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

The effects of long-term (19 years) tropical pasture (GP) and market-gardening (MG) on the distribution of soil organic matter (SOM) and carbohydrates across primary organomineral separates were investigated on a Vertisol (clayey, smectitic, Leptic Hapludert) of Martinique (French West Indies). Surface samples were collected in GP and MG plots, separated by wet-sieving into five particle-size fractions (200–2000, 50–200, 20–50, 5–20, 0–5 μm), and analyzed for carbon (C), nitrogen (N), and monosaccharides (MS). Long-term cultivation resulted in significant declines in total SOM present in the soil and caused changes in SOM distribution when compared to the grazed pasture (GP). A major difference between GP and MG was that the proportions of the soil organic C present in the primary organomineral separates were considerably greater in GP than in MG. Losses of C upon cultivation were higher in the coarse sand-sized fraction 200–2000 μm compared with the 0- to 5-μm fraction although the majority of SOM was associated with this one. Concomitantly, cultivation affected significantly the proportion of soil C present as monosaccharides. The finer separate F 0–5 μm accounted for most of the soil monosaccharides and displayed a microbial character. The conditions of hydrolyses used for extracting the monosaccharides revealed the dominance of hemicellulosic sugars in the bulk soils and their organomineral separates <200 μm. On the opposite, the coarser separates F 200–2000 μm of plant character were characterized by cellulosic sugars.