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

Highly altered rock fragments (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 fine earth. In addition, they display similar or higher pH and exchangeable cations than the <2-mm fraction. Data on microbial biomass-C (C_mic) and basal respiration showed that the highly altered rock fragments 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 rock fragments showed a higher metabolic efficiency than those of the fine earth. In particular, the rock fragments from the deepest horizons, contained a percentage of organic C present as microbial biomass (C_mic/C_org) decidedly higher than in the fine earth: 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 rock fragments was confirmed by the low metabolic quotient (qCO₂) registered throughout the two profiles.