RICERCA ITALIANA     

Evolution, Stability and Dynamic of Soil Organic Components for a Genetic and Functional Definition

Università Cattolica del Sacro Cuore

Abstract

Humic substances are a group of macromolecules involved in numerous chemical and biochemical processes, important for soil fertility and for environmental equilibria and they represent an important source of carbon, essential for soil biomass and, more generally, for the ecosystem (Stevenson, 1994).
The quali-quantitative characterization of OM employing chromatographic, spectroscopic, potentiometric, thermogravimetric, differential scanning calorimetry (DSC) and isotipic techniques, besides the humification parameters, can give an essential contribution to an update classification of diagnostic horizons.
The total organic carbon (TOC) is believed a "static indicator" of processes at the equilibrium, in that only allows the determination of changes in the long run. Other indicators, defined as "dynamic" are particularly suitable for pointing out changes in short times and, among these, are the kinetics of mineralization of total organic carbon of microbial biomass.
In order to complete the characterization of humic substances will be evaluated also the stability rate of organic-mineral compounds, the microbial biomass, enzymatic activities, the availability of heavy metals and their relations as useful tools for achieving the objectives of this research program.
The quantification of dynamic of soil organic matter and its role in better understanding the pedological processes and their properties, is particularly important since it can be stated as a basis to date the genetic and operative definition of some soils particularly important for taxonomic purposes. <<<

Principal Investigator

Sandro SILVA Università Cattolica del Sacro Cuore

Research Objectives

Individuation, characterization and dynamic evolution of organic matter (OM) in all its forms in the pedogenesis of the investigated profiles.
1. Identification of the most representative types of soils to investigate; characterization of the most important horizons for the O.M. dynamics through quali-quantitative balances of the organic components and humified fractions; interlaboratory check among the analytical units for what concerns some official methods of analysis (C org. N Kjeldahl, particle size analysis) for the determination of accuracy and precision;
2. General picture of the chemical, biological and microbial characterization coming from the estimation of suitable parameters/indicators determined by the analysis of soil material from pedogenetic horizons of the investigated profiles;
3. Evaluation of the new indicators for agricultural and forestry soils, for dating genetic and operative riclassification criteria, in order to integrate the classic parameters (e.g., color, C org content) of the most commonly used taxonomic classification. <<<

First Results

Step 1:
Characterization of soil profiles representative of the present study, with horizons typical of the type of chosen soils. One of the expected result will be the evaluation of accuracy and precision of the methods employed to determine some standard soil parameters by all the Working Units.Step 2.
Expected result in to come to an analytical multidisciplinary (chemical, biological, microbiological) approach which allows a better characterization and identification of the investigated soils.Step 3.
Ridefinition or riclassification of the investigated soils on the basis of indicators characterized in the step 2, even in terms of enviromnental sustainability. <<<

Timescale

24 months

National and international background

The release of gases produced by human activity has led to a significant increase in the atmospheric temperature of the Earth with dramatic consequences for the whole planet.
The most important responsible is carbon dioxide, mainly released into the atmosphere when fossil fuels are burnt,and whose concentration in the atmosphere has increased by 30% with respect to the pre-industrial era.
Deforestation has also substantially added to the increase in CO2. In 1990, the World Resources Institute reported that CO2 emissions caused by deforestation amounted to 6 Gt (gigatons).
This alarming data call for urgent intervention at different levels by means of mitigation actions that can reduce and absorb a high quantity of carbon dioxide, as established by the Kyoto agreement and the Bonn Conference on climatic change. Since the soil represents the most important C sink, a good knowledge of the evolution, stability and dynamics of its organic components could drive to guarantee its conservation, so reducing the levels of carbon dioxide in the atmosphere and preserve those soil properties which are strongly affected by organic matter (OM), so contributing to a complete guarantee of its functions.
The OM plays a fundamental role in guaranteeing a good soil quality in that it represents a nutritive and energy substratum for living organisms (Schnitzer, 1986; Stevenson, 1994) , and a nutrient source for plants, has a positive action on soil c.e.c., affects the biodegradability and chelation of nutrients and the detoxification of heavy metals. It guarantees a good soil structure through the increase of porosity and the stability of aggregates, and this reflects in a good drainage and aeration, increase of water retention and reduction of erosion. The OM content has been found to be a good environmental indicator since it is related to the productivity and sustainability of the agro-ecosystem as well as to environmental conservation (Smith et al., 2000).
The study of evolution and dynamics has to take into account a deep characterization of the structure and composition of OM, which, in consideration of its complexity and heterogenity, is often object of scientific studies and debates. The quail-quantitative characterization of soil organic carbon can moreover give a fundamental contribute to a correct classification of those soils where pedogenetic processes are strongly affected by OM.
Soil OM can be divided into non-humic and humic (HU) substances. The former comprehend all those materials with chemical characteristics still recognizable, as carbohydrates, proteins, amino-acids, nucleic acids, lipid, waxes, resin; humic substance don't belong to any group of biochemicals and it's difficult to find out a definition which describe them well (Hayes and Clapp, 2001; MacCarty, 2001). They are generically natural substances, present in soils, sediments and natural waters, which come from the decomposition of plant or animal debris (MacCarty 2001; Schnitzer and Khan, 1972) and from the activity of synthesis of microrganisms (Schnitzer, 1978).
Humic substances (HS) are macromolecules involved in various chemical and biochemical process of great importance both for soil fertility and environmental equilibria. Soil HS represent about 50% of the organic carbon available on the Earth, therefore forming an important sink of carbon refractory to mineralization, essential for the survival of soil living organisms, also in adverse conditions, and for the correct working of the whole ecosystem (Stevenson, 1994).
HS are made of amorphous substances, with a colour from yellow to brown, of mixed nature, aromatic and aliphatic, chemically complex, with characters of polyelectrolyte and with molecular mass ranging from some hundeds to several thousands of Daltons (Schnitzer, 1978).
Some scientists believe that they are not molecules of large size, but compounds with average molecules weight, bound with weak links (Burdon, 2001; Piccolo 2001). These compounds show high resistance to chemical-physical degradation, higher than that of the starting compounds. This characteristics has, in part, to be ascribed to a close association of these molecules with soil colloids and to their trapping inside the aggregates, that hinders the access to microorganisms and enzymes (Schnitzer, 1978). Conventionally and operatively soil OM is divided into several fractions on the basis of their solubilities in an aqueous medium as function of pH: dissolved organic matter (DOM), fulvic acids (FA), humic acids /HA), humin.
In order to characterize humic and humic-like humic substances, fractionated on the basis of their solubilities in an aqueous medium, or on the basic of the different nominal molecular mass (NMW) different analytical methods have been used, such as chromatographic (IEF and CE), spectroscopic (DRIFT; ; 1H-, 13C-, 31P-NMR e Raman-SERS) and potentiometric techniques.
Additional information is obtained by thermal analysis (TG, DSC and DTA) which can give useful information on the stability rate of soil OM, since the values of weight loss, due to decomposition and thermal oxidation are directly correlated with the different composition of OM and allow estimations of the relative abundance of the labile component respect to the more stable one. On the basis of the available information from literature, there seems to be an increasing interest in analysing complex molocular structures with different analytical methods which allow a complete quali-quantitative characterization of this complex matrix. On the other hand, nevertheless, it's important to specify simplex indexes which allow, through easily feasible analysis to obtain information about OM dynamics, the effects on soil properties in particular on reactions of OM stabilization, and the effect on pedogenetic processes.
The evaluation reliability of simple indexes through the complete analytical characterization of soil OM, could permit a great enhancement in the confidence level for the applicability of these indexes. Among the most used indexes are the humification parameters which allow to explain with more immediacy the analytical data of total, labile, extractable and humified organic carbon (Sequi, 1986).
The humification rate (HR) is a strictly quantitative parameter and provides information on the humified fraction of the soil ( HA+FA) with respect to the toatal organic matter.
By taking off the unextracted humic fraction (humin), this parameter can be substituted with the total humification value (HU). The humification degree is a quail-quantitative parameter and provides information on the quality of the extracted organic matter. This value ranges from 0 to 100 is so higher as good is the ability of a soil to humify the available organic matter. The humification index (HI) highlights the incidence of humification processes against mineralization ones. This parameter can be considered as an index of both the humification activity of a given soil and of the availability of labile, non-humified organic fractions.
These parameters can give suggestions on the capacity of a soil to behave as a carbon sink, in that it's possible to distinguish a conservative system, where we can found an accumulation of OM, from a system where the microbial activity on the contrary, is mainly addressed to a fast energy recovery due to OM mineralization, with detriment towards the formation of stable organic compounds.
These facts affect the most general role of soil OM, so these parameters can be used to evaluate soil quality and they could be employed for the classification of some diagnostic epipedons together with or in substitution of the parameters at present requested. (colour, TOC content) allowing, for example, the evaluation of transition characteristics between mollic and umbric epipedons.
The processes of stabilization of OM in the soil are not ascribed only to intrinsic properties, dependent on molecular characteristics, but also to the interaction between OM and inorganic components which can change its degradability and to the accessibility of organic molecules to microbial and enzymatic attack (Sollins et al 1996). If molecular characteristics are well described through the above mentioned methods, the evaluation of all the stabilization mechanisms cannot do without the study of the dynamics of OM in the pedogenetic processes. High quantities of OM can be found in the deepest mineral horizons B and C (Rumpell, 2002) as function of soil age and of the other factors of pedogenesis. In fact, the OM dating (14C) has put in evidence that the youngest C, structurally simpler and so more exposed to microbial attack, is located in the top horizons (Kogel Knabner 1993), but if eluvial phenomena are particularly severe, as occurs in Podzols, present in Italy in mountain environments, high rates of OM with low molecular weight can be present in deep horizons too, e.g. Bh and Bhs. If there is the co-presence of different pedogenetic processes, as occurs in Cambisols, very spread in the Padan Plain, in mineral horizons are observed forms of inorganic C, more bound to root turnover and to microbial activity (Rumpell et al., 2002). In soil where aploidization is the prevalent process (e.g. Vertisols) it's believed that the distribution of OM along the profile, and its dynamic, are affected by pedoturbation processes.
Nevertheless recent studies seem to hinder this hypotesis (Kovda et al., 2001; Dell'Abate et al., 2002) and to point out a more complex mechanism, influenced not only by chemical characteristic of the profile, but also by microbial activity (Skjemstad et al., 1996) and by mineralogy (Krull e Skjemstad, 2003).
These dynamics are effectively studied by isotopic techniques, above all through 13C/12C ratio. The 13C isotope represents about 1,11% of C atoms, and its abundance is expressed as delta, deviation of isotopic ratio of the sample respect to that of an arbitrary standard. The delta-13C value of plant varies according to the photosyntetic cycle and it is smaller in C3 plants (from -22 to -23 per mil) than in C4 ones (from -9 to -16 per mil) (Piccolo, 1996). By using these differences, Romkens et al. (1999) studied OM variations provoked by the transformation of a soil cultivated with maize to a grassland-pasture, noticing how the increase was present mainly in the most gross fractions and in the indecomposed organic remainings and how these fractions showed a turnover much faster than that relative to OM bound to the finest fractions.
The OM stabilization takes place also through the interaction with metallic ions present in the soil solution where problematic are the relationships between OM and metals; on one hand the presence of these last ones can affect the microbial biomass activity (Brookes, 2000; Leita et al. 1999; Bragato et al., 1998; Leita et al. 1997), dynamic describer and more suitable than the total Corg content to put in evidence variations in soil OM in the short time; on the other hand the mobility and "bioavailability) of heavy metals for plant roots is influenced by the various forms of OM present (DOM, HA, FA, etc). These last ones through mechanisms as complexation, chelation, adsorption, desorption, etc. can provoke changes of the chemical form of the metal, so changing dynamically in the time its activity in the soil solution and the relative accessibility for plant roots. In order to evaluate the "bioavailable quota" revealed itself more useful than the total quota for toxicological aims, the past studies, based on the different selectivity of chemical extractants, used alone or in a sequence, in comparison with their affinity towards the different forms of a metal present in a soil (soluble, bound to OM, adsorbed on clays, bound to carbonates, occluded), in these recent years were supported and/or substituted by researches with the use of more advanced techniques and models, based e.g on speciation, on the determination of the activity of free ions in solution (Cancès et al. 2003; Temminghoff et al. 2000; Peijnenburg et al. 2003; Sauvé 2003) which are revealing very promising in view of a deeper and exhaustive comprehension of the mechanisms involved in this subject. It's therefore believed that the studies with the use of chromatographic techniques besides to the most innovative ones, today available, have to be improved to reach a better understanding of the influence of organic fractions on "bioavailability" of heavy metals.
By the point of view of the study of C dynamics in the soil, particularly useful are those indicators which take into consideration the smallest organic pool (less than 2% of the total C) is the very most dynamic and therefore more sensitive towards changes of environmental factors and to soil management practices. The microbial biomass is the metabolic active component of soil OM and takes part in transformation processes of nutrients, their accumulation in organic combinations and in their release in forms often readily available for plants.
Since it shows a very short turnover time (less than one year) it is a sensitive indicator of environmental changes. For this reason the microbial C content, the microbial C/TOC ratio, the metabolic quotient (qCO2) defined as the basal respiration of the microbial biomass/biomass C ratio and the enzymatic activity are reported as indexes sensitive to changes of soil OM in the short time (Nannipieri, 1984; Powlson et al.,1987).
Particularly important are the studies about the enzymatic activities, in particular about those who attend the biogeochemical cycles of the elements in plant nutrition. The employment of the dosage of some enzymatic activities is, for example, particularly useful to investigate the impact on soil of pesticides, heavy metals, urban wastes, animal manures, fertilizers, etc.
The biological activities represent, together with the traditional chemical-physical analysis, a powerful tool whose integrated use aims to a complete understanding of the soil in terms of globality and dynamics (Brady e Weil, 2002). The agronomical practices together with the degradation activities can exert a direct influence on these parameters and any agent or phenomenon which lowers the efficiency of the metabolism of soil organisms, damages the whole system.
All the results, coming from all types of these studies, will help in building up an up-to-date and organic picture of the current topics concerning the evolution, stability and dynamics of soil organic components, in view of formulating working hypothesis, able, through a better comprehension of the mechanisms and processes involved, to improve soil management with aims of environmental sustainability. <<<