Biodegradation and Stabilization after 2 Years of Specific Crop, Lignin, and Polysaccharide Carbons in Soils

The purpose of this study was to follow the biodegradation and stabilization in two soils of specific carbons of model and cornstalk lignins, lignin monomer alcohols, wheat straw, and the polysaccharide fraction of wheat straw over a 2-year incubation period using ¹⁴C-labeled substrates. Incubations were carried out in a closed, constantly aerated system and the ¹⁴CO₂ released determined at specific intervals. After incubation the amounts of residual ¹⁴C in various soil humus fractions, in the biomass, and that solubilized by acid hydrolysis were determined. During the 2-year period, about 40% of the ring and two-side chain carbons of coniferyl and coumaryl alcohol units in model and cornstalk lignins had evolved as CO₂. Losses of CH₂OH and three-side chain and of OCH₃ carbons varied from 52 to 69%. The degradation rate was most rapid during the first 3 to 6 months of incubation. total C losses from whole wheat straw and cornstalks for comparison at 2 years were about 72% and for the polysaccharide portion of the wheat straw, 83%. During the first 6 months about 47% of the two-side chain and ring carbons and 61 to 66% of the other carbons of free coniferyl alcohols had evolved as CO₂. During the next 18 months these values had increased only about 11%. A major portion of the residual lignin carbons was recovered in the HA fractions, relatively small quantities were lost upon 6M HCl hydrolysis, and < 1% was estimated to be in the biomass. Most of the residual wheat straw C and especially the polysaccharide carbon was solubilized upon 6M HCl hydrolysis, 3 to 8% was still present in biomass, and 50% remained in the NaOH extracted soil. The results indicate that the major portions of the lignin carbons are incorporated into the more resistant or aromatic portions of the soil humus. The polysaccharide carbons, on the other hand, are metabolized and utilized for energy and synthesis of cell polymers, primarily proteins and polysaccharides, some of which are stabilized in the humus but are still susceptible to acid hydrolysis. Loss as CO₂ and distribution of the residual carbons of free lignin alcohols in HA and losses upon 6M HCl hydrolysis were intermediate between the lignin and polysaccharide carbons.