Impact Of Long-term Fertilizations On Mn Oxide,Phosphorus Fractions And Their Interactions In Paddy Soil Aggregates

Phosphorus(P)is known to be an essential nutrient for plant growth and development but simultaneously also one of the most limiting nutrients in terms of accessibility,as major proportion of applied P is rapidly modified into insoluble forms with low solubility in soils.Phosphorus consists of different fractions which are important,especially in the transformation process of P in soils.The distribution of these P fractions is also to a large extent dependent on soil aggregate sizes.Studies on how P becomes inaccessible to plants through adsorptive properties of mostly aluminum(Al)and iron(Fe)oxides,especially in acidic soils are ubiquitous.However,studies focusing on possible interactions of P with manganese(Mn)oxide,though also present in acidic soils is rather scarce,especially at the soil aggregate level.In an attempt to verify possible interactions between P fractions and Mn oxide within paddy soil aggregates,three different locations in southern China which included Nanchang(NC),Qiyang(QY)and Suining(SN)were selected,where paddy soils with different soil types,fertilizations and cropping systems were utilized.The general objectives covering all three sites of this study were to:1)assess how various long-term fertilizer compositions/ratios affected the distribution patterns of Mn,Al and Fe oxides,2)examine how P fractions and Mehlich 3-Mn(M3-Mn)oxide were distributed in paddy soil aggregates under varied soil types,nutrient compositions and cropping systems,and 3)assess the magnitude of relationships between these P fractions and pools with M3-Mn oxide in the studied areas.Fertilizer treatments for NC included:unfertilized control(CK),mineral nitrogen,phosphorus,potassium(NPK),30%mineral NPK+70%organic fertilizer(30F),50%mineral NPK+50%organic fertilizer(50F)and 70%mineral NPK+30%organic fertilizer(70F),with pig manure being the organic fertilizer source.QY treatments included:CK,NPK,cow manure(M),NPKM,PKM and NPM whiles SN treatments comprised:CK,NPK,pig manure(M)+NPK(NPKM),chemical nitrogen plus pig manure(NM)and mineral nitrogen,phosphorus plus pig manure(NPM).The various treatments from all studied locations were arranged in a ramdomized complete block design(RCBD)under paddy conditions.Assessment of distributions of Mn pools in bulk soil and aggregates under the different inorganic and organic fertilizer ratios listed above at both surface and subsurface depths was carried out in NC.In this study,a pattern of 30F>50F>70F>NPK>CK was established with positive and significant correlations existing among total organic carbon(TOC),total nitrogen(TN)and p H.Fertilizer treatment30F recorded the highest TOC and TN which were observed in the macro-aggregates(>2.0 and 2-0.25mm).The distribution of all forms of Mn were equal in all aggregate sizes with the exception of oxalate extracted manganese(Mno),where the 0.25-2.0 mm sizes recorded significantly higher Mno(6.14 to79.75 mg kg^-1)compared to other sizes in all fertilizer treatments,particularly,in the surface layer.With the same fertilizer treatments and location,we conducted a similar study to mainly investigate the interactive effects of these fertilizer ratios,size fractions and soil depth on the distributions of Al³⁺and Fe^2+..Conclusions from this study were that 30F compared to other treatments created more macro-aggregates and recorded higher mean weight diameter(MWD)(1.04 and 0.94 mm)and geometric mean weight(GMW)(1.08 and 0.92 mm)in both surface and subsurface layers.Fertilizer treatment,soil depth,aggregate size and their interactive effects were significant on the distribution of all oxides with the exception of dithionite extracted aluminum(Ald).This study was imperative as Fe²⁺in particular is identified to share similar chemical properties with Mn oxides.All three sites were combined to evaluate the impact of long-term fertilizations on P fractions,Mn oxide and their interactions in these paddy soils.Compared to other treatments,total P of 842.1(30F;>2 mm),744.4(NPKM;2-0.25 mm)and 670.2 mg kg^-1(NPKM;>0.053 mm),respectively for NC,QY and SN sites in the surface layer were displayed.Compared to other treatments,total P values of 806.4(30F;>2mm),350.4(NPKM;>2 mm)and 593.9 mg kg^-1(NPKM;>2 mm),respectively for NC,QY and SN site in the subsurface layer were noticed.The inorganic moderately labile pool of sodium hydroxide P(Na OH-P_i)was the dominant fraction under all fertilizer treatments.Concentrations of 232.3(30F;<0.053 mm),202.1(NPKM;0.25-0.053 mm)and 137.8 mg kg^-1(NPKM;>2 mm)of Na OH-P_i that represented 30.9,27.04 and 20.6%of total P,respectively for NC,QY and SN sites were noticed in the surface layer.In the subsurface layer,concentrations of Na OH-P_i(217.5;30F;<0.053 mm),residual-P(57.3;NPKM;>2 mm)and dilute hydrochloric acid(1MHCl-P_i)(113.8;NPKM;>2 mm),representing33.6,16.35 and 19.2%of total P,respectively for NC,QY and SN sites were observed.We noticed positive relationships between the various pools of P and M3-Mn oxide and concluded from this multi-locational study that admixing manure with NPK as a normal practice from the studied sites should be approached cautiously to avert excessive accumulation of both labile and moderately labile pools of P as well as Mn oxides since prolong build-up of these elements could have dire environmental consequences.Finally,a flooded incubation study was conducted to examine the input of Mn oxide on the distribution of various fractions and pools of P using treatments from all experimental sites.From NC site 30F generally recorded significantly higher concentrations of all fractions of P compared to other fertilizer treatments in both surface and subsurface layers.The>2 mm compared to other aggregate sizes recorded significantly higher organic P fraction of concentrated hydrochloric acid(conc HCl-P_o)and residual-P fractions while there was no significant difference between aggregate sizes in accumulating resin-P and Na OH-P_o fractions with the rest of the P fractions being significantly accumulated by the2-0.25 mm aggregate size in the surface layer.In the subsurface layer,the>2 mm aggregate size accumulated significantly higher fractions of P with the exception of conc HCl-P_o and residual-P fractions which were significantly accumulated by the 0.25-0.053 and<0.053 mm aggregate sizes,respectively.In both surface and subsurface layers of the QY site,the NPKM treatment generated significantly higher P fractions with the exception of resin-P and organic P fraction of sodium bicarbonate(Na HCO-P_o)fractions which were liberated by PKM fertilizer treatment.The 2-0.25 mm aggregate size accumulated significantly higher non-labile pools of P while the 0.25-0.053 mm accumulated significantly higher labile and moderately labile pools of P with the<0.053 mm also accruing significantly higher Na OH-P_o fraction compared to other aggregate sizes.In both surface and subsurface layers from the SN site,NPKM treatment just like that observed from QY liberated significantly higher fractions of P with the exception of Na HCO-P_o(surface)and conc HCl-P_i(subsurface)which was liberated by NPK.Labile and moderately labile pools of P were significantly accumulated in the 0.25-0.053 and<0.053 mm aggregate sizes while the non-labile pools were accumulated in significantly higher concentrations in the>2 and 2-0.25 mm aggregate sizes in the surface layer.In the subsurface layer,the>2 mm aggregate size accumulated all fractions of P with the exception of conc HCl-P_o and residual-P which were accumulated in significantly higher amounts in the2-0.25 and 0.25-0.053 mm aggregate sizes,respectively.Similarly,positive relationships were observed between Mn oxide and the various pools of P from all studied sites.From both field and incubation experiments,it was revealed that Mn oxide is able to interact with P through chemical bonding between the oxidized elements of Mn oxides and P,translating into the removal of P from solution,hence the reduction of P solubility caused by the presence of Mn oxide.This mechanism is through the initiation of Fe-Mn complex formation by co-precipitation between Fe and Mn in soils,particularly under anaerobic conditions.