Alberto Agnelli

Mineralogical, physical, and chemical properties of rock fragments in soil

During sample preparation, the coarse fraction (> 2 mm) of soils is commonly excluded from further analytical determinations. Our results demonstrated, however, that the coarse fragments of soils derived from sandstone in Tuscany, Italy, are not chemically inert. From an intensive study of the clasts from three profiles in the Vallombrosa Forest 50 km east of Florence, we have established that the clasts of this fraction, especially those altered and partially altered, manifest properties that in some cases equal or surpass those of the more reactive fine earth. The profiles are underlain by Arenaria del Falterona, a sandstone of the Oligocene time intercalated with siltstone. Our research shows that the coarse fraction participates in a sequence of weathering events that involves dissolution of carbonates and formation and transformation of secondary minerals. Although fresh sandstone and siltstone display distinct mineralogical composition, these differences are not maintained once they undergo accelerated weathering in the soil. Dissolution of carbonate brings indirect enrichment of the noncarbonatic components. Formation of HIV (hydroxy interlayered vermiculite) and HIS (hydroxy interlayered smectite) is the dominant process in the fine earth and clasts. This process becomes particularly important in stabilizing the micropores of the clasts and maintaining a porosity that allows the soil solution to be adsorbed and circulate. Weathering processes thus create voids, release nutrient cations, and render the most weathered clasts similar to fine earth. The progression of weathering into the clasts is demonstrated by the levels of extractable Fe and Al obtained by selective dissolution. Furthermore, our research shows that the rock fragments contain C and N in considerable quantities and have a higher pH and a mineral assemblage less weathered than the fine earth. In addition, clasts constitute a reservoir for nutrients and possess a capacity for proton consumption. Moreover, although the fine earth, including organic matter, dominates the effective cation exchange capacity (ECEC) in the upper horizons, in the B and BC horizons the rock fragments represent from 20 to 55% of the total ECEC. In conclusion, our findings show that the clasts of these reactive lithologies are not inert but play all important role in soils as: (i) reservoirs of nutrients, including N, (ii) sources of cation exchange capacity, (iii) water supplies, (iv) adsorbers of organic pollutants, and in the consumption of protons.

Carbon dioxide efflux and concentrations in two soils under temperate forests

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.

Chemical and spectroscopic characterization of the humic substances from sandstone-derived rock fragments

The characterization of soil organic matter, and of humic substances in particular, has always been made on fine earth, i.e., the <2-mm fraction. The fraction larger than 2 mm, known as rock fragments or skeleton, is commonly discarded. We have extracted the humic acids (HAs) and fulvic acids (FAs) from fine earth, rock fragments, and rock fragments washing (i.e. the fine material adhering to the rock fragments) of the upper forest soil horizons, A1 and A2. The substances were characterized using wet-chemical analyses, FT-IR, and liquid state 13C NMR spectroscopies. HAs of the rock fragments have higher N and H content, paraffinic chains, aliphatic-OH, and highly substituted aromatic groups than those of the fine earth. These features indicate that carbohydrates, lipids, and proteinaceous residues are incorporated in the humic acids of the rock fragments. The fresh biological material residues present in the structure of the skeleton HAs may be attributable to: (i) the selective preservation from microbial and chemical attack offered by the skeletal environment; (ii) the more rapid cycling of the organic matter inside the rock fragments compared with that of the fine earth. The FAs show greater homogeneity than the HAs, probably because of their mobility among the different soil compartments. However, the FAs of the rock fragments have more carboxyl and acidic-OH groups than found in the fine earth. Greater differences are also observed between the two horizons. The FAs extracted from the A2 horizon show less mineralization than those from the A1 horizon. The humic substances extracted from the rock fragments washing show characteristics more similar to those of the skeleton than to those of the fine earth. This suggests that most rock fragments washing originate from the weathering of the rock fragments and, therefore, may be regarded as an intermediate phase between the skeleton and the fine earth.

Exchangeable Ca, Mg, and K of rock fragments and fine earth from sandstone and siltstone derived soils and their availability to grass

Rock fragments (particles >2 mm, are usually considered chemically inert for plant growth. In this paper, the potential fertility in terms of exchangeable Ca, Mg, and K of rock fragments from sandstone and siltstone derived soils from northern Apennines (Italy) is reported and contrasted with that of the fine earth (particles <2 mm). The results show that rock fragments are a source of Ca, Mg, and K. When expressed on a volume basis, the abundance of these exchangeable nutrients sometimes may equal or surpass that of the fine earth. The plant uptake of Mg and K has been demonstrated in growth experiments with Agrostis under controlled conditions.

Early stages of podzolization under Corsican pine (Pinus nigra Arn. ssp. laricio)

Abstract To assess the effects of the plants on pedogenesis, two arboreal species, Corsican pine ( Pinus nigra Arn. ssp. laricio ) and silver fir ( Abies alba Mill.) were compared. We observed that at the base of the largest Corsican pines, patches of bleached soil occur on top of the A horizon. These eluviated areas lie at the outlet of wide bark channels through which the stemflow reaches the ground. In addition, an apparently less bleached material, in the shape of collars, is present around the big roots that collect and conduct the stemflow into the soil. On the contrary, patches and collars are absent in the soil under firs. To understand the role of the two species in the formation of the bleached material, throughfall, stemflow and forest floor solutions were collected for pine and fir. For pine, the stemflow moving along a wide bark channel was collected separately. All these solutions plus the rainfall were obtained and analysed seasonally for 1 year. Also, a number of profiles were excavated in the vicinity of the trunk base of pines and firs. Two profiles were selected for sampling and for the attendant analyses. The results indicate that under silver fir, the processes induce in the formation of an A horizon. The Corsican pine, on the other hand, in 50 years, has been able to produce, even if in small areas, a separate pedogenic process responsible for the bleaching of an existing A horizon. Because the bleaching process is at an initial stage, the chemical differences between the E material and the surrounding A horizon are small. Nevertheless, the E material is poorer in organic matter and in the most mobile inorganic elements than the A horizon. The presence of E material exclusively at the trunk base of the pines is due to the strong acidity of the stemflow — about 30-times higher than that of the fir. The most evident differences between the soils under pine and fir concern the features more directly related to the dynamics of the soil organic matter. Mineral assemblage, which needs longer time for its evolution, is very similar in the two soils; nonetheless a small amount of pedogenic smectite has been recognized in the E material.

Purification and isotopic signatures (δ13C, δ15N, Δ14C) of soil extracellular DNA

The aim of this work was to obtain pure extracellular DNA molecules so as to estimate their longevity in soil by an isotope-based approach. Extracellular DNA molecules were extracted from all horizons of a forest soil and purified by the procedure of Davis (Purification and precipitation of genomic DNA with phenol–chloroform and ethanol. In: Davis LG, Dibner MD, Battey JF (eds) Basic methods in molecular biology. Appleton & Lange, Norwalk, 16–22, 1986) without (DNA1) or with (DNA2) a successive treatment with binding resins followed by elution. The two differently purified DNA samples were compared for their A260/A280 ratio, polymerase chain reaction (PCR) amplification and natural abundance of stable (13C and 15N) and radioactive (14C) isotopes. The purity index and the PCR amplification did not differentiate the efficiency of the two purification procedures. The isotopic signature of DNA was more sensitive and was strongly affected by the purification procedures. The isotopic measurements showed that the major contaminant of extracellular DNA1 was the soil organic matter (SOM), even if it is not possible to exclude that the similar δ13C, δ15N and Δ14C values of DNA and SOM could be due to the use of SOM-deriving C and N atoms for the microbial synthesis of DNA. For extracellular DNA2, extremely low values of Δ14C were obtained, and this was ascribed to the presence of fossil fuel-derived substances used during the purification, although in amounts not revealed by gas chromatography-mass spectrometry analysis. The fact that it is not possible to obtain contaminant-free DNA molecules and the potential use of soil native organic compounds during the microbial synthesis of DNA make it not achievable to estimate the age of soil extracellular DNA by radiocarbon dating.

Changes induced by the roots of Erica arborea L. to create a suitable environment in a soil developed from alkaline and fine-textured marine sediments

Background and aimsWe report on the modifications induced by the roots of Erica arborea L. on a soil derived from alkaline and fine-textured marine sediments.MethodsPhysical, chemical, mineralogical and biochemical properties of bulk soil and of the rhizosphere of Erica were characterised to evaluate its role on soil development.ResultsOnce 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.ConclusionsThe activity of Erica roots modified soil properties so to produce more favourable conditions for itself and the rhizosphere microflora.

Experimental discrimination and molecular characterization of the extracellular soil DNA fraction

We experimentally discriminated and quali-quantitatively characterized the extracellular fraction of a forest soil DNA pool. We sequentially extracted and classified the components of extracellular DNA by its strength of interaction with soil colloids as: (1) extractable in water, free in the extracellular soil environment or adsorbed on soil colloids; and as (2) extractable in alkaline buffer after previous extraction in water, bound on soil colloids. The comparative molecular analysis (fluorometer, gel electrophoresis, genetic fingerprinting) of directly and sequentially extracted extracellular DNA revealed quantitative and qualitative differences, also in terms of genetic information about microbial communities. The sequential extraction of extracellular DNA revealed differences in molecular weight, indicating a relationship between DNA fragment length and strength of interaction with soil colloids. The sequential extraction was also suitable to assess the presence of tightly bound DNA, providing information about the DNA-colloid interactions naturally occurring in the soil environment.

Evaluation of Erosion Intensity and Some of Its Consequences in Vineyards from Two Hilly Environments Under a Mediterranean Type of Climate, Italy

Giuseppe Corti1, Eugenio Cavallo2, Stefania Cocco1, Marcella Biddoccu2, Giorgia Brecciaroli1 and Alberto Agnelli3 1Dipartimento di Scienze Ambientali e delle Produzioni Vegetali, Facoltà di Agraria. Università Politecnica delle Marche, Ancona; 2Istituto per le Macchine Agricole e Movimento Terra, Consiglio Nazionale delle Ricerche, Torino; 3Dipartimento di Scienze Agrarie ed Ambientali, Facoltà di Agraria. Università degli Studi di Perugia, Perugia; Italy

Composition and mean residence time of molecular weight fractions of organic matter extracted from two soils under different forest species

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.