Soil Organic Matter Evolution Research Articles

Agro-ecological services delivered by legume cover crops grown in succession with grain corn crops in the Mediterranean region

Abstract Grain corn is the main cereal produced in Portugal. It is grown in intensive monoculture cropping systems that may have negative effects on soil quality, affecting long-term fertility and productivity, and, therefore, the sustainability of the production. A promising management practice to mitigate soil degradation is to grow winter cover crops used as green manure. This study examined the effectiveness of six legume cover crops (LCCs) species in providing agro-ecological services for grain corn systems in the Mediterranean region, specifically in terms of nutrient leaching, nutrient recycling, weed control, and soil fertility. The study was performed in Central Portugal during 2 years, and it assessed legumes/weeds dry biomass yield, their nutrients content, and soil organic matter evolution. Results show that, in general, LCC are well adapted to Mediterranean conditions, yielding large amounts of biomass (up to 8 ton/ha for some clover species). In terms of nutrient leaching mitigation, the overall N–P–K nutrient uptake was 176–20–172 kg/ha. Green manure services enabled a reduction of 35% of N, 50% of P, and 100% of mineral fertilizers for a grain corn production of 12 ton/ha. Weed control by LCC was effective only in the second year of the study and for three clover species (crimson, balansa, and arrowleaf) due to their early establishment and/or high biomass production competing with weeds. Soil fertility was not improved in the short term, LCC incorporation into the soil to a slight depletion of the soil organic matter content.

Contribution of Tamarix aphylla to soil organic matter evolution in a natural semi-desert area in Tunisia

A soil, classified as Arenosol (Eutri-Aridic Arenosol (Calcaric)), located in Neffatia (Tunisia) and populated by the shrub tamarisk (Tamarix aphylla), was studied to assess how the litter deriving from tamarisk can affect its characteristics. Several parameters were considered: particle size distributions (PSD), pH, cation exchange capacity (CEC), total CaCO3 content, total nitrogen (TKN), phosphorus and total organic carbon (TOC). Down the soil profile the pH increased from 7.83 to 8.32 as a probable consequence of salts accumulation deriving from the mineralization of the organic matter and the limited leaching due to low rainfall. As expected, TOC and TKN decreased, from the top downwards, and the two parameters were well correlated (TOC vs TKN: R2 = 0.98; p < 0.05; n = 3). CEC assumed progressively lower values reflecting the decreasing organic matter content (CEC vs TOC: R2 = 0.93; p < 0.05; n = 3). PSD showed that the presence of roots influenced the quantity of fine particles down the profile and the PSD cumulative curves were indicative of an aeolian origin for the soil parent material, confirming the hypothesis that tamarisk interacts with the environment, trapping sediment and forming the so-called phytogenic dunes. By chemical and spectroscopic analyses, it was possible to assess that tamarisk plant residues directly contributed to the soil organic matter (SOM) accumulation and characteristics. Stable SOM (ligno-humic fraction) closely resembles that of the plant (leaves and stems) and is chemically lacking in the more easily degradable organic components such as fats, hemicellulose, cellulose and proteins. 13C CPMAS NMR spectroscopy showed that the so-called soil ligno-humic fraction consists of aromatic molecules such as tannins, and aliphatic carbon (i.e. cutins and suberins) already present in the plant and preserved by mineralization processes because they are the most resistant to biological degradation.

Soil organic matter content and chemical composition under two rotation management systems in a Mediterranean climate

The crop rotation system in organic farming is a determinant factor to accumulate and preserve soil organic matter (SOM), and in depth knowledge on its effects is still lacking. Tillage intensity in particular is crucial to maintain soil aggregates and protect SOM from degradation. The evolution of SOM was tested in two adjacent fields under two different rotation cropping systems (low-intensity tillage and high-intensity tillage), and the effect of a further cultivation of legume in both fields was evaluated using 13 carbon (C)-nuclear magnetic resonance (NMR) and elemental analysis of samples isolated through combined aggregate size and density fractionation. The two adjacent fields had been managed using the following organic farming methods for 13 seasons since 1998: i) alfalfa-based, with nitrogen (N) enrichment and low-frequency tillage with alfalfa (Medicago sativa) (9 seasons), winter wheat (Triticum durum) (3 seasons), and broad bean (Vicia faba) (1 season) and ii) cereal-based, with N depletion and annual tillage with barley (Hordeum vulgare) (7 seasons), sunflower (Helianthus annuus) (2 seasons), broad bean (Vicia faba) (3 seasons), and bare fallow (1 season). Soil sampling was carried out at the end of the 13-year rotation (T0, November 2011) and after winter wheat and chickpea cultivation in both fields over two subsequent years (T1, July 2013). Bulk organic C was significantly higher in the alfalfa-based system than in the cereal-based system at both T0 and T1, with SOM occluded in soil aggregates and associated with mineral particles. In terms of the macroaggregates heavy fraction at T0, the alfalfa-based field contained twice the organic C of that in the cereal-based field, as well as three times the organic C in the occluded particulate organic matter (POM). The occluded POM (oPOM) had a lower aryl/O-alkyl C ratio in the alfalfa-based system than in the cereal-based system, suggesting that oPOM undergoes a lower degree of decomposition during low-intensity management. The aryl/O-alkyl C ratios of the macro- and microaggregate oPOM decreased from T0 to T1 in the cereal-based system, suggesting increased protection of these fractions by soil aggregates. Thus, including legumes in crop rotation appears to positively affect the accumulation of SOM associated with mineral particles and within soil aggregates.

Characteristics and Driving Mechanism of Soil Organic Carbon Content in Farmland of Beijing Plain: Implication for the Fate of Engineered Polymers in Soil

Soil organic matter (SOM), as a kind of natural polymers, affects the migration and transport of ions and particles in soil system due to its surface characteristics and interaction and then causes significant changes in soil quality such as soil fertility loss and pollutant transfer. It is of great importance to study the temporal and spatial evolution of soil organic matter and its driving mechanism for soil health management. This study aims to fully reveal the evolution characteristics and driving mechanism of soil organic carbon (SOC) in farmland of the Beijing plain based on a six-year site monitoring. According to the research results, there is a significant difference in the overall soil organic content during the 6-year period. The temporal stability of SOC is moderate, and it is inversely proportional to SOC content in terms of spatial distribution. SOC content increases as organic fertilizer input rises, and an extra unit (15 kg·ha−1) of organic fertilizer input leads to an increase of 0.057 g·kg−1 in SOC content. The soil with higher clay content exhibits higher SOC content. The organic carbon content in different soil texture types increases with time, and there is a significant difference between the increases in medium loam and light loam. The grain field plantation system exhibited declining SOC content, while the protected vegetable fields, open vegetable fields, and orchards all showed an increase in SOC content. According to our results, the SOC content of farmland in the plain areas of Beijing is largely dependent on the input of organic carbon if other conditions remain unchanged or exhibit insignificant changes.

Evolution of Dissolved Organic Matter Along a Septic System Plume: Evidence of Sustained Biogeochemical Activity in the Groundwater Zone

AbstractDissolved organic matter (DOM) is an important energy source for biogeochemical reactions. However, DOM is often assumed to be recalcitrant in the groundwater zone due to extensive microbial processing in the overlying soil zone and long groundwater residence times. Consequently, further heterotrophic microbial processes proceed at inherently low rates. Septic systems provide an ideal opportunity to study the evolution of groundwater DOM due to the development of confined and easily identifiable plumes of known age. Here we use measures of DOM composition (size‐exclusion liquid chromatography, ultraviolet absorbance, and C:N) to quantify DOM evolution along an exceptionally well‐characterized septic‐impacted groundwater plume. Elevated concentrations (5 to 8 mg C/L) close to the tile bed decrease to values below 2 mg C/L with lower UV‐absorbing components normalized to overall DOM concentration (specific ultraviolet absorbance) along the six‐year‐old plume flow path. The humic substance fraction (HSF) comprises the largest percentage of DOM and decreases in concentration along the septic plume. HSF properties also change with travel time becoming lower in molecular weight, specific ultraviolet absorbance, and C:N. Denitrification continues along the length of the plume concomitant with changes in DOM and HSF composition. Thus, HSF, typically considered recalcitrant in subsurface environments, is actively cycled by microorganisms on multiyear time frames in accordance with the recent paradigm shift for soil organic matter evolution. Lastly, measures of DOM composition indicate changes in DOM that are not evident from measures of concentration alone.

Seasonal evolution of soil organic matter, glomalin and enzymes and potential for C storage after land abandonment and renaturalization processes in soils of NE Spain

Rates of recovery of soil organic matter (SOM) in abandoned fields of Mediterranean Spain are affected by spontaneous succession in the vegetation, the occurrence of periodic wildfires and lack of forest management. Soil organic matter has a relevant role together with the renaturalization process of shallow soil under vines (V) and olive groves (OL), cork oak (S) and pine (PI) trees, pasture (PR), Cistus (MC) and Erica (MB) scrub in abandoned areas of NE Spain that are sensitive to fire. Physical, chemical, biological and biochemical properties of the soil determined season by season warned against its fragility. Pyrolysis-GC products showed in general moderate to strong positive and negative correlation with soil organic carbon (OC), easily extractable glomalin (EE-GRSP) and total glomalin (GRSP), β-glucosidase and protease, which strengthens the hypothesis that both less and more recalcitrant SOM fractions might be inferred and so there is a scale of potential carbon storage in the environments studied.Recently abandoned V and OL soil had small OC contents (mean 2.51±0.25 and 15.26±1.39gkg−1) and GRSP (mean 1.04±0.22 and 2.84±0.53gkg−1) and large contents of aromatic compounds. By contrast, PR and MB soil had large OC (mean 36.24±0.75 and 37.66±2.38gkg−1) and GRSP (mean 4.85±0.87 and 3.88±0.76gkg−1) content and smaller contents of aromatic compounds. This suggested a renewal of active OC in older soil. In fact, EE-GRSP and GRSP of more organic soil were positively correlated with the mineralization indices N/O (Furfural/Pyrrole) and O/Y (Pyrrole/Phenol) from active organic compounds (P<0.05 and P<0.01 respectively), and negatively correlated (P<0.05) with the humification index B/E3 (Benzene/Toluene) from recalcitrant organic compounds. Principal component analysis (PCA) identified those environments where soil renaturalization is better. Our findings might indicate the potential for carbon storage in different abandoned shallow soil environments under renaturalization in Mediterranean climate conditions.

Dynamics of soil organic matter based on new Rock-Eval indices

This paper aims to develop a new approach to interpret soil organic matter (SOM) dynamics from Rock-Eval pyrolysis. Rock-Eval standard parameters (TpS2, HI, OI) are limited when applied to SOM, as they were defined for tracking the origin of sedimentary organic matter (i.e. terrestrial vs aquatic and lacustrine vs marine). This study proposes new Rock Eval based indices, projected on a new diagram plotting >1300 samples, including litter and soil horizons, pure compounds and organic material. These new parameters reflect the thermal stability of SOM rather than its bulk chemistry. Their calculations are based on the contribution of four different areas (A1 to A4) integrated below the S2 pyrogram (amounts of released hydrocarbon compounds during the pyrolysis step). Results demonstrate that the relative values of A1 to A4 parameters can be used to survey the evolution of SOM during pedogenesis. These Rock-Eval parameters revealed a consistent thermal differentiation of SOM with depth, from thermally labile biogenic SOM in soil organic horizons to thermally stable mineral-associated SOM compounds in organo-mineral and mineral soil horizons, indicating a pedogenetic stabilization of SOM. Finally, newly defined I- and R-indices integrate the respective parts of A1 to A4 parameters into SOM dynamics, the I-index emphasizing the degree of transformation of the immature organic fraction (related to SOM stabilization), the R-index highlighting the contribution of the most refractory fraction or persistent SOM (related to pedogenic and inherited contributions). Analyses of a wide range of soils and reference samples (cellulose, lignin, humic substances, lignite, charcoal, coal, etc.) allow end-members as well as particular trends (humic, spodic, inherited) to be drawn. Specific examples are provided in order to illustrate some applications and uses for this new Rock-Eval based I/R diagram, from the study of grain size fractions to the evolution of SOM in soil chronosequences.

Effect of Organic Amendments on the Evolution of Soil Organic Matter in Soils Stressed by Intensive Agricultural Practices

Losses of soil organic carbon often occur because of intensive agricultural practices that lead to removal of organic carbon following harvest production and to insufficient inputs. Organic amendments can be very appropriate for enhancing organic carbon content in very stressed agricultural soils. By enhancing soil organic matter, they generally play an important role in environmental matrices due to their capacity in retaining water and in enhancing nutrient availability to plants. Therefore, understanding the mechanisms by which organic amendments interact with other chemicals in the environment is of paramount importance. The attention was focused on farms near Sele river (Campania region, Italy), an area characterized by intensive agriculture. Two farms, with contrasting geopedologic properties (clay vs. sandy), were selected in order to study the effect of different doses of organic amendments containing compost from municipal solid wastes and wood-wastes (scraps of poplars pruning) at different ratios. The organic fractions were extracted from soil, after one and twelve months from amendment, and were analysed in order to establish their elemental composition and properties by FTIR, CPMAS 13C NMR spectroscopy and FFC-NMR relaxometry. Results showed an important role of soil geopedologic characteristics and experimental time in the evolution of humic acids. In less aerobic conditions as occurs in clay soils, organic matter was more stable, rich in carbonyl groups belonging to aldehydes and ketones, whereas under more aerobic conditions, occurring in sandy soils, organic matter had undergone a faster degradation, due to oxidative conditions and more intensive microbial activity.

Post-thinning soil organic matter evolution and soil CO2 effluxes in temperate radiata pine plantations: impacts of moderate thinning regimes on the forest C cycle

Forest ecosystems can act as C sinks, thus absorbing a high percentage of atmospheric CO2. Appropriate silvicultural regimes can therefore be applied as useful tools in climate change mitigation strategies. The present study analyzed the temporal changes in the effects of thinning on soil organic matter (SOM) dynamics and on soil CO2 emissions in radiata pine ( Pinus radiata D. Don) forests. Soil C effluxes were monitored over a period of 2 years in thinned and unthinned plots. In addition, soil samples from the plots were analyzed by solid-state 13C-NMR to determine the post-thinning SOM composition and fresh soil samples were incubated under laboratory conditions to determine their biodegradability. The results indicate that the potential soil C mineralization largely depends on the proportion of alkyl-C and N-alkyl-C functional groups in the SOM and on the microbial accessibility of the recalcitrant organic pool. Soil CO2 effluxes varied widely between seasons and increased exponentially with soil heating. Thinning led to decreased soil respiration and attenuation of the seasonal fluctuations. These effects were observed for up to 20 months after thinning, although they disappeared thereafter. Thus, moderate thinning caused enduring changes to the SOM composition and appeared to have temporary effects on the C storage capacity of forest soils, which is a critical aspect under the current climatic change scenario.

Evolution of soil organic matter after prescribed fire: A 20-year chronosequence

Vegetation fires influence the properties and turnover of soil organic matter in numerous ecosystems. Thermal alteration tends to increase the stability of soil organic matter. Fire induces the destruction of above‐ground biomass, which is associated with a large production of new partially-charred litter. Plant regeneration is directly affected by the fire itself, and by the ensuing modifications to the C, N and nutrient cycles associated with soil organic matter transformations. The objectives of the present study were to determine 1) changes in soil organic matter composition induced by fire, and 2) the dynamics of soil organic matter recovery in the first two decades following a fire event.. In a Florida scrub oak ecosystem, a chronosequence of soils protected from vegetation fire for 1 to 20years was studied. The bulk organic matter and oxidation resistant elemental carbon were quantified in depths 0–5cm, 5–15cm, and 15–25cm. The soil organic matter was characterized by solid-state 13C NMR spectroscopy and Curie point pyrolysis.The combination of these techniques allowed us to identify three steps in soil organic matter evolution: first the un-charred litter brought by the fire degraded between 1 and 4years after fire; second the contribution of aryl carbon, and most probably the pyrogenic carbon, significantly decreased between 4 and 11years after fire; Thirdly, after 11years, the soil organic matter quality appeared driven again by the fresh litter input from regenerating plant ecosystem. In this study, the soil organic matter did not appear strongly modified by the fire. The pyrogenic carbon did not dominate the soil organic matter composition and underwent significant degradation at the decadal timescale. Our results also highlight a potential underestimated effect of dead root input to soil organic matter after the fire.

Soil organic matter evolution after the application of high doses of organic amendments in a Mediterranean calcareous soil

Purpose We investigate the coevolution of soil organic matter (SOM) and soil properties in a semiarid Mediterranean agroecosystem, as well as the 1-year evolution of the different pools of soil organic and inorganic carbon and their influence on soil respiration after the application of a single high dose of three different organic amendments.

Indicators of change in the organic matter in arid soils

Soil organic matter is a key component in ecosystems, as it is the essential part of a set of relevant processes and constitutes an important carbon pool contributing to Global Change. The design of environmental monitoring programmes should include indicators of the current status of ecosystems, alerting to incipient changes in them. In this context, a sampling scheme has been designed taking into account the main processes and soil uses affecting the dynamics of soil organic matter. Well-tested parameters were determined in order to assess which of them are most useful as indicators of soil organic matter evolution in arid soil, such as that in the "Cabo de Gata-Níjar" Natural Park (SE Spain). The parameters characterising the lability of the different fractions indicate changes in soil organic matter triggered by changes in soil use and soil dynamics. Changes in soil use, when drastic, are best reflected by those fractions comprising a high percentage of the total soil carbon, while the processes having slower dynamics are best demonstrated by the labile fractions. As a result of the sensitivity analysis of parameters versus changes, and taking into account the operational difficulties for determining them, the following indicators are proposed for a monitoring programme: total organic carbon, active fraction of the organic carbon and ratios of this fraction versus total organic carbon (%) (as given by the lability index proposed).

Evolution of soil organic matter changes using pyrolysis and metabolic indices: A comparison between organic and mineral fertilization

The aim of this study was to evaluate chemical and biochemical changes of organic matter in fertilized (ammonium nitrate) and amended (vermicompost and manure) soils using pyrolysis and metabolic indices. The metabolic potential [dehydrogenase (DH-ase)/water soluble organic carbon (WSOC)], the metabolic quotient ( qCO 2) and the microbial quotient (Cmic:Corg) were calculated as indices of soil organic matter evolution. Pyrolysis–gas chromatography (Py–GC) was used to study structural changes in the organic matter. Carbon forms and microbial biomass have been measured by dichromate oxidation and fumigation–extraction methods, respectively. Dehydrogenase activity has been tested using INT ( p-Iodonitrotetrazolium violet) as substrate. The results showed that organic amendment increased soil microbial biomass and its activity which were strictly related to pyrolytic mineralization and humification indices (N/O, B/E3). Mineral fertilization caused a greater alteration of native soil organic matter than the organic amendments, in that a high release of WSOC and relatively large amounts of aliphatic pyrolytic products, were observed. Therefore, the pyrolysis and metabolic indices provided similar and complementary information on soil organic matter changes after mineral and organic fertilization.

Method for assessing impacts on life support functions (LSF) related to the use of ‘fertile land’ in Life Cycle Assessment (LCA)

Degradation of soil quality is a major concern due to the scarcity of fertile land, and needs to be properly addressed in the environmental assessment of agro-forestry systems. This paper addresses the main issues arising in assessing the impacts of fertile land use on “Life Support Functions” in Life Cycle Assessment (LCA). These issues include the assessment of occupation and transformation impacts, the references against which such impacts should be measured, and the concept of natural relaxation versus backup technology for recovery from the impacts. The alternative (or reference) situation is defined using the concept of consequential LCA, which facilitates clarification of the allocation issues arising due to successive land uses. This paper presents soil organic matter (SOM) as a robust indicator for soil quality; even though it does not fully consider all aspects of soil functioning, SOM has been often recognised as the best stand-alone indicator for soil quality. Impacts on biodiversity are not included in this indicator and should be assessed in parallel. Alternative data sources for practical implementation of the method are suggested in a hierarchical way, including locally specific data sets describing changes in SOM; mechanistic models for the prediction of SOM evolution; and general data sources for simplified analyses. Thanks to its flexibility in data collection the method is applicable to any agricultural or forestry LCA to fill an important gap in Life Cycle Impact Assessment (LCIA) of land use.

Evoluzione della sostanza organica in suoli rappresentativi dell’altopiano della Sila

Abstract: Organic-matter evolution of soils from Sila uplands (Calabria, Italy). Soil organic matter (SOM) has long been known for its central role in chemical, biochemical and physiological soil processes. Efforts in optimizing crop production, minimizing environmental pollution, and enhancing soil quality all require a better understanding of the nature and evolution of soil organic matter for its proper management. Four sites (S1 ’ S4), developed on an area of about 9000 ha, in the Sila Mountains, were evaluated to study the turnover of the soil organic matter. The SOM of four sites studied undergoes to a fast mineralization process in the sites S1, S3 and S4, a pastureland, a grass-pastureland, and a cropland, respectively. An opposite trend is observed in the S2 site, under mixed forest of coniferous and broadleaf, where prevailed the humification process.

Labile soil organic matter as a potential nitrogen source in golf greens

The loss of fertilizer N from golf greens can be high depending upon management (irrigation schedule, N source, rate and timing of fertilizer application) as well as soil conditions. Although soil organic matter (SOM) is acknowledged as a major source of N and other nutrients, its potential as an N source seems to be neglected in the management of golf greens. The susceptibility of SOM to degradation is one indication of how active a role SOM plays as a nutrient source. An extraction method developed by Olk et al. [Geoderma 65 (1995) 195] distinguishes humic acid fractions by their binding to dominant stabilizing soil cations and separates them into calcium-bound (CaHA) and non calcium-bound or mobile (MHA) fractions. Mobile humic acid is a relatively young, N-rich HA fraction that does not appear to form stable complexes with Ca. The MHA could therefore play a greater role in nutrient availability than CaHA. We determined C and N distributions within SOM extracted from these two HA fractions in 11 golf greens ranging in age from 4 to 28 yr. Because SOM in golf greens is recently formed, and MHA is an N-rich fraction representing an early stage of SOM evolution, we hypothesized that the MHA fraction would account for a larger proportion of soil organic N than CaHA. The amounts of both HA-C and HA-N increased significantly with green age. MHA accounted for a larger proportion (20–27%) of total soil C than CaHA-C (8–14%). MHA was also enriched in N compared to CaHA with consistently smaller C-to-N ratios. Thus, the greater abundance of MHA and its higher N concentration accounted for a larger proportion of soil organic N (24–45%). The equivalence of MHA-N ranged between 250 kg N ha −1 for a 4 yr-old green and 775 kg N ha −1 for a 21 yr-old green. Thus, soils of established greens contain significant quantities of labile SOM rich in N that could through mineralization supply part of the fertilizer N requirement of turf grass. A greater understanding of the dynamics of this resource is needed if we are to manage golf greens for optimal use without negative consequences to the environment.

Spatio‐Temporal Simulation of the Field‐Scale Evolution of Organic Carbon over the Landscape

The spatial or temporal variability of soil has been extensively considered in the literature using either experimental or modeling approaches. However, only a few studies integrate both spatial and temporal dimensions. The aim of this paper is to present a method for field‐scale simulations of the spatio‐temporal evolution of topsoil organic C (OC) at the landscape scale over a few decades and under different management strategies. A virtual landscape with characteristics matching part of Brittany (France) was considered for the study. Stochastic simulations and regression analysis were used to simulate spatial fields with known spatial structures: short‐range, medium‐range, and long‐range variability. These were then combined using an additive model of regionalization. Agricultural land use was simulated considering four different land uses: permanent pasture, temporary pasture, annual cereal crops, and maize (Zea mays L.). Land use evolution over time was simulated using transition matrices. Evolution of soil organic matter was estimated each year for each pixel through a rudimentary balance model that accounts for land use and the influence of soil waterlogging on mineralization rates. This spatio‐temporal simulation approach at the landscape level allowed the simulation of several scales of soil variability including within‐field variability. Spatial variability decreased drastically over time when only the influence of land use was considered. This effect on soil variability over the landscape may have implications for site‐specific soil management and precision agriculture. The presence of redoximorphic conditions was found to maintain soil spatial variability.

Humic substances along the profile of two Typic Haploxerert

In Vertisols, organic matter contributes to soil colour by formation of organo-mineral complexes and affects morphological, physico-chemical, biological and biochemical properties. Turbation may affect the chemical and structural composition of the most stabilised fractions of soil organic matter (SOM), i.e., humic substances (HS). The objectives of this study were to: (1) characterise SOM in two Vertisols (V1 and V2) developed under Mediterranean climate in Italy, using some HS characteristics as indicators of SOM turnover in Vertisols, and (2) explore possible differences related to the pedomorphologic conditions of the area under which two soils have formed. SOM evolution along the soil profile was investigated by applying different techniques to chemically extracted HS. Humification parameters, thermal methods, such as Differential Scanning Calorimetry (DSC) and Thermogravimetry (TG) together with isoelectric focusing (IEF) were used. The results showed a significant difference in the chemical characteristics of SOM in the considered Vertisols. A higher amount of total and humified carbon was found in pedon V2. In particular, the results indicated an accumulation of humified compounds in deeper horizons of V2: this effect is particularly significant since both soils were under the same agricultural management and the same climatic conditions, but were different in physiografic position, slope and colour. The higher amount of not humified SOM in the pedon V1 could explain the differences found in the shape and size of soil structure between the two pedons. The different SOM evolution occurring in the two Vertisols apparently influenced the structure and composition of the HS fraction. Thermal analysis and isoelectric focusing data provided evidence of a different level of stabilisation of HS from the two Vertisols. The HS from pedon V2 have a higher molecular weight structure, which would imply a higher level of SOM stabilisation. Conversely, HS in the pedon V1 appeared to be richer in aliphatic and phenolic structures, which may imply the presence of less stabilised HS fractions. These findings agree well with the different water dynamics occurring in the two sites, which confirmed the influence exerted by water on chemical and structural composition of HS and, consequently, their effect in determining Vertisols colour. In conclusion, thermal and isoelectric focusing characterisation of HS appear to shed some light in understanding the structural complexity of Vertisols. The results obtained in the present study, if confirmed on a larger number of Vertisols, suggest the possibility of using HS as indicators of SOM turnover in relation to pedomorphology, use and management of soils.