Long-term Phosphorus Inputs Regulate Phosphorus Transformation And Acid Phosphatase Activity In The Profile Of Red Soils

Phosphorus（P）is an important element required for all life on earth,and concerns over the future of its global supply have been on the rise.Soil P could be assimilated by microbial immobilization or adsorbed by soil colloids into a relatively stable form in the soil.Also,P can be immobilized by microorganisms to form microbial biomass P,and then can be further mineralized or assimilated into more complex organic P forms.In particular,assimilation by microorganisms into microbial biomass P has become one of the important processes of soil P transformation and improving soil fertility.Fertilizer application can regulate soil P transformation by affecting soil properties,microbial biomass and extracellular enzymes activity.Phosphatase enzymes activities in acidic soil play a key role in sustaining long-term P availability through efficient organic P cycling.However,the link between phosphatase activity and soil P fractions remains unknown under varied long-term fertilizations,soil depths,and soil conditions.Red acidic soil are highly weathered and characterized by low p H,hence,long-term fertilization can significantly influenced their properties and nutrient efficiency.Therefore,it is important to elucidate the dynamics of soil P fractions,its driving factors and underlying mechanisms at both surface and subsurface layers.In this study,the experiment was conducted at two red soil sites with the same climate type and parent material.At both sites upland and paddy soils were selected.Four fertilizer treatments including control（CK）,chemical nitrogen and potassium fertilizer（NK）,chemical nitrogen,phosphorus and potassium fertilizer（NPK）,and chemical NPK plus manure（NPKM）were selected,based on the long-term experiments in Jinxian and Qiyang.Qiyang paddy has no NK alone,so NKM was selected,as it can be compared with NK.At Jinxian,34-year（upland）and 39-year（paddy）fields were selected,whereas,at Qiyang 30-year（upland）and 38-year（paddy）fields were selected.Firstly,soil organic carbon（SOC）,p H,soil microbial biomass C and P,soil P fractions and related enzyme activities in the0-80 cm were investigated.Secondly,the effect of exogenous C（glucose）and P（potassium dihydrogen phosphate）addition on soil P transformation was also investigated to clarify the underlying mechanism.The treatments were glucose（+C）,phosphorus（+P）and their combination（+CP）.Finally,the relationships between soil chemical,microbial and enzymes activity with soil P fractions were established by linear regression models,redundancy analysis（RDA）,and structural equation modeling（SEM）to provide a theoretical basis for an in-depth understanding of P transformation in red acidic soils.The main findings of the three experiments were as follows:1.The NPKM increased the quantities of resin-P,Na HCO₃-Pi,Na HCO₃-Po,Na OH-Pi,Na OH-Po,and dil.HCl-Pi fractions in the 0-80 cm at both sites.At the 0-80 cm,NPKM had larger concentrations and fractions of labile and moderately labile P pools than NPK.At Qiyang,Ac P activity was significantly higher under NK than in the NPK.At both sites,Ac P activity decreased with increasing soil depth.The predominant soil P fractions under NPK were Na OH-Pi,dil.HCl-Pi,and conc.HCl-Po,whereas resin-P,dil.HCl-Pi,and conc.HCl-Pi prevailed under NPKM.According to RDA results,the primary factors influencing variations in soil P fractions at different soil layers were SOC,SMBC,SMBP,BG（β–Glucosidase）,and Ac P activity.Different soil P pools were positively associated with Ac P activity at different soil layers,except at the 40-80 cm,where Ac P activity was inversely related to non-labile P pool.Organic manure inclusion can increase the labile P concentrations of different soil layers,which should reduce the inorganic P fertilizer need for crop growth at topsoil but may increase the danger of P loss from subsurface to ground water if the P is not properly absorbed by the crops.2.The NPKM enhanced the amounts of resin-P,Na HCO₃-Pi,Na HCO₃-Po,Na OH-Pi,Na OH-Po,and dil.HCl-Pi fractions in the 0-80 cm at both sites.In the 0-80 cm soil profile,NPKM had larger concentrations and fractions of labile and moderately labile P pools than NPK.Non-labile P pools（conc.HCl-Pi,conc.HCl-Po,and residual-P）were more prevalent in the P-added treatments than in the CK and NK at both sites.The NPKM had greater fraction of non-labile P pools in soils treated with P fertilizer.The NK had lower Ac P activity than NPK at Jinxian site,whereas NKM had substantially higher Ac P activity than NPK at Qiyang site.Ac P activity decreases with increasing soil depth at both sites.According to RDA results,soil p H,SOC,SMBP,CBH（1,4β–Cellobiosidase）,and Ac P activity were the major elements influencing soil P fractions in the topsoil and subsoil.according to the linear regression model,different pools of soil P were positively related with Ac P activity at 0-80 cm.Na HCO₃-Pi and conc.HCl-Pi were the dominant soil P fractions under NPK,whereas resin-P,Na HCO₃-Pi,Na OH-Pi,and conc.HCl-Pi prevailed under NPKM.These data suggest that long-term double-rice cropping with P fertilization（particularly manure incorporation）may result in soil P fraction accumulation at both the surface and deeper layers.Soil organic matter enrichment can also have an effect on the amount of soil microbial biomass,which can contribute to the accumulation of non-labile P components.3.For both soils,+C and+CP addition increased Ac P activity while decreasing labile-P pool.On days 14 and 28,moderately labile-P pool was significantly higher for both upland and paddy soils when compared to labile and non-labile P pools.The concentration and proportion of Na HCO₃-Po and Na OH-Po were higher in upland soil compared to Na HCO₃-Pi and Na OH-Pi,whereas the opposite was observed in paddy soil.Ac P activity in both soils was significantly lower on days 14 and 28 in the+P than in the+C and+CP.The RDA results showed that RDA-1 and RDA-2 contributed 96.27%and2.77%,respectively,in upland soil and 95.62%and 1.67%,respectively,in paddy soil and that SMBP content alone accounted for more than 80%of the variations in soil P fractions.The SEM demonstrated that the increase in the moderately labile-P pool after+CP addition was principally explained by an increase in SOC.This study therefore indicates that resin-P,Na HCO₃-Pi,Na HCO₃-Po,and Na OH-Pi were more advantageous to soil microorganisms.Overall,the mobilization and lability of labile and moderately labile P fractions in the profile of upland and paddy red soils could be increased by the addition of organic amendments generated from pig manure and cow dung.However,by improving phosphorus uptake and use efficiency,the prospective P loss and accumulation brought on by elevated labile and moderately labile P fractions could be hindered.