plus straw(NPKS);and NPK plus manure(NPKM)at 1990,2000/2002 and 2013,by using the Jiang and Gu method,31P-NMR,modified Hedley method and the batch sorptionr-desorption experiments,changes in soil P fractions and P sorption-desorption characteristics were analyzed,and the mechanisms of difference in P availability and PUE in three soils were explored.The main remain results were summarized as follows:(1)The Olsen P contents,P activation coefficient(PAC)and degree of P satuation(DPS)values were significantly increased in all three types of soils,after 23 years of long-term application of P fertilizer.Compared with the basic soils in 1990,the average Olsen P contents increased by 6.8,2.7,and 6.1 times,and the average PAC values increased by 3.8,1.3,and 1.4 times,and the DPS values increased by 6.3,6.5,and 6.6 times,at black soil,fluvo-aquic soil and red soil,respectively.There were big difference in PUE among soil types and fertilization treatments:the fluvo-aquic soil had the highest PUE,the black soil was the second,and red soil was the lowest;and the NPK and NPKS had a higher PUE than NPKM.(2)The application of P fertilizer can increase PAC in all three soils,mainly by regulating the ratio of inorganic P to organic P(Pi/Po),soil organic carbon content(SOC),and the ratio of carbon to organic P(C/Po),and also affected by changes in P sorption-desorption charateristics and soil physicochemical properties.After long-term P application,the proportion of labile P(Resin-P,NaHCO3-Pi and NaHCO3-Po)and moderate labile P(NaOH-Pi,NaOH-Po and Di1.HCl-Pi)in total P increased,and the proportion of non-labile P(Conc.HCl-Pi,Conc.HCl-Po and Residual-P)in total P decreased.Long-term P application can increase the ratio of Pi/Po,increase soil SOC content,and decrease C/Po value,especially under NPKM treatment.Stepwise multiple regressions showed that PAC was positive with Pi/Po and SOC,while negative with C/Po and CaCO3.In addition,the variance partitioning analysis showed that more variance of PAC is explained by soil factors(48%)than by P input(5%)and climate(12%)factors.(3)The application of P fertilizer can increase DPS,mainly due to the changes in NaHCO3-Pi,NaHCO3-Po and SOC,and also affected by P sorption-desorption behaviors and soil physicochemical properties.In all three soils,the application of P fertilizer can increase the contents of SOC,NaHCO3-Pi and NaHCO3-Po,decrease the P sorption energies(k)and the maximum buffer capacity(MBC),but the variation trend of the maximum P sorption capacity(Qm)varies with the soil type and the fertilization treatment.Principal component analysis showed that DPS was positively correlated with SOC and most of the P fractions,and negatively correlated with Qm,k,MBC,and Dil.HCl-Pi content.Moreover,there were change points at 25.4%,34.5%and 24.9 of DPS corresponding to readily desorbable P(RDP),NaHCO3-Pi and NaHCO3-Po,respectively.Regardless of soil types,DPS values of 25%can be used as threshold values to monitor whether the soil has reached a P leaching risk level.(4)The PUE was different in each soil type and fertilization treatment,and can be affected by soil properties,P fractions,and P sorption-desorption behaviors.Stepwise regression showed that PUE was mainly controlled by soil specific surface area(SA).Considering the post-effect of phosphate fertilizers,it is more reasonable to calculate the cumulative phosphorus use efficiency(PUEa)under long-term fertilization.At the same time,the PUEb(calculated by the balance method)is close to the PUEa,and this calculation is simpler and has certain reference value.In all three soils,the response of PUEa to Olsen P was analyzed,and it can be observed that:When the Olsen P content in the soil exceeds the agronomic critical value of Olsen P,PUEa remains unchanged or decreases with increasing Olsen P content.Especially when the Olsen P content exceeds the turning point of soil P fertility rate,the Olsen P content will increase sharply with the increase of total P content,which may lead to a rapid decline in PUEa.In order to achieve higher P utilization,crop yield and soil fertility,the Olsen P level in the soil should be adjusted between the agronomic threshold and the turning point of soil P fertility rate,with 13.4-25.2,10.4-26.5,and 29.4-40 mg kg-1 for Gongzhuling,Zhengzhou,and Qiyang,respectively.