Fiorenzo C. Ugolini

IONIC MIGRATION AND WEATHERING IN FROZEN ANTARCTIC SOILS

Soils of continental Antarctica are forming in one of the most severe terrestrial environments. Continously low temperatures and the scarcity of water in the liquid state result in the development of desert-type soils. In an earlier experiment to determine the degree to which radioactive NaCl36 would migrate from a shallow point source in permafrost, movement was observed. To confirm this results, a similar experiment involving Na22Cl has been conducted. Significantly less movement of the Na22 ion was observed. Ionic movement in the unfrozen interfacial films at mineral surfaces in frozen ground is held to be important in chemical weathering in Antarctic and other desert soils.

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.

PEDOLOGICAL, ISOTOPIC, AND GEOCHEMICAL INVESTIGATIONS OF THE SOILS AT THE BOREAL FOREST AND ALPINE TUNDRA TRANSITION IN NORTHERN ALASKA

On well-drained sites in northern locations, Podzols have traditionally been observed in association with forest vegetation and the less leached Arctic Brown soils with tundra vegetation. The present study shows that this vegetation-soil association is not a ubiquitous feature in northern Alaska, suggesting that Podzol occurrence beyond the modern tree line is not sufficient evidence to infer past forest expansion. This study was initiated at the boreal forest and alpine tundra transition at Walker Lake, Alaska, to determine whether the lichen-mixed-heath assemblage of the tundra was capable of producing Podzols. Morphologically, soils developed under lichen-heath cover could not be distinguished from those under spruce trees. Standard pedologic parameters, such as pH, CEC, and dithionite (Fed) and pyrophosphate (Fep) extractable iron, also failed to distinguish between these soils. An altitudinal trend unrelated to vegetation cover was apparent in these data. The Fep:Fed ratio, as well as Fep and organic carbon in the B2hir, decreases with altitude. Both stable carbon isotopes and lignin oxidation products were used to investigate the plant sources of carbon in the B horizons. Altitudinal increases of both B horizon 13C/12C and present day plant cover 13C/12C indicate that morphologically similar soils may have developed under isotopically and taxonomically different plant communities. The lignin oxidation products from the B2hir of the forest soil are consistent with the prevailing gymnosperm trees growing on the site, and the oxidation products of the B2hir of the tundra soil are consistent with the angiosperm species that predominate at the tundra site. These results and other field evidence suggest that trees have not been more extensive in the past at Walker Lake. Albic horizons in the Arctic are, therefore, not conclusive evidence of a forest environment. Additional evidence, such as relict arboreal material, is needed before inferring tree line fluctuations and associated climatic change.

THE ROLE OF PROTON DONORS IN PEDOGENESIS AS REVEALED BY SOIL SOLUTION STUDIES

A chemical model that stresses the role of proton donors in soil genesis is proposed. The model proposes that the abundance, strength, and ability of the conjugate bases to participate in chemical reactions are keys to weathering and pedogenic processes. The study of soil solution has been instrumental in determining the role of proton donors and has led to the development of this chemical model. Conventional pedological investigations only analyze the solid phase and consequently do not allow the identification of the proton donors. Soil-forming processes are defined as a combination of weathering and transportational processes occurring in the absence or presence of living and dead biota. Proton donors initiate weathering reactions and participate in transport of ions released during weathering. While the concept of proton and ligand-promoted dissolution is not new, our recent work with solutions collected from different ecosystems has revealed which acids are important in different soils and throughout the horizons in a given profile. The perspective here illustrates the role of proton donors by describing models of soil genesis. In surveying the proton donors present in the soil, it is recognized that not only is the strength of the acid as indicated by the pKa important in weathering reactions but also the ability of the conjugate base to form complexes. On this basis, the acids are classified as weak, strong, and complexing acids. Reactions involving these acids are presented indicating their tendency to cause congruent and incongruent dissolution. The influence of the soil-forming factors (climate, biota, parent material, topography, and time) on the source and distribution of proton donors is presented. Finally, three case studies are presented to reveal how studies involving soil solutions have been used to identify the major proton donors and to determine the present soil-forming processes.

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.

Pedogenesis induced by Genista aetnensis (Biv.) DC. on basaltic pyroclastic deposits at different altitudes, Mt. Etna, Italy

The paper deals with the role of Etnean broom [Genista aetnensis (Biv.) DC.] on the early stages of pedogenesis on basaltic pyroclastic deposits (Mt. Etna, Italy) of different age and altitude previously not vegetated. After a few decades, this plant has been capable to arrest erosion and produce some soil features in both Entisols. The soil of Mts. Rossi, at a lower altitude, formed from a centenary parent material and hosted a broom plantation of about 50 years old. Here, the regimes of soil moisture (ustic) and temperature (mesic) limited the diffusion of the grass to the projection of the broom crowns, but favoured the diffusion of microorganisms and pedofauna. These conditions favoured a generalised alteration of the parent material and induced a certain horizon organisation. The soil of Mt. Vetore, at higher altitude, formed from a millenary parent material, and hosted a broom plantation of about 35 years old. At this site, the soil moisture (udic) and temperature (frigid) regimes favoured the formation of a thick and continuous carpet of gramineae. Yet, these conditions limited the activity of microorganisms and pedofauna, thus inducing a poorer horizon organisation. In this soil, because of the higher mean annual precipitation and root activity, most of the chemical modifications of the parent material occurred at the level of the rhizosphere, which acquired a thickness of 2–3 cm. From a chemical and mineralogical point of view, horizontal variations between rhizosphere and matrix were more evident than those among horizons. The most striking change occurring in the rhizosphere was the accumulation of secondary minerals such as oxalates and easily reducible Fe-oxyhydroxides. We also inferred that, in the environment of Mt. Etna, the excretion of oxalic acid from the roots of the broom could represent a strategy of nutrient uptake, in particular P, Mg and K.

Pedogenesis in disturbed alpine soils (NW Italy)

In alpine environments, natural or man-induced disturbances are fairly common and acknowledged as determining factors in pedogenesis and soil distribution. We have selected a representative alpine valley in the Italian Northwestern Alps to evaluate the effects of perturbations of different intensity and frequency on soil development and weathering during the last few centuries. In order of decreasing disturbances, we selected: (i) an active avalanche shoot; (ii) a man-built terrace; (iii) a park-like forest; and (iv) a coniferous forest. In the two most disturbed sites, independently of the intensity and frequency of the disturbances, the active pedogenic processes are restricted to the topsoil. At site 1, the acidity induced by the invading pioneer N2-fixing tree species determines a certain degree of mineral dissolution, while at site 2, plowing and manuring influence the properties of organic matter and limit its mobility. At site 3, where coniferous species are associated to the herbaceous cover, the weathering proceeds further and a cambic horizon was detected in the field and supported by evidences of K depletion from illite. The higher polarity of the organic substances of mixed origin is certainly responsible for this more intense weathering. In the coniferous forest, the least disturbed situation, the vegetation is more representative of the climax in the area and pedogenesis further proceeds leading to the appearance of podzol-like features. These, even if scarcely displayed by a very shallow and discontinuous E horizon, are clearly manifested by the formation of interstratified minerals and by the amount of pedogenic iron oxides. This can be related to the organic matter dynamics, with a high FA/HA ratio and accumulation in the Bw horizons of acidic and highly polar aromatic molecules. However, podzols are not common at this elevation and this steady state condition seems to be very vulnerable to disturbances even of low intensity. In conclusion, in an alpine valley, the impact of different disturbances on soil development, independently from their intensity and frequency, leads to a pattern of development of soils in which the vegetation plays a key role.

The contrasting effect of broom and pine on pedogenic processes in volcanic soils (Mt. Etna, Italy)

Abstract Effects of Etnean broom ( Genista aetnensis (Biv.) DC.) and Corsican pine ( Pinus nigra Arn. ssp. laricio Maire) on the morphological, mineralogical and chemical properties of volcanic soils from Mt. Etna (Italy) were compared and contrasted. For this purpose, we studied the rhizosphere and the bulk soils under adjacent 30 years old pure plantations of both species. Morphology of the soil under broom differs from that under pine for (i) a higher accumulation of organic matter in the topsoil, (ii) an incipient formation of E material around the base of the stem, and (iii) the presence of yellowish collars around the primary roots. Mineral horizons of the two soils are made of plagioclases, pyroxenes, magnetite and glass. The yellowish colour of the collars is attributed to a root effect that results in a confined alteration of primary volcanic glass and also iron-bearing minerals, leading to the precipitation of amorphous Fe-oxides. Under pine, we observed a more widespread weathering of primary minerals throughout the profile, and a depletion of base cations and a release of Al in the topsoil. On the whole, therefore, Corsican pine—commonly planted in the last decades on the pyroclastic deposits and lava flows of the Etna volcano—seems to play a detrimental role on these soils.