Amino acids and amino sugars as molecular indicators of the origins and alterations of organic matter in buried tephra layers
2020
Hobara, Satoru | Ogawa, Hiroshi | Benner, R. (Ronald)
Deep soils contain more organic carbon than surface soils, and deep organic carbon has a longer carbon mean residence time. Relatively little is known about the degradation of organic matter in deep soils, especially over millennial time scales. Tephra (volcanic ash) deposition occasionally buries surface soils and forms deep soils rich in organic matter. In order to investigate the alteration and dynamics of buried organic matter in deep soils, we characterized common biochemicals, combined amino acids and amino sugars, in tephra layers deposited over millennial timescales in Hokkaido, Japan. Chemical analyses indicated relative abundances of specific amino acids (e.g. leucine and phenylalanine) and amino sugars, (e.g. glucosamine and galactosamine) changed with tephra soil age, indicating microbial alterations over millennial time scales. The observed changes in chemical composition were much slower than those typically observed during litter decomposition. Adsorption characteristics of the tephras developed with weathering and contributed to substantial organic carbon accumulation in the deep, aged tephra layers, such as Ta-c (deposited 1640 yr BP) and Ta-d (deposited 8940 yr BP). These observations indicate rapid microbial degradation and alteration during early stages of litter decomposition followed by much slower degradation following burial, when physicochemical processes such as sorption and occlusion and changes in environmental conditions influence organic matter accumulation and composition. Thus, microbial processes during organic matter decomposition and physicochemical processes during tephra diagenesis shape the chemical characteristics, reactivity and age of organic matter in buried tephra soils.
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