Lignin, carbohydrate, and amino sugar distribution and transformation in the tropical highland soils of northern Thailand under cabbage cultivation, Pinus reforestation, secondary forest, and primary forest

Abstract

Structure and transformation processes of soil organic matter (SOM) are extremely complex, but advancing our knowledge on SOM cycling is a prerequisite for a sustainable soil management. To get a better insight to this issue, we determined the vertical distribution of lignin, carbohydrates, and amino sugars in bulk soils and NaOH-extracts using wet chemical techniques. These results were compared with those obtained by solution ¹³C nuclear magnetic resonance (NMR) spectroscopy after alkaline extraction. Soil samples were taken under a primary forest, a secondary forest, a 20-year-old Pinus kesiya (Royle ex Gordon) reforestation established following 15 years of cultivation, and a cabbage cultivation site in northern Thailand. Significantly lower contents of organic C and N at the cabbage cultivation and reforestation sites indicated that the replacement of forests by arable land at the reforestation and cabbage cultivation sites about 30 years ago resulted in enhanced breakdown of SOM. This means that after 20 years of Pinus growth, reforestation did not lead to a significant build-up of organic matter in the mineral soil. With increasing soil depth the sites showed comparable decreases in soil organic matter, exhibiting a typical pattern of decomposition expressed by a higher degree of side chain oxidation, increasing carboxyl functionality, and a decrease of lignin-derived phenols and aromatic compounds. Microbial contribution to SOM was determined using the carbohydrate and amino sugar biomarker approach. The amino sugars were predominantly of fungal origin in the organic layer. In the mineral soil, bacterial amino sugars dominated and the relative contribution of amino sugars to SOM increased with depth. Comparison of results from wet chemical analyses and of liquid-state ¹³C NMR signatures requires that alkaline-extractable organic matter is representative for bulk soil organic matter. This seemed to apply to lignin-derived phenols and amino sugars but not to neutral sugars and uronic acids. Significant correlations were found for lignin-derived phenols with phenolic C (R = 0.74, P < 0.01) for the bulk forest site samples and amino sugars with O-alkyl C (R = 0.93, P < 0.001) for the mineral soil horizons, whereas the carbohydrate contents did not show any clear correlation. Therefore, we concluded that most of the phenolic C signal intensity might be attributed to lignin, and the enrichment of O-alkyl C with depth may be a result of bacterial resynthesis with a significant contribution of amino sugars.