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Background and aims: We report on the modifications induced by the roots of Erica arborea L. on a soil derived from alkaline and fine-textured marine sediments. Methods: Physical, chemical, mineralogical and biochemical properties of bulk soil and of the rhizosphere of Erica were characterised to evaluate its role on soil development. Results: Once the upper horizons had been decarbonated because of geomorphic and pedogenic processes, Erica colonised the soil and progressively modified it through the activity of roots. In the upper horizons, there was no difference between rhizosphere and bulk soil for pH, organic C and exchangeable Al and H. At depth, pH, organic C and exchangeable Al and H differed between rhizosphere and bulk soil. The weathering reactions induced by the Erica roots caused a relative quartz enrichment in the rhizosphere compared with the bulk soil. In the E, EB and Bw horizons, the microbial community of the rhizosphere appeared better adapted than in the underlying 2Bw horizons, where the rhizospheric microorganisms were poorly adapted as these horizons represented the boundary between acid and sub-alkaline soil environments. Conclusions: The activity of Erica roots modified soil properties so to produce more favourable conditions for itself and the rhizosphere microflora. [ABSTRACT FROM AUTHOR]

The organic matter extracted from various mineral horizons of two forest soils, one under silver fir (Abies alba Mill.), the other under European beech (Fagus sylvatica L.), was fractionated by dialysis into three fractions, 100–1000, 1000–8000, and >8000 Da. On a C basis, in all horizons the recovered organic matter amounted to less than a half of the total and was mainly composed of molecules >8000 Da. The 100–1000 Da fraction had a principal elemental composition profoundly different from the other two fractions, which, instead differed from each other significantly only for the S content and the molar ratio of C with N. No significant difference in this regard was found between soils. The richness in O and some typical absorption bands in the FT-IR spectra indicated that the 100–1000 Da fraction had a lot of carboxyl moieties. The spectroscopic (13C NMR) investigation showed that the 1000–8000 and >8000 Da fractions had a prevalently aliphatic nature and signals attributable to polysaccharides (O-alkyl C) revealed overall a high presence of non-humic biopolymers. These latter were significantly more abundant, suggesting a lower degree of humification, in the >8000 Da fraction than in the 1000–8000 Da fraction. Comparing soils, that under beech appeared significantly richer in O-alkyl C than that under fir. The organics extracted from the A horizon of both soils had positive Δ14C values, indicating recent synthesis mainly due to the present forest cover. The mean residence time (MRT) of the combined 100–1000 Da and 1000–8000 Da fractions and the >8000 Da fraction increased with depth, even to about 5000 years in the more than 1-m deep BC horizons under beech. In some cases, and especially in the soil under fir, despite higher values of δ13C denoting stronger microbial decomposition, the 100–8000 Da fraction showed a higher MRT than that of the >8000 Da fraction, perhaps due to its ascertained lower content of non-humic biopolymers. [ABSTRACT FROM AUTHOR]

The carbon dioxide efflux to the atmosphere and the concentrations at various depths in two soils were measured, for more than a year, under pure stands of silver fir ( Abies alba Mill.) and European beech ( Fagus sylvatica L.) in central Italy. Microbial biomass and activity at the monitored depths were determined in the laboratory and the CO2 evolved from incubated samples was submitted to radiocarbon analysis to assess the mean residence time of the organic matter degraded by microorganisms. The CO2 efflux showed similar trends in the two soils, with highest values in October and lowest in January. The efflux depended more on air and soil temperatures than soil moisture, and was related to these variables better under fir than under beech. In both soils, the CO2 concentration increased with depth: in the top horizon it was low and similar to that of the atmosphere, while in the deeper horizons it often amounted to considerable values (up to more than 1% by volume in the BC horizon under fir). The subsoil of the fir stand generally showed much higher CO2 concentrations than that of the beech. The basal respiration as determined in the laboratory was at a maximum in the topsoil and decreased sharply downwards. Therefore, the high CO2 concentrations measured in the field at the bottom of the profiles—where roots were few, and microbial biomass and available C pool were at a minimum—appeared to be due more to slow diffusivity of the soil matrix rather than to heavy release of the gas by the biota. The organic matter respired by microorganisms in incubated soil samples showed positive values of Δ14C that revealed a recent synthesis. The estimated mean residence time increased with depth, suggesting a generally higher degree of stabilisation of the organic pool in the subsoil. [ABSTRACT FROM AUTHOR]