Effects Of Forest Conversion On Quantities And Spectroscopic Characteristics Of Soil Dissolved Organic Matter In Subtropical China

DOM(Dissolved Organic Matter,include dissolved organic carbon,dissolved organic nitrogen,dissolved organic phosphorus and so on)is a very active component of organic matter,it offers a lot of nutrients which can be easily utilized by soil microorganisms,and it plays an important role in global carbon cycle,its availability is mostly influenced by its structural and chemical characteristics.Castanopsis kawakamii was the most common seen broadleaved tree species in Sanming,Fujian province,mid-subtropical China,while many natural forests of Castanopsis kawakamii have been transformed into plantations in many areas of southern China these decades.Many studies focused on dynamics of soil organic matter and biomass after forest conversion,while there is little known about how these changes will affect the soil DOM.In this study,we investigated the quantities and spectroscopic characteristics of soil DOM in soil lays of 0—5 cm and 5—10 cm from natural forest of Castanopsis kawakamii(NF)(forest age more than 150a,area more than 700 hm2)and five plantations of Castanopsis kawakamii(CAK),Schima superba(SCS),Phoebe zhennan(PHZ),Castanea henryi(CAH)and Fokienia hodginsii(FOH)in a national forest park in Sanming,Fujian province by means of Dissolved Organic Carbon(DOC),Dissolved Organic Nitrogen(DON)content,Special Ultraviolet-Visible Absorption(SUVA),Emission Fluorescence Humification Index(HIXem),Synchronous Fluorescence Humification Index(HIXsyn),Fluorescence Efficiency(Feff)and Fourier Transform Infrared(FTIR)spectroscopy.Spectroscopic techniques are widely used to investigate the structures of organic matters these years,and we can get a lot of useful information by using these methods.The results showed that,in 0—5 cm soil layer,DOC content decreased 15%,66%,60%,70%and 29%respectively after the NF transformed into CAK,SCS,PHZ,CAH and FOH,while DON content had no significant change(P>0.05).The aromaticity and the humification indices(HIXem,HIXsyn)of soil DOM from NF were significantly(P<0.05)higher than those of plantations,and in all forest types they decreased with depth.Feff of DOM from SCS,PHZ,and CAH was significantly higher than those of other forests.The main emission peaks were aromatic amino acid,aromatic aliphatic and lignin-derived fluorophores with a high degree of conjugation in the synchronous fluorescence spectra of all soil DOM samples.FTIR spectra showed that soil DOM from all forests featured—OH from hydrogen bond,aromatic C=C,O—H and COO-from carboxylic acid,carbohydrate C—O,C—N from amino acid and aliphatic C—O—C.Our results indicate that,in comparison with plantations,soil DOM from natural forest is nutrient richer and contains more complex and highly condensed compounds,therefore soil from natural forest is better for the accumulation of organic matter,and molecules with more complex structure are more likely to be absorbed onto upper soil surface other than migrating to deeper soil layer due to their poor mobility,among the five kinds of plantations,CAK and FOH are better for the regaining of soil fertility.