Biodegradation, Stabilization in Humus, and Incorporation into Soil Biomass of 2,4-D and Chlorocatechol Carbons

¹⁴C-labeled catechol, 4-chlorocatechol, and 4,5-dichlorocatechol readily linked into model peroxidase humic acid-type polymers, with 84 to 96% recovery of the ¹⁴C-activity in the reaction mixtures. The 4,5-Dichlorocatechol was the least reactive. 2,4-D did not link into the polymers. The biodegradation of labeled catechol, 4-chlorocatechol, and 4,5-dichlorocatechol, free and linked into model polymers and of 2,4-D, a fungal melanin, ferulic acid, wheat straw, and glucose were followed over 6-month and 1-year incubation periods in four soils, and the distribution of the residual activity in biomass, new humus, and 6N HCl hydrolyzable substrates was determined. The rate and extent of degradation depended upon the substrate, the concentration, and the soil. The 2,4-D was rapidly degraded with 86 to 94% of the side chain and 73 to 85% of the ring-¹⁴C evolved as CO₂ over 1 year. Only 38 to 50% of the catechol, 22 to 63% of the 4-chlorocatechol, and 22 to 62% of the 4,5-dichlorocatechol carbons were lost over 1 year indicating substantial stabilization of these compounds, probably by enzymatic polymerization reactions. Catechol, 4-chlorocatechol, and 4,5-dichlorocatechol incorporated into model polymers were still more stable with 6 to 22, 8 to 42, and 13 to 43%, respectively, of the ¹⁴C evolved as CO₂. After 1 year, about 3 to 8, 2 to 14, 1 to 7, 0 to 8, and 2 to 15% of the residual ¹⁴C activity from catechol, 4-chlorocatechol, 4,5-dichlorocatechol, the ring portion, and the side chain of 2,4-D, respectively, was present in the biomass. The amount of residual C in the biomass from the catechols incorporated into the model polymers was very low and varied from 0 to 3%. The amounts of the residual ¹⁴C solubilized upon 6N HCl hydrolysis were related to the extent of biodegradation of the substrates. With highly degraded substrates, such as side chain 2,4-D, glucose and wheat straw carbons the major portion of the residual C was lost upon hydrolysis. With highly resistant substrates such as catechol, the model polymers, and the fungal melanin, the major portion was not lost upon hydrolysis. The other substrates gave intermediate values.