Tillage has been reported to reduce organic matter concentrations and increase organic matter turnover rates to a variable extent. The change of soil climate and the incorporation of aboveground C inputs within the soil lead to no unique effect on biodegradation rates, because of their strong interaction with the regional climate and the soil physical properties. The periodical perturbation of soil structure by tools and the subsequent drying±rewetting cycles may be the major factor increasing organic matter decomposition rates by exposing the organic matter that is physically protected in microaggregates to biodegradation. This paper reviews the assessed effects of tillage on organic matter, the scale, extent and mechanisms of physical protection of organic matter in soils. #

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Understanding the chemical composition of soil organic matter (SOM) requires the determination of the dynamics of each class of compounds. We measured the dynamics of carbon in neutral carbohydrates by use of natural 13 C labelling in an experimental wheat and maize sequence extending over 23 years. The isotopic composition of individual neutral monosaccharides was determined in hydrolysed particle-size fractions by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) of trimethylsilyl (TMS) derivatives. The sensitivity in terms of 13 C/ 12 C ratios ranged between 1 and 2% depending on the monosaccharide. The age distribution of neutral sugar carbon was very similar to that of total soil carbon. Particulate organic matter (POM) was characterized by the predominance of glucose and xylose of vegetal origin. In POM > 200 mm, the mean age of sugar-C (5 years) was slightly less than that of total carbon (7 years). Xylose was younger than glucose. The fine fraction 0-50 mm contained mainly glucose, arabinose, galactose, xylose, fucose and mannose, which had predominantly microbial origins. The mean age of carbohydrate carbon in the fraction 0-50 mm was between 60 and 100 years and was similar to that of total organic carbon (OC). No difference in the age of carbon between the individual monosaccharides was found. The POM fraction 50-200 mm had an intermediate signature and turnover. Considering the typical lability of carbohydrates, the relatively great age of carbohydrate carbon may be explained by physical or chemical protection from degradation, as well as by recycling of soil organic matter carbon by soil microbes.