Decomposition in Soil of 14 C‐labeled Coumaryl Alcohols; Free and Linked into Dehydropolymer and Plant Lignins and Model Humic Acids

Side chain and ring‐ 14 C‐labeled coumaryl alcohols and coumaric acids and ring‐ 14 C‐labeled ferulic, caffeic, protocatechuic, vanillic, and p ‐hydroxybenzoic acids and coniferyl alcohol were synthesized. The coumaryl alcohols and phenolic acids were linked into model phenolase humic polymers. Labeled corn ( Zea mays L.) stalk lignins were prepared by injecting the labeled coumaric acids as lignin precursors into growing corn plants. Model DHP lignins were prepared by polymerizing the labeled coumaryl alcohols with unlabeled coniferyl alcohol using peroxidase and hydrogen peroxide. Decomposition of the labeled carbons in the phenolic compounds and in the polymers was followed in a neutral sandy loam soil. After a 28‐week incubation period about 58, 44, and 47% of the CH 2 OH and 2‐(side chain) and ring‐ 14 carbons, respectively, of applied coumaryl alcohols had evolved as 14 CO 2 . Losses of ring C from other common benzenoid compounds ranged from 68 to 84%. Linkage of coumaryl alcohols into model humic acids reduced losses of ring carbons to 5 to 12%. The decomposition percentages for the CH 2 OH and 2‐(side chain) and ring‐ 14 carbons of the coumaryl alcohols linked into model lignins were 33, 18, and 20%, respectively, while comparable values for the same carbons in corn stalk lignins averaged 7% higher. About 40 to 60% of the residual 14 C activity from the lignins and model polymers was recovered in the humic acid fraction of the soil. The amounts of coumaryl alcohol and other phenolic compounds linking into model phenolase polymers were inversely related to the degradability of the compounds in soil.