| Humic acid(HA) can reduce fixation and increase movement of P in the soil,enhance the efficiency of phosphate fertilizers.The function of HA is influenced by its structure and composition.In our research,weathered coal HA were used as materials to prepare different molecular weight HA(>100 k Da,10-100 k Da and<10 k Da)by ultrafiltration,and humic acid phosphate fertilizers(PHA)were prepared by adding mixed,high,middle and low molecular of humic acids in phosphoric acid and KOH.The detailed structure of HA fractions and PHA were characterized by the potentiometric titration and spectroscopic technologies.A series of experiments were conducted to analyse the wheat root growth,transformation and movement of P in the soil,wheat yield and the utilization efficiency of phosphate fertilizer on different HA.The main results are as follows:1.The HA fractions with different structures could be obtained by ultrafiltration.HA of ultrafiltration were divided three fractionations.Quality distribution of high molecular weight HA accounted for 73.28%,while middle and low molecular weight of HA was 26.72%.High molecular weight HA was abundant in C content,aromaticity structure,and metal ion binding sites of aromatic ring.Middle and low molecular weight of HA contained high content of O,N,O/C,H/C,N/C,E2/E3,E4/E6,COO/N-C=O,Alkyl C,O-alkyl C,Carbonyl C,while low aromatization index.2.Structures of both HA and phosphate fertilizer could be changed when HA with different structures was added into phosphoric acid and KOH.The content of COO/N-C=O,Alkyl C and Carbonyl C in HA increased while O-alkyl C and Aromatic C decreased when HA was combined with phosphate fertilizer.Aromatic C-O,COO/N-C=O and Carbonyl C were negatively correlated with the molecular weight of HA,while the content of Alkyl C was positively correlated with the molecular weight of HA.The phosphorus form of HA contained phosphate phosphorus orthophosphate,phosphate monoester,pyrophosphate,orthophosphoric acid ester.The content of orthophosphate diester was positively correlated with the molecular weight of humic acid.Compared with ordinary phosphate fertilizer,PHA could effectively prevent P fixation,increase P migration in soil,delay the conversion of available P to slow available P.Tthe effect of high molecular weight PHA was the best.3.Different molecular weights HA had a differrnt regulation effect on wheat roots.HA significantly increased the growth of wheat root system,promoted the amount of carbohydrate and protein substances in wheat roots and sugar,ester,protein,polysaccharide,nucleic acids,cellulose in polysaccharide in wheat shoots.The content of protein mwas significantly positively correlated with O,O/C,E2/E3,E4/E6,COO/N-C=O,Carbonyl C,and negatively correlated with Aromatic C,Aromatic C-O,ARI.Low molecular humic acid had a better effect on promotaterial in wheat seedling root was negatively related to the molecular weight of HA,while cellulose esters and polysaccharides nucleic acids were positively related to the molecular weight of HA.The root fresh weight and root dry weight were significantly positively correlated with O,O/C,COO/N-C=O and negatively correlated with Aromatic C.Root activity Nitrate reductase activity Root glutamine synthetase activity ing wheat growth.4.Regulation of different HA generated various effects on wheat yields and efficiency of phosphorous fertilizer.There was a positive interaction between humic acid and phosphorus.PHA significantly increased grain yields of wheat,and the effect was better complied with low and medium molecular weight PHA.Compared with common P treatment,the wheat yields in PHA_M and PHA_Ltreatments were significantly increased by 18.84%and 21.37%,the P partial productivity and agronomic efficiency increased by18.82%,21.35%and 20.18%,23.03%,and the P recovery efficiency increased by4.52,5.41 percentage points,respectively.Wheat grain yield and P recovery efficiency were positively correlated with H/C,O/C,COO/N-C=O,O and Carbonyl C of humic acids. |
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