Fig. 2. The carboxyl and phenolic acid content of the hydrophilic and hydrophobic DOM fractions of the plant biomass- and animal manure-derived DOM before (left bar) and after decomposition (right bar). *, **, and *** designate significant difference between the fresh and decomposed total acidity content at the p = 0.05, 0.01, and 0.001 levels using the student's t test. NS indicates that the mean difference is not significant.