Effect Of Phosphogypsum On Acidic Yellow Soil And Its Mechanism

As the largest area of all zonal soil,the yellow soil covers 738.4×10⁴hm² accounting for 46.4%in total soil area in Guizhou Province.The yellow soil whose pH is below 5.5 occupied 41.2%in total yellow soils.The excessive acidity of yellow soil causes not only crop yields decline because of its anomalously growing,but also the loss of nutrient elements(such as potassium,calcium,magnesium,molybdenum and boron),microbiological activity degression,and heavy metal's increasingly activity.The aluminum toxicity and nutritional deficiency have become one of the major limiting factors of agricultural development and environmental conservation in region of acid soil.Improving acidity of yellow soil is always a problem in the development and ultilization of yellow soil resource.Guizhou Province is one of the mountainous areas lacking of plain,so the land resources are very scarce in Guizhou Province.Development and ultilization of yellow soil resource would have an important influence on agricultural development and food safety in Guizhou Province.In the long term,applying lime in acid soil is considered as a useful and economic method of ameliorating Al toxicity.It usually decreases acidity of topsoil and increases concentration of exchangeable Ca in topsoil.But the movement of surface-applied lime downwards the soil profile is extremely slow.If lime is applied in a long time or a great mass,it will form carbonate concretion which causes the disorder of balance between calcium, potassium and magnesium of soil and crop's output reduction.Phosphogypsum(PG) is a by-product of phosphate chemical industry,the amount of which is very enormous in Guizhou Province.There is 3 million ton PG in Guizhou Weng-Fu phosphorus fertilizer factory.PG is not being extensively utilized in building material for its hygroscopicity,so utilization ratio of PG is only about 10%.In present,the preservation of PG is mainly piled up in the open air.It occupies large space of land and is likely to become latent pollution source.It is reported that PG is used as amendment of acid soil and effect of surface-applied PG is superior to effect of surface-applied lime in alleviation of acid subsoil infertility.If PG is largely used for improving acidic yellow soil in Guizhou,it is very important to increase crop output and relieve stress of PG to environment in the air open.Nowadays, amending acid soil by lime has been a widely accepted practice to alleviate soil acidity,but the studies on the alleviation of Al toxicity by PG are few and mechanisms of ameliorating Al toxicity by PG are not clear. Therefore,it is vital to study on the mechanisms of amending acid soil by PG.In this research,the control experiment on effects of several amendments was first conducted to verify effects of ameliorating Al toxicity by PG in the field.Then by means of pot experiment and chemical analysis systematical research on effects of crop growth,nutrient balance and physiological response in plant and soil chemical property on PG is fulfilled so as to probe why PG's effect is more than lime's effect. Third,the research on components and concentrations of Al in yellow soil and influence factors were carried out by sampling outside and indoor chemical analysis.Fourthly,systematical research on effects of Al forms on applying PG was done to make clear the mechanisms of alleviating Al toxicity acid soil by PG. In the end,evaluation of using PG to soil environment was fulfilled based on the state criterion in order to put forward scientific support of meliorating acid soil by PG.The Following are the results of the research:The control experiment results showed the output of maize and wheat of applying PG,phosphatic slag (PS),flying coal ash(FCA) and lime(L) has increased by 6.9%～12.6%,5.4%～6.9%,4.0%～5.4%,2.2%～2.8%as compared with control treatment in acidic yellow soil,respectively.Compared with using lime,ratios of maize increase of using PG,PS,and FCA have risen to 4.0%,2.3%and 1.2%, respectively.The maize biological parameters(such as plant height,row per ear,grain per row,weight per 100 grain) of using PG were significant difference with these of control treatment.But one parameter (weight per 100 grain) of using lime was significant different from that of control treatment.Adding PG could increase concentration of available nitrogen(N),phosphorus(P) and potassium(K) in acid soil.The content of soil available P of adding PG was twice that of control treatment.The capacity of increasing available nutrient of PS was lower than that of PG.There were no difference between FCA,L and control treatment in content of soil available nutrient.Obviously,PG was superior to lime in improving acid soil.Results on pot experiment of applying PG in acidic yellow soil indicated that:first,main growth index of broomcorn plant and root have obviously increased in accompany with increase of PG dosage.Plant height of broomcorn and dosage of PG were quadratic correlative.The regression equations were: y_PG=14.99+9.39x-0.96x²(R=0.986);y_L=14.98+7.92x-0.77x²(R=0.984).Plant dry weight of broomcorn and dosage of PG and L were similarly quadratic correlative.The regression equations were: y_PG=6.88+11.92x-1.65x²(R=0.983);y_L=6.88+6.39x-0.72x²(R=0.986).The value of b₁ and b₂ in two equations reveals that ratio of increase of plant height or plant dry weight through using PG were larger than these through using lime.There were similar correlative between root dry weight of broomcorn and dosage of PG or lime.Comparing with control treatment,the contents of nutrient elements such as N,P,K, Ca in broomcorn plant were very significant increase,while content of Al in broomcorn plant was obvious decline through using PG in yellow soil.Among four nutrient elements,the increase of P was the highest, reaching 117%～200.0%.So the absorption of N,P,K,and Ca in broomcorn plant by using PG is higher than that by using L.Both of them were higher than that of control treatment.Secondly,applying PG could regulate the balance of N,P,K and Ca in broomcorn plant as a result of increasing ratio of N/Al,P/Al,K/Al and Ca/Al in broomcorn plant.The ratios of N/Al,P/Al,K/Al and Ca/Al in broomcorn plant by using PG were 74.6～77.6,10.5～11.4,56.8～64.3 and 24.8～30.3,respectively,which have multiplied 1.9～2.0,5.8～6.3,1.8～2.1 and 2.0～2.5 times as against contrast,respectively.Meanwhile,the contents of available P and K of using PG in acid soil have increased 41.0%～114.2%and 59.4%～67.5%respectively more than those of using L,while soil exchangeable Al(Ex-Al) has decreased in an extent through using PG or L. Thirdly,the physiological active indexes such as chlorophyll meter readings(SPAD), superoxide-dismutase(SOD),peroxidase(POT),catalase(CAT) of broomcorn leaves were raised by PG. SPAD reading of broomcorn leaves has obviously increased in accompany with increase of PG dosage. SPAD reading of broomcorn and dosage of PG were quadratic correlative.The regression equations were: y_PG=16.37+7.32x-1.25x²(R=0.981);y_L=16.02+4.06x-0.87 x²,(R=0.977).The activity of SOD and CAT of broomcorn leaves has also heightened by using PG or L,and the content of proline of broomcorn leaves is reduced by 13.6%～34.9%and 11.5%～32.1%respectively by using PG or L.These results mean that protected enzyme system in broomcorn plant was recovered through improving excessive acidity of soil by applying PG or L.Results on Al forms in strongly acidic yellow soil illuminated that:the range of Ex-Al in the yellow soil covered 0.005～1.079 g/kg,average of Ex-Al in the yellow soil was 0.326±0.305g/kg (n=20,C.V.=93.5%).The concentration of Ex-Al about 70%yellow soil was over 0.2 g/kg,which was much superior to tolerant dose of general plant to water-soluble Al.The average of hydroxyl-polymeric Al(Hy-Al) and organic-ligand Al(Org-Al)were 1.039±0.353g/kg and 0.291±0.064g/kg respectively,accounting for about 2.5% in total Al of soil,which were in dynamic equilibrium with Ex-Al and cnange with soil condition.In addition,Ex-Al in yellow soil had very significant positive correlation to exchangeable acid(Ex-A) in soil and the linear equation was y_Ex-A=0.212+1.828x_Ex-Al(R=0.870).Ex-Al in yellow soil had very significant negative correlation to active acid(pH) in soil and the linear equation was y_pH=5,024-1.828x_Ex-Al(R=-0.900). Likewise,Org-Al in yellow soil had very significant positive correlation to organic matter(OM) in soil and the linear equation was y_Org-Al=0.188+0.003x_OM(R=0.668).The amount of Ex-Al,Hy-Al and Org-Al was called as active Al,accounting for about 3.0%in total Al of soil.The changes of active Al in yellow soil should be concerned because the ratio of active Al in yellow soil was higher than that in red soil.Results on incubation trial and chemical analysis of using PG in strongly acidic yellow soil revealed that:with increase of PG's dosage,the amount of Ex-Al and Hy-Al in yellow soil decreased by 43.2%～46.1%and 7.0%～21.1%,respectively.However,the amount of Oog-Al in soil had slight augment with increase of PG's dosage.Accumulation of decline rate of Ex-Al and Hy-Al in yellow soil had parabola correlation with PG's dose.The correlative coefficient of eqution reached 0.979 and 0.989 respectively. Similarly,accumulation Org-Al in yellow soil had parabola correlation with PG's dose.The correlative coefficient of eqution added up 0.988.The extremum of parabola eqution indicated that the suitable dose of PG was 4.5g/kg;the suitable temperature of using PG was 20～30℃and the period when effect of PG's improving lasted was 2～4 months after applying PG.Results on valuation of impact of applying PG to soil environment indicated that:the concentration of five heavy metals(Hg,Cd,As,Pb,Cr) in PG were under limit of two state criterion(GB8172-87 and GB4284-84).The contents of heavy metals in grain of maize,peanuts and wheat were lower limit of state feed criterion after using PG.Moreover,the contents of heavy metals in soil has no obvious increase after using PG.Applying PG in extent of 1500～2250kg/hm²(equal to 100～150 kg/667m²) was safety to people and animal,which did not cause the second-pollution to soil and crop grain.Results in this research pointed out that there were a great deal of Ca²⁺,SO₄^2-,PO₄^3- etc.in PG.These ions could react with Ex-Al[Al³⁺,Al(OH)²⁺,Al(OH)₂⁺]in soil.These reactions included:1) Ion exchange, whereby OH^- released as a result of ligand exchange between SO₄^2- and OH^- resulted in polymerization or precipitation of Al;2)Precipitation of aluminum sulfate minerals due to an increase in SO₄^2- concentration in soils;and 3)Formation of the AlSO₄⁺ ion-pair,which was less toxic to root growth than the other monomeric species.These reactions alleviated Al toxicity in yellow soil by transforming Ex-Al into inert Al.In addition,concentration of Ca²⁺ in soil retarded absorption of Al in plant due to ion competition. These maybe was the mechanisms that PG is different with lime in ameliorating acid soil.