Study On The Formation Factors And Loss Control Of Colloidal Phosphorus In Typical Agricultural Soils Of Zhejiang Province

Loss of soil phosphorus（P）from agricultural fields has long been a global challenge,and is one of the main factors responsible for the eutrophication of rivers and lakes in southern China.Existing studies have shown that the formation of colloidal P（CP）in agricultural soil is a key factor of P migration to the external water environment.However,the current understanding of the formation,distribution,and loss control of the colloidal P in agricultural soil are still unclear.This study first compared the separation and determination methods of CP in soil.Then,the influence mechanisms of soil aggregation and dispersion on the content,form,and release potential of CP were clarified;moreover,the size fractions of CP and its linkage with organic carbon in 15 farmland systems in Zhejiang Province were further analyzed on both regional and field scales;Furthermore,the evaluation method of CP loss index was established to evaluate the CP loss poteitial of 19 regions in Zhejiang Province;finally,the loss characteristics and loss load of CP and other P fractions in runoff were calculated,and the actual effect of carbon-based fertilizer input（Carbon-based soil conditioner+organic fertilizer）in reducing the loss of CP in runoff was investigated.The main findings are as follows:（1）By comparing different CP determination methods and characterizing CP,this study found that ultracentrifugation had minimum errors and was easy to recover colloids,but it was time-consuming.Ultrafiltration had the best separation effect,high precision,and simple and reliable separation steps,but the mechanical error was large.The field flow separation method can achieve continuous nondestructive separation and elemental determination of colloids and nanoparticles of different sizes,but it is not easy to collect colloids.The P in soil solution was mainly carried by fine colloidal particles with sizes less than 220 nm;The content of CP accounted for 7.3%-88.6%of the total P in different soil colloidal solutions,and most was molybdenum blue reactive P.Colloidal P particles are consist of a complex of silicate minerals and organic matter,but the smaller size nanoparticles differed in chemical composition from the larger size colloidal particles;colloidal mineral crystals were consist of polyhydric kaolinite and muscovite;colloidal solutions were dominated by humic-like fulvic acid,while the truly dissovled solution had the highest relative content of tryptophan;compared to the original soil particles,and colloidal particles showed a higher P monoester and P diester concentration.（2）To elucidate the effects of aggregate size on the potential loss of CP in agricultural systems,soils（0–20 cm depth）from six land-use types were sampled in the Zhejiang province.The aggregate size fractions（2–8 mm,0.26–2 mm,0.053–0.26 mm and<0.053 mm）were separated using the wet sieving method.CP and other soil parameters in aggregates were analyzed.In acidic soils,the highest CP content was observed in the 0.26–2 mm sized aggregate,while the lowest was reported in the<0.053 mm（silt+clay）-sized particles,the opposite of that revealed in alkaline and neutral soils.Aggregate-associated total carbon（TC）,total nitrogen（TN）,C/P,and C/N had significant negative effects on the contribution of CP to potential soil P loss.The CP content of the aggregates was controlled by the aggregate-associated TP and Al content,as well as the soil p H value.The potential loss of CP from aggregates was controlled by its organic matter content.The concentrations of various P forms in different soil particles increased with the decrease of particle size.In the colloidal particles,the concentration of orthophosphate was as high as 2272–4027 mg kg^-1;the proportion of organic P was about 50%of the total P concentration.Among them,the concentration of P monoester increases exponentially,indicating that the dispersed colloidal particles has a high affinity with organic P.In contrast,among the aggregates of various sizes,the large-size aggregates（2–8 mm）had higher P concentration in various forms,while the small-size aggregates had lower P content in various forms,especially for the concentration of P monoester.Therefore,soil aggregation process leads to the P immobilization,especially for organic P.In summary,soil aggregation and soil particle dispersion cause P enrichment in two dimensions.Therefore,increasing the size of aggregates and reducing the dispersion of particles can effectively reduce soil P loss.（3）Surface soils（0?20 cm depth）were collected from 15 agricultural fields,including two sites with different carbon input strategies,in Zhejiang Province,China,and water-dispersible nano colloids（NC）,fine colloids（FC）,and medium colloids（MC）were separated and analyzed using Asymmetric Field-Flow-Field Fractionation（AF4）technique.Three levels of fine CP content(3.5–6.1,0.8–2.6,and 0.5–0.7 mg kg^–1)were identified at the regional scale.This study found that the nano CP consists mainly of organic carbon(C_org),calcium（Ca）and P,while the fine CP component is a complex of C_org,clay and P.The organic carbon controlled CP saturation,which in turn affected the P carrier ability of colloids.Field-scale organic carbon inputs did not change the overall morphological trends in size fractions of water dispersible colloids.However,they significantly affected the fraction based peak nano,fine and medium CP concentrations.Application of chemical fertilizer with carbon-based solid manure and/or modified biochar reduced the soil nano,fine and medium CP content;however,biogas slurry boosted CP formation.（4）The CP index equation was established by the multivariate statistical analysis method.the"risk of soil CP loss"can be divided into low risk（<-0.82）,medium risk（-0.82 to-0.22）,high risk（-0.22 to 0.83）and very high risk（>0.83）;the"risk of loss of TSP loss"can be divided into low risk（<-0.73）,medium risk（-0.73 to-0.13）,high risk（-0.13 to 0.88）and very high risk（>0.88）;The soil P index equation was obtained as follows:"risk of soil CP loss"=(-0.263×Z_GMD)+(-0.479×Z_TOC)+(-0.188×Z_{p H})+(0.422×Z_AP)+(0.448×Z_Clay);"risk of loss of TSP loss"=(-0.549×Z_GMD)+(-0.205×Z_TOC)+(0.629×Z_{p H})+(0.426×Z_AP)+(-0.147×Z_Clay);the risk of CP loss in typical farmland systems in 19 fields of Zhejiang Province was evaluated using the above P index equation.The results showed that,there are higher loss risks of CP and truely dissolved P in farmland soils in Zhejiang Province.（5）Here,to cotrol the CP loss,we established three monitoring stations in Zhejiang Province,China,in each of which the land-use types were double cropping rice,rice–wheat rotation,and vegetable cultivation.Field experiments were conducted with four treatments and three replications over a whole planting year.The four treatments at the same P application rate were:1)no fertilization（control;CK）,2)chemical fertilization（CF）,3)substitution of 30%chemical P in CF by organic P fertilizers（solid sheep manure or liquid biogas slurry）（OF）,and 4)substitution of 30%chemical P in CF by organic P fertilizers（solid sheep manure or liquid biogas slurry）+1.5 t ha^-1 modified BC（PSB）.PSB treatment significantly reduced the concentration of P fractions in the runoff in all three land-use types,while the OF treatment significantly reduced the particulate P and CP concentrations.However,BS treatment increased particulate P and CP concentrations.Compared with CF treatment,PSB treatment reduced total P loss by 41.1%,29.7%,and 37.8%in the double cropping rice,rice–wheat,and vegetable systems,respectively.The PSB and OF treatments significantly reduced the particulate P and CP losses,while BS treatment increased the loss of various P forms.Compared with CF,the PSB treatment decreased the CP contents by 26.7%–51.4% in the soils examined.