Effect of land use change on contents and distribution of monosacharides within density fractions of calcareous soil

In the last century, many calcareous soils in Castilla León (northwestern Spain) have been transformed from natural Quercus ilex forest to cropped land. During the past 40 years, cropping was abandoned and some of these soils has been reforested with Pinus halepensis. We studied how thEse land use changes affected the soil organic matter amount (C stocks) and characteristics. Density fractionation combined with ultrasonic dispersion enables separation and study of soil OM fractions, considered on the basis of the mechanisms of physical protection: non-physically protected OM (FF), OM occluded into aggregates (OF), and OM stabilized in organo-mineral complexes (DF). In the present study, whole soils and density fractions were analyzed for organic C amount (OC) and for carbohydrate content. The study aimed to describe the carbohydrate content in whole soils and its SOM density fractions to find out whether the carbohydrates can be used to explain the origin of SOM fractions and if they can depict its response to land management. We found a loss of 67% of the topsoil OC as a result of converting the natural Quercus ilex forest to cropped land, ∼100 years ago. After crop abandonment and soil reforestation with Pinus halepensis, the stock of topsoil OC increased. After ∼40 years of pine plantation, an average recovery of 71% of the previously lost OC had been recovered (43% loss of reference topsoil). The changes in OC stocks affected mainly the free fraction (FF) but also the organo-mineral (DF) one. Our results point to the role of physical protection in the C stocks but also that organo-mineral associations could be disrupted under a land use change. Monosaccharide content was significantly different (p < 0.01) among land uses. Gas chromatography analysis revealed significant differences in monosaccharide composition between land uses and also among density fractions. Whatever the fraction and land use considered, glucose was the dominant sugar monomer, followed by mannose and xylose.