Tracking the fate of dung-derived carbohydrates in a temperate grassland soil using compound-specific stable isotope analysis

Carbohydrates are major organic components of dung and are likely to contribute substantially to increased carbon stocks in manured soils. To investigate this hypothesis, a field-scale experiment was conducted on a temperate grassland site in Devon, UK. C-4 dung (bulk delta C-13 value -12.6%) was applied to a temperate grassland C-3 soil (bulk delta C-13 value -30.3 parts per thousand) in April and the surface soil beneath cow pats sampled at seven dates over a year. Total carbohydrates were extracted as their monosaccharide components and analysed as the alditol acetates using gas chromatography. The delta C-13 values of the major monosaccharides glucose (-11.5 +/- 0.6 parts per thousand), xylose (-10.4 +/- 0.4 parts per thousand), arabinose (-10.4 +/- 0.5 parts per thousand) and galactose (-8.3 +/- 1.6 parts per thousand) extracted from the C-4 dung via acid hydrolysis were indicative of their source. Their weighted mean delta C-13 value was -10.8 parts per thousand, 1.8 parts per thousand more C-13-enriched than the bulk dung value. The delta C-13 values of individual monosaccharides recovered by acid hydrolysis in the 0-1 cm and 1-5 cm soil horizons beneath C-4 cow pats, compared with control soils determined over 372 days, allowed assessment of the extent of incorporation and fluxes of dung-derived monosaccharides. A maximum of 60% of the dung C in soil was derived from carbohydrates after 56 days, declining to around 20% after 372 days. Incorporation dynamics varied between monosaccharide species. Glucose, xylose and arabinose behaved in a similar manner because of their predominantly plant cell wall derived provenance in the dung, whilst dung-derived galactose and mannose appeared to have a microbial source in the soil. The dynamics of total dung-derived monosaccharides in the top 5 cm was comparable to incorporation and flux of bulk dung C, previously estimated using bulk delta C-13 values. The movement of dung-derived carbohydrates into the soil was inequivalent between the 0-1 cm and 1-5 cm horizons. The lack of a significant difference in concentration, but the evidence for the persistence of dung-derived monosaccharides in soil based on delta C-13 values, indicated replacement of existing pools in the soil, suggesting that the ability of this particular soil to sequester further C derived from carbohydrates was limited. (C) 2009 Published by Elsevier Ltd.