| Humic substances, as one of the most important organic carbon compounds in the ecosystem systems, its formation, transformation, decomposition and death have a close relationship with the changing of the global climate and the emission of the greenhouse gas. Improving the formation and stability of humic substances in the agro-ecosystems is becoming one of the most important methods to alleviate the changing of global climate. Paddy field ecosystms is a typical agricultural ecosystem. Humic acid is not only one of the most important components of the ecosystms, but although is the intermediate of the soil material and energy circulation. Recently, the research of the paddy humic acid is primary about soil fertility, soil stability and the environmental rehabilitate, but merely about the formation of soil humic acid under the long-term farming condition. So, to thoroughly understand the regulation of the formation and transfortation of the humic acid could abound the carbon cycle theory of paddy field agro-ecosystems.According to the experiment of straw decomposition firstly, elemental analysis, Fourier transform infrared spectroscopy and solid-state cross polarization-magic angle spinning 13C-nuclear magnetic resonance spectrometry (solid-state CPMAS 13C-NMR) were used to study the humic acids in purple paddy soil applied with rice straw residue. On this basis, by using dynamic light scattering and static light scattering techniques, the characters and variations of molecular structure and shape of humic acids isolated from purple paddy soil in Sichuan basin under a long-term field experiment, including conventional tillage (rice-fallow), water-upland rotation (rice-rape)and water-upland rotation (rice-rape), were analysed and determinated. Then the adsorption behavior of humic acids on montmorillonite were different form each other, which were interpreted by the formation of humic acids in purple paddy soil and the variations of molecular structure and shape under conservation tillage.Chemical compositions of humic acids extracted from purple paddy soil with straw residue were changing. The content of soil organic carbon and humus were increased about 31.71% and 51.93%, which were the largest during straw decomposition in the 90th day. In the 45th of decomposition of rice straw residue, the PQ was significantly lower, and at the time of the 90th and 135th, the PQ had changed little. After using straw residue, the⊿logK of the humic acid extracted from purple paddy soil decreased, the RF increased rapidly. In the elemental composition of humic acids, the carbon was the main contents (about 52%), the oxygen followed. H/C decreased, O/C increased continuously. Carboxyl and phenolic-hydroxyl groups were the main acidic groups. The content of Carboxyl group was reduced firstly, with the decomposition of paddy Straw, which was going up; the content of phenolic-hydroxyl group changed contrarily with carboxyl group, carbonyl group increased. In the first phases of decomposition of rice straw residue, all the FTIR spectra of humic acids absorption peaks intensities which included 3364,2933,1653, 1599,1508,1461,1421,1331,1225,1126 and1033 cm-1 weakened obviously; alkyl, polyhydroxy and aryl resonance peaks of 13C-NMR spectra weakened clearly, and that the change of methoxy was opposite. With the process of decomposing, all the FTIR spectra of humic acids absorption peaks intensities weakened obviously; alkyl, aryl and carbonyl resonance peaks of 13C-NMR spectra were reinforced. In the final of decomposition, the intensity of FTIR spectra absorption peak that absorption frequency was 2933cm-1 of humic acids was lower, as well as the intensity of absorption peaks that absorption frequency were 1651, 1599,1508,1461,1422 and 1224 cm-1 were stronger slightly; alkyl resonance peaks of 13C-NMR spectra was flagging while methoxy resonance peaks were different. Therefore, the regular changes of the intensity of FTIR spectra absorption peaks and 13C-NMR spectra resonance peaks reflected the formation process of humic acids in purple paddy soil passed through three phases. And the formation process of humic acids in purple paddy soil was accorded with lignin theory.Particle size distribution, fractal dimension, hydrodynamic diameter, molecular weight, root mean square radius of the roundabout, and the second virial coefficient were caused by conservation tillage. Humic acid is a stable decentralized system when pH was 5.5 and c was 100 mg-L'1. The fractal dimension of humic acid under conventional tillage together with no tillage and ridge culture increased with the depth decreasing of soil. The fractal dimension of humic acid at depth of 20～40 cm under water-upland rotation was the largest (df=2.77). Molecular weight, hydrodynamic diameter and root mean square radius of roundabout of humic acid under conventional tillage reduced with the increasing depth of soil. The change of humic acid under no tillage and ridge culture was in opposition to conventional tillage. Molecular weight, hydrodynamic diameter and root mean square radius of roundabout of humic acid at depth of 20～40 cm under water-upland rotation were all the least (Mw=4.69×105Da,2Rh=141nm, Rg=102nm). The second virial coefficient of conventional tillage, no tillage and ridge culture, water-upland rotation were opposite to the trend of their molecular weight. The redistribution of straw and the level of microbial activity in soil affected the distribution of Particle size distribution, fractal dimension, hydrodynamic diameter, molecular weight, root mean square radius of the roundabout, the second virial coefficient. Adsorbance and Adsorption isotherm of humic acids isolated form purple paddy soil on montmorillonit were impacted significantly by conservation tillage. When pH of humic acids was 5.5, Adsorbance of humic acids on montmorillonit was increased gradually with the increase of humic acid concentration. The variation of adsorbance reduced gradually as the increase of humic acid concentration. The adsorbance of humic acid at depth of 40～60 cm under conventional tillage on montmorillonit was the largest, depth of 0-20 cm followed, depth of 20～40 cm end. The adsorbance of humic acids of different depths under water-upland rotation and no tillage and ridge culture on montmorillonit were diversity as a result of the concentration of humic acid.Fruendlich equation and Langmuir equation were taken to fit the adsorption of humic acids isolated form purple paddy soil under long-term conservation tillage on montmorillonit.Facing the Fitting results of adsorption of humic acids in depth of 20～40 cm under conventional tillage and depth of 0-20 cm under water-upland rotation on montmorillonit, Langmuir equation is better than Fruendlich equation, others were that Fruendlich equation is better than Langmuir equation. So we can say that adsorption is decided by the shape and the content of carboxyl group of humic acids.Consequently, the formation process of humic acids in purple paddy soil was accorded with lignin theory. The molecular weight and shape of humic acids had changed causing by conservation tillage and adsorbance and adsorption of humic acids on montmorillonit also had changed. The formation and molecular characteristics of humic acids in purple paddy soil were studied form the Molecular level, in order to understand the formation and transformation of humic acids. |
