Dissolved organic matter release and retention in ultisols in relation to land use patterns

The application of organic fertilizer to maintain soil fertility and crop yield has been practiced for thousands of years in China. This practice improves soil carbon sequestration, due to the high level of dissolved organic matter (DOM) in organic manure. In this study, batch equilibrium studies were conducted to examine the capacity of three ultisols from areas under different land use patterns to retain dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) extracted from rape cake and chicken manure. The results showed that the amount of DOM removed or released in solution by the soil was a linear function of the initial amount added to the soil-water system; therefore, analysis of sorption isotherms was best conducted using the initial mass isotherm IM method. The ultisol retained, on average, 19.9% of the total DOC and 41.7% of the total DON in solution, suggesting that ultisol has a relatively low DOC adsorption capacity. The ultisol from a bamboo forest was found to have a higher capacity than that from a pear orchard to retain DOC and DON. The adsorption affinities of DOM according to soil type were in the following order: bamboo forest (BF)>tea garden (TG)>pear orchard (PO). These results suggested that the continuous application of high doses of organic manure, particularly rape cake, may saturate the DOC adsorptive sites, thereby permitting increased leaching of DOC and the possibility of ground water contamination. Furthermore, we note that amorphous Fe and Al oxides play an important role in the adsorption capacity of both DOC and DON in ultisols.