| Soil organic matter is an important part of the soil, is not only a source of energy for the plant nutrient sources and soil microbial activity, but also improve soil physical structure, and is a important indicator of soil fertility and soil quality. At the same time, soil organic matter is complicated by the different components, the conversion rate,carbon storage capacity,soil nutrient supply and maintain soil structure of the organic matter of different components were different. The tobacco is a special leaf crops of high economic value, the amount and composition of soil organic matter is one of the key factors affecting the yield and quality of tobacco. Therefore, to understand the quantity and quality situation and composition characteristics of soil organic matter in typical tobacco-growing area, Chongqing, and provide the basis for the regulation of tobacco-growing soil fertility,carbon management and the production of high quality tobacco.In this study, using the soil samples from the Pengshui,Fengdu and Wushan tobacco-growing areas in Chongqing as test materials,the distribution and relationship between the soil organic matter and labile organic matter were studied by the methods of chemical oxidation.In addition, the soil samples that organic matter content lie in the appropriate range (15～30 g kg-1) from the three tobacco-growing areas were selected to group particle size and density; then selecting some samples of high organic matter content (> 30 g kg-1), middle organic matter content (15～30 g kg-1), low organic matter content (<15g kg-1)were used to chemical group and analysis the spectral characteristics of dissolved organic matter(DOM).The purpose was to analysis the existing characteristics of soil organic matter(SOM),labile organic matter(LOM), heavy fraction organic matter(HFOM), light fraction organic matter(LFOM) and DOM. The results suggested that as follows:(1) The content of SOM ranges between 3.35 and 53.94 g kg-1 in the typical tobacco-growing area,Chongqing.The content of SOM in pengshui showing from the southwest to the northeast gradually increased, in Fengdu gradually increasing from west to east, Wushan was manifested lower in northeast, medium in southwest and high in southeast. Among them, Pengshui, Fengdu, Wushan three tobacco-growing areas in the appropriate range(15 ～30 g kg-1) accounted for 67.73%,80.13%,69.25%, accounting for the rich and very rich(> 25 g kg-1)were 59.26%,33.78%,42%, and them were the lack of lack of(<15 g kg’)accounted for 3.67%,4.64%,10.25%, respectively. Showed that most of the area in Chongqing is more suitable for the cultivation of tobacco, but the higher the proportion of high SOM content in PengShui, while the higher proportion of low SOM content in Wushan. So should be regulated according to the specific distribution of SOM to produce a better quality of flue-cured tobacco.(2)The contents of three fractions of labile organic matter named as high labile organic matter(HLOM), middle labile organic matter(MLOM) and labile organic matter (LOM),respectively,were determined using 33,167 and 333 mmol·L-1, respectively. Their contents were 0.10～8.55 g kg-1,0.60～16.58 g kg-1 and 1.82～23.42 g kg-1, mainly distributed 2～6 g kg-1,5～10 g kg-1 and 10～15 g kg-1,respectively. And in the range followed the order of Fengdu> Wushan> Pengshui.Among them, three kinds of labile organic matter content of Penshui and Wushan exhibit the same distribution with organic matter content, but in Fengdu the HLOM content increases from west to east, the MLOM and LOM content high in around, low in the middle. The three of labile organic matter in the SOM followed the order of Wushan>Fengdu>Pengshui, Fengdu>Wushan> Pengshui and Fengdu>Pengshui>Wushan, respectively. The content of HLOM, MLOM, LOM were significantly and positively correlated with the content of SOM, demonstrating that the active ingredient of soil organic matter may well reflect the dynamic changes of soil carbon, and therefore can be used as a indicator for the evaluation of soil fertility and soil quality.(3) Aggregates composition was dominated by particles 1 ～ 2 mm which accounted for 40.16% to 42.02% of the whole in the three typical tobacco-growing areas by different particle size separation. Among them, with increasing aggregate particle size,SOM and LOM content were decreased in the three tobacco-growing areas, HLOM and MLOM content also showed decreases with increasing particle size in the tobacco-growing areas of Pengshui and Wushan,but in the tobacco-growing area of Fengdu showed first decrease and then increase with increasing particle size. However, these aggregate particles were 1 ～2 mm maximum,to the distribution of soil organic matter and three kinds of labile organic matter,and 1～2 mm aggregates contributing 29.99～31.58% and 18.46 ～33.36%,respectively.Therefore,it can be assumed that particles of aggregate size 1～2 mm hold the majority of soil matter fixed in the typical tobacco-growing area,Chongqing.In addition,the soil total organic matter and total labile organic matter content with organic matter and labile organic matter content of <0.25 mm aggregates were highly significantly positively correlated,respectively. This means that the use of organic matter and labile organic matter of particle aggregates as the sensitivity factor is feasible for judge the change of soil organic matter in Chongqing Tobacco-growing area.(4) The content of LFOM in Pengshui,Fengdu,Wushan were 1.52～11.76g kg-1, 1.26～10.46 g kg-1,1.87～10.02 g kg-1, accounting for 5.55～58.50%,4.64～50.21%, 6.80～57.78% of SOM, respectively,the content of HFOM were1.34～33.29 g kg-1, 10.05～27.71g kg-1,4.47～35.75 g kg-1,accounting for 40.00～93.87%,48.28～93.97%, 39.51～93.02% of SOM, respectively by density separation. In addition,the SOM content with LFOM,HFOM content and HFOM accounting for SOM were highly significant positive correlation,but with LFOM accounting for SOM was highly significant negative correlation. That showed LFOM content was disproportionately increased with SOM content, HFOM content increased in proportion with the SOM content.(5) The DOM accounts for the proportion of SOM does not necessarily changes with SOM content. Among them,the content of DOM in Fengdu,Pengshui, Wushan were 0.86～2.08gkg-1,0.9～2.81g kg-1,0.81～1.8g kg-1, accounting for 4.22～10.05%,3.53 ～ 23.49%,5.45 ～10.40% of SOM,respectively,It mainly distributed between 2 and 6 g kg-1 was 60%. It was only a small part of the soil quality, but it was the main energy of soil microorganisms, has an important role in terms of providing soil nutrients. The relationship between the SOM and DOM in Chongqing tobacco-growing area,indicating that SOM content and DOM content showed a significant positive correlation.But between SOM content with DOM accounts for the proportion of SOM showed a irrelevantion. That showed DOM content was disproportionately increased with SOM content.(6) In the DOM Ultraviolet-visible(UV-Vis)spectra, through the results of specific absorption parameters,it was suggested that the aromaticity,hydrophobicity and humification degree were lower in the soils of high organic matter content than the soils of middle and low organic matter content. And the absorption spectral slope(S) values increased with increasing wavelength. Furthermore,the ratio of specific spectral slope(SR) indicated higher organic matter content, the light degradation activity was also higher, which may had special meaning in soil carbon turnover, tobacco nutrient supply, heavy metals activation, it was worth further study.(7) In the DOM three-dimensional fluorescence spectra, the soils of high and middle soil organic matter content were found fluorescence peaks D and E,the soils of low organic matter content was found fluorescence peaks C in the tobacco-growing area of Fengdu and Wushan;and the soils of high and middle soil organic matter content were found fluorescence peaks A, the soils of low organic matter content was found fluorescence peaks D and E in the tobacco-growing area of Pengshui. Analysis of soil DOM fluorescence intensities showed that soil protein was the main source of soil DOM, and the humus-like material in the soil DOM from the tobacco-growing area was mainly humic substances of large molecular weight. A poor correlation between the Humic-like fluorescence peaks and organic matter, but there is a good correlation between the protein-like fluorescence peaks and organic matter. Analysis of the HIX values in soil DOM indicated that the high organic matter content soil DOM had a higher degree of humification and aromaticity. |
