| Water-level fluctuation zones of Three Gorges Reservoir Region has been become an sensitive ecosystem environment because of its unique characteristics of alternation of wetting and drying, since it retained water,it has been widely concerned. Water-level fluctuation zones of Three Gorges Reservoir Region is an active region of energy, material and information transfer and transformation of water,land ecological system,which has the function of buffer and vegetation. Furthermore,it is sensitive to the change of external environment. Water-level fluctuation zones of Reservoir Region is main steep farmland plays an important role for ecosystem environment quality.The dissolved organic matter has an important influence on global climate change as an important part of the global carbon cycle, meanwhile it is the most important ways of material and energy cycle in the ecosystem and that is closely related to the important link of ecological system.Because of the active effect of DOM in environment,it has a significant impact to the sdjacent water which will no doubt further affect the environmental behavior of the nutrient elements, Heavy metals, organic pollutants and greenhouse gases in agricultural watershed. Therefore, a typical small agricutral watershed of Three Gorges Reservoir Region was selected to illustrate the differences of the geochemical characteristics of DOM, combined with different using types of land and different types water, and the seasonal change of water DOM of the agricultural watershed with ultraviolet-visible(UV-Vis) absorption spectra, three-dimensional fluorescence spectroscopy. The results showed that:(1)Results showed significant differences of DOM samples amongst different land uses were observed, and DOM from forest had the highest aromaticity and humification degree, then DOM from cropland was followed. Although DOM from vegetable field and residence showed the highest dissolved organic carbon(DOC) concentration(average values 80.69 mg·L-1 and 89.20mg·L-1 respectively), but aromaticity was lower indicating lower humification, which further suggested non-chromophoric component in these DOM samples contributed significantly to total DOM compositions. Additionally, in all DOM samples that were independent of land uses, fluorescence index(FI) values were between 1.4(terrigenous) and 1.9(authigenic), evidently indicating both of allochthonous and autochthonous sources contributed to DOM characteristics. Meanwhile, r(T/C) values in most of samples were higher than 2.0, suggesting soil DOM in this agricultural watershed was heavily affected by anthropogenic activities such as agricultural cultivation, especially vegetable field was a good example. Additionally, sensitivities of different special spectral parameters for reflecting the differences of DOM characteristics amongst different land uses, were not identical. For example, spectral slope ratio(SR) and humification index(HIX) both could not clearly unveil the various geochemical characteristics of soil DOM from different sources. Thus simple and single special spectral parameter cannot comprehensively provide the detailed information of DOM, and combined application of UV-Vis and fluorescence spectroscopies were highly recommended.(2)As a key geochemical factor in earth system, dissolved organic matter(DOM) plays an important role in controlling environmental quality of watersheds. Results showed that largely spatial variances of aquatic DOM in this watershed, within signifincantly differences in compositions and sources. After calculation of ag*(355) for indicating proportion of chromophoric DOM in bulk DOM, the order of DOM was paddy rice field> ditch> pond> well > outlet point. DOM samples from paddy rice field and ditch showed higher SUVA254 suggesting higher aromaticity. DOM from this watershed showed typical 2 types(4 peaks A, C, B and T) of fluorescent components including humic-like and protein-like component. Dual contributions from autochthonous(e.g., microbial or alga production) and allochthonous both heavily affect the DOM characteristics. Besides active microbial activities due to highly organic and nutrients inputs for agricultures, discharge of sewage and water used in agricultural production could contribute proteins possibly inducing ascending proportion of protein-like component as showed in fluorescence analysis. DOM samples from the same sampling points but in different crop plantation seasons were collected to compare, for understanding the differences between two planting seasons. It clearly suggested protein-like component was the key variable to control the DOM dynamics. After land-use changing from rice/corn into mustard plantation, all of DOC, CDOM and r(T/C) showed significantly differences, but no such observations were observed in FI, BIX and r(A/C).(3)Results showed that significant differences of DOC Pool amongst different land uses were observed with different geochemical characteristics,and DOC Pool from vegetable field had the largest value and the complex structure, then DOC Pool from cropland had the minimum value.The non-chromophoric component in these DOM samples contributed significantly to total DOM compositions. The different extraction column of CDOM and DOC didn’t show significant correlation(P>0.05). The result of principal component analysis showed that the change of DOC Pool was influenced by Microbial activity and exterior human factors.Four types of soil DOC Pool and the Spectral parameters of DOM had the different significant correlation,because of it was influenced by the common factor of External light degradation and human factors, Thus simple and single special spectral parameter cann’t comprehensively provide the detailed information of DOC Pool, and combined application of UV-Vis and fluorescence spectroscopies were highly recommended.Ks and [DOC] had shown significant correlation(P<0.05),the soil which had biggish adsorption constant will had the bigger DOC Pool at the same time.(4)The FDOM and CDOM of DOM in water of the year both had shown significant correlation with DOC(P<0.05). DOC concentrations in different sampling time were in order of April> November> July> January. DOC concentrations changed from1.22 mg·L-1 to 165.00 mg·L-1 of all the year. CDOM concentrations [a(355)] ranged in 0.08~24.47 m-1,And FDOM concentrations [Fn(355)] ranged in 2063.80 ~102811.99m-1. The fluorescence intensity of protein-like component(Peak B and Peak T) in different sampling time were in order of April> November> July> January. Fluorescence index(FI) values ranged in 1.55~1.75 and values were between 1.4(terrigenous) and 1.9(authigenic),DOM was influenced by both terrigenous and 1.9 authigenic. The authigenic,aromaticity,hydrophobic components,molecular weight of Sub-1 and sub-2 agricultural watershed both showed significant differences. DOM from water of agricultural watershed in the summer had the highest aromaticity, humification degree and class protein components,and higher biological availability; the humification degree of DOM from water in the spring and autumn was in the middle,and the weakest in winter.The total mercury in water and spectral slope ratio(SR) which represent the molecular weight size of agricultural small watershed had shown significant positive correlation(P<0.05),but it had shown significant negative correlation with Fluorescence index(FI). The methyl mercury(DHg) and a(355),and SR had shown significant positive correlation(P<0.05),and significant negative correlation with Fluorescence index(FI),as well.(5)Dissolved organic matter(DOM) was an important component in water eco-systems. It played a crucial role in the environmental fate of pollutants such as mercury(Hg),because of its specific characteristics and functional structures. Importantly, no clearly differences between concentrations of Hg species and SUVA254 and DOC respectively were observed, however the chromphoric component and molecular weight size controls the dissolved Hg and reactive Hg in water. Additionally, highly organic matter accumulation and transfer resulted from primary productivity in water may be a substantial reason to explain the methylmercury changes. |
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