Fractionation Of Dissolved Organic Matter During The Decaying Process Of Rice Straw And Its Environmental Significance

Dissolved organic matter(DOM),also called water soluble organic matter,is defined as it can be extracted by water,acid and alkali.The operation definition of DOM is defined through filtration,the size limit,which is used to differentiate DOM from particulate organic matter,is somewhat arbitrary,but there is an almost universal consensus that it is around 0.45μm.Because of its structural characteristics and physicochemical properties, research has proven that DOM is an active and important chemical component of soil ecosystem.In soil(sediment) environment,DOM could affect the form,fate, transformation and sedimentation of organic compounds in soils through a series of interactions,such as ion exchange,absorption,ligation,chelating,pectisation, oxidation-reduction and so on.For more attention was payed to organic pollutants, studying the relationship between DOM and organic pollutants will be the frontier of soil science,ecology and environment science.Fraction of DOM derived from various stages of crop straw decomposition was conducted by using XAD-8 resin into hydrophobic bases (HOB),hydrophobic acids(HOA),acid-insoluble matter(AIM),hydrophobic neutrals (HON) and hydrophilic matter(HIM).The dynamics of DOM fraction have been studied in this research,its effects on the sorption behavior of probe organic pollutants in soil had also been studied,the one is polycyclic aromatic hydrocarbons(Pyrene, PYR).The main results as follows:(1) The dynamic changes of DOM derived from crop straw decomposition was studied by using XAD-8 resin.The results showed that the content of DOM fractions was in dynamic changes at all the incubation stages.During the decomposition period(0～90 days),the content of HIM decreased,while AIM,HOA and HON was on the contrary, HOB content changed little.(2)Fourier transform infrared(FTIR) spectroscopy was used to study the chemical composition of DOM fractions.FTIR spectroscopy indicated that the DOM derived from crop straw decomposition had similar structure characteristics,such as methyl,methylene, methine,carboxyl,hydroxyl et al.The compounds in DOM were saccharide,aliphatic and aromatic organic acid,hydroxybenzene et al.However,it was showed that the absorption strength of different functional groups of DOM fractions were obvious differences,the functional groups were changed with the progress of DOM decomposition.(3) With and without DOM fractions,the thermo-dynamics and mechanisms of sorption of PYR to soil were investigated.The results suggested that PYR distribution coefficient(Kd) between soil and water were negatively correlation to temperature,and the processes of sorption were spontaneous exothermic reactions.Adding DOM fraction, the free energy of Pyrene sorption to soil was changed;the adsorbed free energy of pyrene in soil were all less than 40 kJ.mol^-1,suggest that the sorption behavior is dominated by dispersive force,H-bonds,van de Walls force,dipolar action force.The sorption happened in soil because of the decrease of free energy.(4) Batch experiments were conducted to investigate the effects of different concentrations of DOM fractions on the sorption behavior of PYR in a soil-water system.It was found that DOM fractions reduced sorption of PYR significantly onto the tested soil.Within the selected concentration range of the PYR and DOM fractions in this study,the sorption quantity has negative linear correlativity with the concentration of DOM fractions,and there is significant difference(p＜0.05) within different concentration treatments besides 200mg/L and 300 mg/L concentration treatments of HON.It is related to complexation of PYR and DOM fractions in water.(5) The linear model can comletely explained the mechanism of PYR sorption. Within the selected concentration range of the PYR and DOM fractions in this study,the sorption capacity of soil PYR reduced significantly after applying DOM fractions into soil. Furthermore,there is significant difference(p＜0.01) comparing to control treatment (i.e,no DOM in sorption-desorption course).The effects of DOM fractions on the sorption capacity of soil PYR were strengthened with longer DOM composition time.The restraint effect was caused by comprehensive factors.The hydrophobicity and structure of fractions were primary related to the PYR sorption.This research was important to direct the reasonable application of organic fertilizer.