Seasonal Changes in the Carbohydrate Pool of an Atlantic Forest Soil under Different Vegetation Types

The seasonal variations of the content and composition of soil carbohydrates, a labile pool of the soil organic matter, were studied in a Humic Cambisol located within the Atlantic temperate-humid zone (Galicia, N.W. of Spain) and developed over basic schists and under different type of vegetation: Quercus robur (climax forest), Pinus pinaster and Eucalyptus globulus. Soil samples from the A horizon (0-15 cm depth) of the three different forests were collected in spring, summer, autumn and winter. The carbohydrate content was estimated by colorimetry after their extraction by a sequential two-step acid hydrolysis method and further purification of the hydrolysates (first hydrolysis fraction, FA, non-cellulosic polysaccharides; second hydrolysis fraction, FB, cellulosic polysaccharides). The total amount of neutral sugars (hexoses and pentoses) from the three forests was in the range of 2.9-27.4 g kg(-1) d.w., and represented between 5 and 12 % of the total organic C. The carbohydrate content was much higher in the FA fraction than that in the FB fraction, hexoses predominating over pentoses in both fractions. Seasonal variations of hexoses and pentoses exhibited the same behaviour pattern, showing that for FA and FB fractions higher concentrations of both neutral sugars were found in spring and winter (mean values: 12 and 14 g total neutral sugars kg(-1) d. w., respectively) than in summer and autumn (7 and 8 g total neutral sugars kg(-1) d. w., respectively). Likewise, for each year's season, the total content of both hexoses and pentoses in both fractions varied in the order: Quercus (16 g neutral sugars kg(-1) d. w.) > Pinus (9 g neutral sugars kg(-1) d. w.) > Eucalyptus (5 g neutral sugars kg(-1) d. w.) forests. Vegetation type clearly affected soil organic carbon, carbohydrate content and aggregate stability, while the ratio neutral sugar C / total organic C was mostly affected by season. Compared to the climax oak forest, the pine and eucalyptus stands exhibited a significant reduction in the content of carbohydrates, total organic C, total N and in aggregate stability, showing a decrease in soil quality.