Carbon and monosaccharides of a tropical Vertisol under pasture and market-gardening: distribution in secondary organomineral separates

Prolonged cultivation may result in declines in total soil organic matter (SOM) and cause changes in the composition of its labile pools (carbohydrates) and in soil aggregation. The effects of long-term (19 years) tropical pasture (GP) and market-gardening (MG) on the distribution of carbon (C) and monosaccharides (MS) (individual carbohydrate monomers) across water-stable aggregates were investigated on a Vertisol (clayey, smectitic, Leptic Hapludert) of Martinique (French West Indies). Our objective was to distinguish whether the carbohydrate components identified, important stabilizing agents for aggregation, were altered after a change in land use. Surface (0–10 cm) soil samples were collected in GP and MG plots, and separated by wet-sieving into four aggregate classes (secondary organomineral separates) prior to analyses of C and monosaccharides. Concurrently, various parts of Digitaria decumbens, the vegetation dominating pasture, were analyzed for monosaccharides. The highest losses of C upon cultivation were observed in the larger aggregates 200–2000 μm where about 41% were in the form of particulate organic matter (POM) and the remaining mainly in the form of an organo-clay 0–5 μm fraction. Cultivation also affected the proportion of soil C present as carbohydrates in the aggregates. All the aggregates of the pasture soil were significantly richer in monosaccharides than those in the cultivated soil, the aggregates 200–2000 μm accounting for most of the soil carbohydrates. Whatever the separates considered in the pasture soil, glucose was the dominant sugar monomer, followed by xylose or mannose. The 0–5 and 5–20 μm aggregates containing more mannose than xylose—a sugar essentially of plant origin—displayed a microbial character. We concluded on a microbial storage of organic matter in aggregates <20 μm. In addition, it appeared that xylose which was highly depleted within macroaggregates after cultivation might be responsible for the management-induced changes in aggregate stability. This sugar, mainly associated with the 0–20 μm fraction, probably originated from plant exudates.