Microbial biomass- C and the basal respiration of fine earth and highly altered rock fragments of two forest soils

Highlyalteredrockfragments (dimension limits between 2 and 10 mm) from two sandstone-derived soils, Cavalla and Buca, of the Vallombrosa Forest, Italy, are porous and manifest chemical properties similar to those of fineearth. In addition, they display similar or higher pH and exchangeable cations than the <2-mm fraction. Data on microbialbiomass-C (Cmic) and basalrespiration showed that the highlyalteredrockfragments are a favourable environment for the presence and activity of the microbial community that survives on the organic matter contained inside this fraction. In both soils the microorganisms inhabiting the rockfragments showed a higher metabolic efficiency than those of the fineearth. In particular, the rockfragments from the deepest horizons, contained a percentage of organic C present as microbialbiomass (Cmic/Corg) decidedly higher than in the fineearth: 8.97 vs 0.57% in the BCb2 horizon of Cavalla and 1.71 vs 0.36% in the BC horizon of Buca. This high metabolic efficiency of the microbial community present in the rockfragments was confirmed by the low metabolic quotient (qCO2) registered throughout the two profiles.