| Urea is the main nitrogen(N)fertilizer in agricultural production.However,its high activity while low use efficiency could cause a large number of noticeable problems in resource waste and environmental risks.Thus,modifying the urea to enhance its use efficiency is one of the priority developments in domestic nitrogen fertilizer industry.The combination of humic acid(HA)and urea has the tremendous potential in improving stability and use efficiency of urea However,there is limited systematic and in-deep study on the effects of HA's structure on regulating urea.Therefore,we used the weathered coal as the raw material,and fractionated it into HA fractions with different structures.The detailed structure of HA fractions was characterized by the potentiometric titration and spectroscopic technologies.Then,the optimal process was explored to manufacture humic acid enhanced urea(HAU).With this process,we manufactured HAUs with HA fractions and urea,and explored the combination effect of HA fractions and urea.Subsequently,a series of experiments were conducted to analyze the transformation process of HAUs in soil,and to investigate the effects of HAUs on crop growth and the fate of fertilizer N.Finally,the above effects were analyzed further when taken the structure of HAs and HAUs into consideration,in order to explore the inherent regulation mechanism.The main results were summarized as follows:(1)The HA fractions with different structures could be obtained by pH-fractionation and ultrafiltration.The main fractions was concentrated in the extraction with pH 3-7,accounting for 90%of total fractions in weight.All fractions derived from pH-fractionation were characterized with higher aromaticity,more protonated aromatic structure.For these fractions,their contents of acidic functional groups,aliphatic carbon(C),protonated aromatic C decreased with the extraction pH raised,while there were opposite tendency between the nonprotonated aromatic C and extraction pH.Meanwhile,there were 72%of fractions in weight with the molecular weight lower than 100 kDa by the ultrafiltration fractionation.There was more significant difference in HA structure and composition among the fractions derived from ultrafiltration fractionation.For these fractions,the aromaticity decreased along with the cut-off molecular weight,while the content of acidic functional groups and aliphatic C showed a reverse trend.(2)More N will be incorporated into HA during the melting process,and different effects will generated when HA with different structures was added into molten urea.The amount of urea-N incorporated into HA during the melting process exceeded that during the blending process by 4.43 times,suggesting that the melting process was more beneficial to the combination effects of N and HA.The combination effect was attributed to carboxylic and phenolic hydroxyl groups in HA,which reacted with urea to generate the complex of urea and HA.The complex contained the structure of secondary amide.The fractions from extraction pH 6?7 and molecular weight with 10?100 kDa combined more urea-N,and there was 13.2%and 12.4%of urea-N in their corresponding complex of urea and humic acid(UHA),higher than UHAs with other fractions.The carboxyl functional groups played an important role during this process.In addition,the degree of protonation enhanced the reaction depth between HA and urea.(3)The application of different HAUs exhibited different effects on the nitrogen transformation.During the incubation period,the soil pH with HAU treatments had the variation of 0.43?0.45 units,lower than that under common urea(U)by 13.2%?18.9%.Thus,HAU application could alleviate the dramatic changes of soil pH after urea addition.HAUs could inhibit the urease activity at the initial stage of incubation.This brought out the amount of accumulated ammonia volatilization reduced by 4.6%?9.2%,and the maximal inhibition ratio happened on the HAU3 synthesized by urea and HA fraction extracted from the extraction with pH 9?10.It is likely attributed to its higher degree of aromaticity and nonprotonation to generate a stable complex.HAU3 also stabilized the urease activity most at the later stage of incubation.The HAUs delayed the transformation from NH4+-N to NO3-N in soil,and the better effects were achieved by the HAU2 and HAU3,which were synthesized by urea and HA fraction extracted from the extraction with pH 6?7 and pH 9-10,respectively.This was beneficial to the increase in residual amount of fertilizer N in soil,resulting in the content of total N under HAU treatments higher than that with U by 3.5%?4.4%.HAU application also enhanced the extracellular enzyme activities.(4)The application of different HAUs promoted the maize/wheat growth and generated various effects on fate of fertilizer N.HAU application enhanced the aboveground biomass of maize/wheat.The increasing effects of grain were attributed to the increase of the total kernel in maize and the thousands weight in wheat.Among all HAU treatments,the maximum of maize grain yield happened on the HAU2 treatment,higher than those under other treatments by 7.3 percent points,which is consistent with the more nitrogen that the raw HA fraction could incorporate.Meanwhile,the maximum of wheat grain yield happened on HAU2 and HAU3,higher than that under the HAU treatment synthesized by urea and HA fraction extracted from the extraction with pH 3?4 by 11.8%.At the same time,HAU2 and HAU3 improved the fertilizer N use efficiency by 4.7?12 percent points for maize and 5.0?5.7 percent points for wheat,respectively.The residual ratio increased by 0-1.3 percent points for maize and 8.5 percent points for wheat,respectively.This is consistent with the effects of HAUs on N transformation.It could be attributed to the more stable complex in HAU2 and HAU3,for there was lower degree of protonation in HA fractions extracted from the extraction with pH 6?7 and pH 9?10.Owing to the above,HAUs enhanced the utilization of fertilizer N,improved its residual amount,and reduced its loss. |
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