Abstract: Dissolved organic matter (DOM) is the most active component of organic matters,and plays an important role in the translocation and transformation of pollutants and nutrients in terrestrial ecosystem.In this paper,the vertical fractionation of DOM in a yellow brown soil under water-dry cultivation was investigated by field experiment and physico-chemical analyses.The results showed that in treatments CK,chemical fertilizer and sewage sludge,the DOM concentration decreased from 145.8,117.7 and 114.8 mg穔g^-1 to 21.57,23.23 and 13.78 mg穔g^-1 with soil depth,respectively.In control and fertilizer-treated soils,the hydrophilic fraction of DOM was increased respectively from 19.01% and 18.47% to 34.97% and 44.37% with soil depth,and the biodegradable rate of DOM also increased with soil depth.In sludge-treated soil,a reverse tendency was found for the concentration of hydrophilic DOM and its biodegradable rate.Hydrophilic fraction decreased from 36.96% to 17.07%,the increase of hydrophobic fraction was from 63.04% to 82.93%,and the biodegradable rate decreased from 53.19% to 30.75% with soil depth.The biodegradable rate of DOM was positively correlated to the hydrophilic fraction of DOM,and UV spectra analysis also suggested this.

Key words: Arable soil, Dissolved organic matter, Hydrophilic, Biodegradable, Vertical fractionation