Submitted to: American Chemical Society Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: 5/12/2011
Publication Date: N/A
Technical Abstract: Lignin is the third most abundant plant constituent after cellulose and hemicellulose and thought to be one of the building blocks for soil organic matter formation. Lignin can be used as a predictor for long-term soil organic matter stabilization and C sequestration. Soils and humic acids from forest, matching silvopasture sites, and an open pasture site from hill-land environments were analyzed for copper oxide lignin-derived phenolics by HPLC. In addition, humic substances were analyzed by 13C NMR for lignin functionalities. Total C-lignin in soils, the sum of the vanillyl, syringyl, and cinnamyl groups, was higher for the silvopasture followed by forest and open pasture sites (583 +/-29, 431 +/- 27, and 376 +/- 17, respectively); however, the humic acids from the forest sites contained more C-lignin, followed by the open pasture and silvopasture sites (1251 +/- 170, 1042 +/- 14, and 722 +/- 16, respectively). The acid: aldehyde ratios of vanillyl and syringyl groups, indicators of lignin degradation, were higher for the forest sites than the silvopasture or open pasture sites, for both the soils and humic acids. NMR spectra suggest that humic acids are dominated by lignin, cutins, and cellulose; silvopasture sites have more carbohydrates, more aromatics and/or unsaturated carbons, and a lesser amount of carbons in alkyl chains than the forest sites. Furthermore, from 22 to 27% of the total NMR-detectable C content in humic acids is accounted by aromatic moieties in the silvopasture and open pasture sites; whereas about 18% is accounted in forest sites. Results indicated silvopasture soils had highest yet youngest lignin content, which derived from both woody and non-woody tissues, suggesting that plant residues in silvopasture sites decomposed faster than in forest systems. Conversely, humic acids from forest soils had the highest yet the oldest lignin content, derived from woody tissues.