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We evaluated the influence of the rhizosphere, soil depth, and altitude on the amount and nature of the density separated soil organic matter (SOM) pools. Samples were collected from the A and AB horizons of European beech (Fagus sylvatica L.) forest soils located at two altitudes (800 and 1000 m) along 1° of latitudinal gradient in central Italy, by using altitude as a proxy for air temperature change. Specifically, we hypothesized that: i) larger amount of labile organic matter, comprising of fresh plant and organism residues and easily degradable molecules, was allocated in the rhizosphere than in the non-rhizosphere soil, and ii) the temperature had a stronger effect on the C pools of the rhizosphere than in that of the non-rhizosphere soil. At both altitudes, we found more organic C (OC) associated with the light fractions of the rhizosphere than in those of the non-rhizosphere soil and, specifically in the rhizosphere free light fraction, larger OC concentrations were observed at 1000 m than at 800 m above sea level. These higher amounts of OC have been attributed to roots, which are one of the main source of particulate organic matter, and their activity and turnover increase when the environmental conditions become more restrictive, as it happens at higher altitude. Conversely, no effect related to rhizosphere and altitude on the OC associated to the heavy fraction was found. The recalcitrance of the OC of the heavy fraction has been ascribed both to its protection due to the tight bounds to mineral particles and to its degradation degree, as indicated by δ13C values, which were greater than those of the light fractions. The similar 14C signature and the presence of recent C in all the density fractions of rhizosphere and non-rhizosphere soil of both A and AB horizons suggested the occurrence of a rapid incorporation of fresh organic matter into the mineral horizons, followed by occlusion into aggregates and adsorption on mineral surfaces. Further, the lack of different Δ14C values between the fractions at 800 and 1000 m could indicate that a temperature change of 1 °C is not sufficient to induce marked changes in SOM cycling.
