Characterization of carbohydrates during early diagenesis of five vascular plant tissues

Long-term changes in the carbohydrate composition of 5 different vascular plant tissues, including black mangrove leaves and wood (Avicennia germinans), cypress needles and wood (Taxodium distichum) and smooth cordgrass (Spartina alterniflora), were measured as these tissues decomposed over a 4 yr period under sub-aqueous conditions. Carbohydrate composition was measured using a molecular-level analysis for neutral sugars and a modified version of the MBTH (3-methyl-2-benzothiazolinone hydrazone hydrochloride) method for colorimetric determination of total carbohydrate yields. Minimal cross contamination from non-carbohydrate vascular plant constituents indicated the MBTH method was highly specific for carbohydrates. The difference between total carbohydrate yields using the MBTH method and total neutral sugar yields revealed a substantial carbohydrate fraction (7–23% of the total plant carbon) in fresh and senescent tissues that was not identified at the molecular level. The molecularly uncharacterized fraction of carbohydrates probably consisted of ketoses, uronic acids and amino sugars. The decomposition series demonstrated certain features about carbohydrate diagenesis not apparent from previous short-term degradation studies. During the latter phase of decomposition (2–4 yr), selective carbohydrate loss relative to bulk tissue was not evident in 2 of 3 herbaceous tissues. This indicates that carbohydrates may be of similar reactivity as bulk tissue in highly decomposed particulate organic matter. The extent and timing of all compositional changes were tissue dependent, yet certain trends emerged which were consistent with geochemical observations. In herbaceous tissues, both glucose and xylose were selectively degraded while deoxy sugars increased in relative abundance. These changes resulted in an increased abundance of initially minor neutral sugars and a general trend towards a more uniform neutral sugar composition. A clear reduction in carbohydrate yields (mg carbohydrate carbon/100 mg organic carbon) among all tissues provided the most consistent indicator of diagenetic status. Total carbohydrate yields, mole percentages of glucose and percent deoxy sugars in highly degraded herbaceous tissues were similar to those measured in particulate organic matter fractions of major world rivers, and provide diagenetic parameters which link relatively fresh plant tissues to their degraded counterparts in aquatic environments.