RESPONSE OF SOIL MICROBIAL BIOMASS TO 1,2-DICHLOROBENZENE ADDITION INTHE PRESENCE OF PLANT RESIDUES

The impact of 1,2-dichlorobenzene on soil microbial biomass in the pre sence and absence of fresh plant residues (roots) was investigated by assaying total vital bacterial counts, vital fungal hyphal length, tot al culturable bacterial counts, and culturable fluorescent pseudomonad s. Diversity of the fluorescent pseudomonads was investigated using fa tty acid methyl ester (FAME) characterization in conjunction with meta bolic profiling of the sampled culturable community (Biolog). Minerali zation of [C-14]1,2-dichlorobenzene was also assayed. Addition of fres h roots stimulated 1,2-dichlorobenzene mineralization by over 100%, wi th nearly 20% of the label mineralized in root-amended treatments by t he termination of the experiment. Presence of roots also buffered any impacts of 1,2-dichlorobenzene on microbial numbers. In the absence of roots, 1,2-dichlorobenzene greatly stimulated total culturable bacter ia and culturable pseudomonads in a concentration-dependent manner. 1, 2-Dichlorobenzene, up to concentrations of 50 mu g/g soil dry weight h ad little or no deleterious effects on microbial counts. The phenotypi c diversity of the fluorescent pseudomonad population was unaffected b y the treatments, even though fluorescent pseudomonad numbers were gre atly stimulated by both roots and 1,2-dichlorobenzene. The presence of roots had no detectable impact on the bacterial community composition . No phenotypic shifts in the natural population were required to bene fit from the presence of roots and 1,2-dichlorobenzene. The metabolic capacity of the culturable bacterial community was altered in the pres ence of roots but not in the presence of 1,2-dichlorobenzene. It is ar gued that the increased microbial biomass and shifts in metabolic capa city of the microbial biomass are responsible for enhanced degradation of 1,2-dichlorobenzene in the presence of decaying plant roots.