The Optical Characteristics And Influencing Factors Of Soil DOM In Purple Soil Area Under Different Land Uses

Dissolved organic carbon?DOM?is a kind of dissolved organic mixture with complex structure,active reaction and easy migration,which is the most active part of soil organic matter.Soil DOM mainly migrates into downstream water bodies through runoff and sediment under rainfall erosion,and also moves into deep soil or the groundwater network with the movement of soil interflow.Soil DOM is an important source of DOM in downstream water bodies.Different components in DOM have different groups and different absorption characteristics under different types and wavelengths of light waves,spectroscopic techniques can clarify the composition and structure of DOM and indicate the source of DOM.However,it is not clear the soil DOM turnover mechanism under different land uses and the exact source of DOM erosion now.To reveal the soil DOM composition structure and its influencing factors under different land uses,and clarify the potential sources of DOM erosion,the soil samples of different soil layers?0-5,5-10,10-15 and 15-20 cm?under three typical land uses in Purple Soil Area were studied.Analyzing the soil physicochemical properties,DOC contents and their optical characteristics and correlations betwwen them under the three land uses.The research results are as follows:?1?The bulk density of forest was significantly higher than that of garden and arable land?P<0.05?,and the electric conductivity and cation exchange capacity of garden was significantly higher than that of forest and arable land?P<0.05?.In the same land use,bulk density and pH increased with soil depth,while water content and electric conductivity decreased with soil depths.The soil carbon?C?,nitrogen?N?,and phosphorus?P?contents and stoichiometry ratios differed significantly among land uses?P<0.05?.In the 0?5 cm soil layer,the C:P and N:P of forest were significantly higher than the garden and arable land than other soil layers?P<0.05?,the P content of the garden land was significantly higher than that of the forest and arable land in each soil depth?P<0.05?.Under the same land use,soil C,N,P contents in the 0?5 cm layer were significantly higher than that of any other soil layer?P<0.05?,but the C:N,C:P,N:P between the soil layers were not significant?P>0.05?.The average values of soil C:N,C:P and N:P in the study area was 10.63,36.08 and 3.32,respectively,which was lower than the average values of China and the global soil,and N was the limiting element of soil in this area.In the study area,soil bulk density was very significaltly negatively correlated with soil C,N content?P<0.01?,and electric conductivity and soil moisture contents had significant positive correlations with soil C,N content,C:N,C:P and N:P?P<0.05?.?2?Land uses significantly affected the contents of soil dissolved organic carbon?DOC?,colored dissolved matter?CDOM?,and fluorescent dissolved organic matter?FDOM??P<0.05?.The order of DOC contents was forest(63.54 mg·L^-1)>arable land(62.79 mg·L^-1)>garden(35.09 mg·L^-1),but soil CDOM and FDOM in garden(6.95 m^-1,4821.95 m^-1)were significantly higher than forest(3.57 m^-1,2 980.76 m^-1)and arable land(5.19 m^-1,2 759.97 m^-1)?P<0.05?,indicating that the content of chromophore and conjugated bond components in garden soil DOM were higher.Soil DOC,CDOM,FDOM contents among soil layers were not significant in forest,garden and arable land?P>0.05?,but the soil CDOM and FDOM contens decrease with the depth of soil layer in the vertical level.There were no significant correlation between soil DOC content with soil C and N?P>0.05?,but CDOM and FDOM had significant positive correlation with soil C,N,C:P and N:P?P<0.05?.There were significant nagative correlation between soli C,N contents?C:P?N:P with CDOM and FDOM?P<0.05?,while significant positive correlation between soil moisture contents and electrical conductivity with CDOM and FDOM?P<0.05?.?3?There were significant differences in DOM composition and structure under different land uses?P<0.05?.The order of size of SUVA₂₅₄,SUVA₂₆₀,a?355?,C:C in DOM were all like garden>arableland>forest,and the order of E2:E3,E4:E6 in DOM were all like forest>arable land>garden,indicating that the aromatic structure,molecular hydrophobic components and colored humus in the garden soil DOM were significantly higher than those in forset?P<0.05?.Under the three land uses?forest,garden and arable land?,SUVA₂₅₄,SUVA₂₆₀,C:C in DOM showed decreasing trend with the increasing soil depth,but were not significant in each soil layer?P>0.05?.Soil bulk density and electrical conductivity had significant positive correlation with soil SUVA₂₅₄,SUVA₂₆₀,a?355??P<0.05?,and soil C and N were significantly positive correlated with soil SUVA₂₅₄,SUVA₂₆₀,a?355?,C:C,E2:E3,E4:E6?P<0.05?.?4?In the studied area,the humification coefficients?HIX?of forest,garden and arable land was 3.14±1.84,2.90±1.62,3.48±1.68,respectively,which were all less than 4.The autogenic source index?BIX?was 0.68±0.07,0.70±0.09,0.69±0.06,respectively,which were among 0.6?0.7.The fluorescence index?FI?was 1.51±0.07,1.46±0.09,1.47±0.09,respectively,which were all among 1.4-1.9.According to the comprehensive analysis of HIX,BIX,and FI,DOM in the study area mainly came from terrigenous?plant litter,soil humus,fertilization,etc.?and had little humification.In forest and garden,HIX had significant positive correlation with soil C,N,P,C:N,C:P,and N:P?P<0.05?.?5?There were extremely significantly positive correlations among SUVA₂₅₄,SUVA₂₆₀,C:C,a?355??P<0.01?,and significantly positive correlations between E2:E3and E4:E6 in DOM?P<0.01?,which indicated that the aromatic substances,colored humus and hydrophobic components in DOM were closely related.BIX was significantly positive correlated with FI,but negatively correlated with HIX?P<0.05?.BIX and FI had a very significant positive correlation with SUVA₂₅₄,SUVA₂₆₀ and C:C?P<0.01?.BIX,FI and HIX had a very significant positive correlation with E2:E3and E4:E6?P<0.01?.There was no significant correlation between soil DOC,CDOM and FDOM?P>0.05?,while CDOM and FDOM showed extremely significant positive correlation?P<0.01?.