Changes in organic matter composition caused by EDTA washing of two soils contaminated with toxic metals

Two soils contaminated with potentially toxic metals (PTMs) contrasting in pH and mineralogy were remediated with CaEDTA, and changes in soil organic matter (SOM) composition were investigated. Previous studies showed no significant loss of SOM from CaEDTA-treated soils, but the results of our study reflected significant decreases (from 46 to 49%) in the free fraction of humic acids (HAs). Remediation affected the composition of the free HA fraction via disturbance of intermolecular bonds — an increase in phenolic and aromatic groups with a simultaneous decrease in carbohydrates — which was confirmed by FTIR spectroscopy in both soils. Because non-radical molecules such as carbohydrates were selectively removed, the concentration of free radicals in the free HA fraction increased in acidic soil. The bound fraction of HAs and fulvic acids (FAs) in SOM, which are important due to their stability and the permanent effects they have on the soil’s physical properties, remained unchanged in both remediated soils. The effect of soil recultivation was observed only in the excitation emission matrix (EEM) fluorescence spectra of HAs. In terms of SOM, CaEDTA soil washing can be considered moderately conservative; however, the restoration of free humic fractions is likely to be a long-term process.

Changes in Organic Matter Composition Caused by EDTA Washing of two Soils Contaminated with Toxic Metals

Two soils contaminated with potentially toxic metals (PTMs) contrasting in pH and mineralogy were remediated with CaEDTA, and changes in soil organic matter (SOM) composition were investigated. Previous studies showed no significant loss of SOM from CaEDTA-treated soils, but the results of our study reflected significant decreases (from 46 to 49%) in the free fraction of humic acids (HAs). Remediation affected the composition of the free HA fraction via disturbance of intermolecular bonds - an increase in phenolic and aromatic groups with a simultaneous decrease in carbohydrates - which was confirmed by FTIR spectroscopy in both soils. Because non-radical molecules such as carbohydrates were selectively removed, the concentration of free radicals in the free HA fraction increased in acidic soil. The bound fraction of HAs and fulvic acids (FAs) in SOM, which are important due to their stability and the permanent effects they have on the soil’s physical properties, remained unchanged in both remediated soils. The effect of soil recultivation was observed only in the EEM fluorescence spectra of HAs. In terms of SOM, CaEDTA soil washing can be considered moderately conservative, however, the restoration of free humic fractions is likely to be a long-term process.

Humic fractions as indicators of soil organic matter responses to clear-cutting in the mountain and lowland conditions of south-western Poland

Clear-cutting means forest removing (stem only) and is the most common type of forest harvesting but undoubtedly has a negative impact on the C budget in soils. This work aimed to describe responses of soil organic matter in the forest soils to forest removing under temperate climate conditions of lowland and mountain regions in south-western Poland. Using advanced instrumental analysis, like EPR, 1H NMR and FT-IR spectroscopy it has been found that clear-cutting, alters C cycling and accelerates decomposition in the forest floor leading to loss of humic fractions in the investigated soils. In the mountain forests the more labile, low-molecular fulvic fraction decreased as the effect of harvesting practice. The transformation of organic matter after clear-cutting resulted in the loss of less humified organic matter containing humic substances of less polymerised molecules. Analysis of the semiquinone radical structures and concentrations showed a decrease in radical concentration observed for HA from mountain clear-cut areas compare to the undisturbed forest. Results presented in this paper have proved less aliphatic character of humic acid molecules from the lowlands, compared to the mountain forest as the effect of clear-cutting. Harvesting practices in mountain regions should be approached with particular care due to the risk of erosion of exposed surfaces and soils containing less humified and less stable organic matter than in the lowlands. Humic fractions of higher solubility, less stability and tendency to migrate through the soil profile may favour the leaching of nutrients and consequently cause the eutrophication of waters.

Chemical diversity and molecular signature of soil humic fractions used as proxies of soil quality under contrasted tillage management

Increasing food production while avoiding the progressive degradation of agricultural soils has become one of the major challenges at a global level. In consequence, the development of sustainable tillage methodologies or cultivation strategies is an important subject of current research. In fact, it has been observed that the implementation of reduced tillage (RT) vs. traditional tillage (TT) in the long term not only improves soil physicochemical properties but also global soil quality in terms of soil health. In particular, the increase of the soil organic carbon (SOC) content under RT conditions is one of the most important factors, but there is little information about the chemical composition and humification level of this carbon, and thus about its persistence at long-term. This is of particular importance considering the policies of carbon sequestration and climate change mitigation, such as the “4 per 1000” initiative. In this study, molecular-level characterization of the humic acid (HA) and fulvic acid (FA) fractions isolated from a soil after 19 years under RT and TT practices was carried out. This study would provide objective descriptors of the impact of these two tillage practices in the chemical composition of the resulting SOC. With this purpose, the potential of Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR-MS) for assessing changes in the molecular assemblages released from the humic fractions (HA and FA) was examined. The results showed enhanced diversity and chemical richness (expressed as number of molecular formulas) in the composition of SOC under RT. Different tillage-specific compound classes were associated with both tillage practices. As a whole, the humic fraction showed a higher proportion of molecular formulas for lipid and hydroaromatic families in the case of RT compared to TT, while the same fraction under TT showed a greater richness of oxidized protein-derived formulas than RT. In the case of FAs, a similar pattern was observed for hydroaromatic and protein-derived formulas, but the proportion of molecular formulas assigned to unsaturated lipids was higher in TT than in RT. In addition, increased number of formulas for aromatic and condensed aromatic compounds was observed in FAs under TT respect to RT.

Humic fractions as indicators of soil organic matter responses to clear-cutting in the mountain and lowland conditions of south-western Poland

Clear-cutting means forest removing (stem only) and is the most common type of forest harvesting but undoubtedly has a negative impact on the C budget in soils. This work aimed to describe responses of soil organic matter in the forest soils to forest removing under temperate climate conditions of lowland and mountain regions in south-western Poland. Using advanced instrumental analysis, like EPR, 1H NMR and FT-IR spectroscopy it has been found that clear-cutting, alters C cycling and accelerates decomposition in the forest floor leading to loss of humic fractions in the investigated soils. In the mountain forests the more labile, low-molecular fulvic fraction decreased as the effect of harvesting practice. The transformation of organic matter after clear-cutting resulted in the loss of less humified organic matter containing humic substances of less polymerised molecules. Analysis of the semiquinone radical structures and concentrations showed a decrease in radical concentration observed for HA from mountain clear-cut areas compare to the undisturbed forest. Results presented in this paper have proved less aliphatic character of humic acid molecules from the lowlands, compared to the mountain forest as the effect of clear-cutting. Harvesting practices in mountain regions should be approached with particular care due to the risk of erosion of exposed surfaces and soils containing less humified and less stable organic matter than in the lowlands. Humic fractions of higher solubility, less stability and tendency to migrate through the soil profile may favour the leaching of nutrients and consequently cause the eutrophication of wate

Application of Brown Coal and Activated Carbon for the Immobilization of Metal Forms in Soil, along with Their Verification Using Generalized Linear Models (GLMs)

Various factors can lead to the transformation of metal forms and to an increase in their solubility and, consequently, their mobility. One solution to the problem of increased solubility is the use of carbons as additives to soil in order to limit the potential migration of contaminants. The aim of this study was to determine the effect of using brown coal and activated carbon on metal forms that are available to plants. The mineral composition of the coals used in a pot experiment was analyzed. Observations were carried out with a JJSM-6380 LA scanning electron microscope (SEM) connected to an EDS electron micro-probe. The total contents of Zn, Cd, Pb and Cu in the assessed soils as well as the content of available metal forms were determined after single extractions with different reagents, namely 1 M NH4NO3, DTPA and 1 M HCl. Generalized linear models (GLMs) were used to evaluate the effectiveness of the stabilization methods in a long-term pot experiment. The carbons reduced the percentage of these forms relative to the total metal content in the soil. After adding brown coal, Zn, Cd, Pb and Cu forms were reduced by up to 32%, 30%, 33% and 43%, respectively. After adding activated carbon, the metal forms of Zn, Cd, Pb and Cu were reduced by up to 47%, 44%, 40% and 50%, respectively. The following order of extracted metal forms with different solutions was found: HCl: Zn > Pb > Cu > Cd; DTPA: Pb > Zn > Cu > Cd; NH4NO3: Zn > Cu > Pb > Cd. Eight years after setting up the pot experiment, the contents of humic substances in soils with the addition of both tested carbons were compared, and the soils with added carbons were found to have a stable content of humic fractions. The costs of remediation through the stabilization method using the tested brown coal and activated carbons do not exceed USD 75/t (taking into account the double doses of both carbons).

Soil quality indicators in areas with different vegetation cover in the Atlantic Forest biome, Southeastern Brazil

The southeast region of Brazil, in the Atlantic Forest biome, has been impacted for a long time, observing today small agricultural areas inserted in a matrix of pastures and forest fragments, the soils being highly modified due to the different forms of use they were submitted. The objective of this study was to evaluate the organic carbon content in the humic fractions on soil organic matter (SOM), the distribution of aggregates in size classes and the porosity and aggregates density variables by means of computed tomography in secondary forest, pasture and agriculture areas in Pinheiral (RJ). Three areas composed of secondary forests with different succession stages and three managed areas were evaluated: Secondary Forest (SF) Initial Stage (SFIS), Medium Stage (SFMS), Advanced Stage (SFAS), mixed managed pasture (MMP), and perennial and annual agriculture (PA and AA, respectively). Three types of samples were collected in different layers: i) deformed samples to quantify carbon in the SOM humic fractions; ii) clods for quantifying the mass of aggregates and later assessing the distribution by diameter classes; and iii) undisturbed samples collected with the aid of an acrylic cylinder to perform computed tomography (CT). Humic substances and aggregate classes below 2.0 mm were the attributes most influenced by the forms of use and/or vegetation cover. The CT technique proved to be efficient in the measurement of the edaphic attribute’s porosity and density of the aggregates, helping in the characterization and separation of the different environments.

CARBON AND NITROGEN IN SOILS AND HUMIC FRACTIONS OF DIFFERENT PEDOFORMS IN THE ATLANTIC FOREST BIOME

Relief is a soil formation factor that can modify the distribution of soil organic matter (SOM) fractions in a landscape. The objective of this study was to evaluate the effect of the relief on SOM fractions, considering their distribution in different topographic positions, segments, in two pedoforms (concave and convex) in areas covered with forest in the Atlantic Forest biome. The two pedoforms were selected in adjacent areas and divided into segments considering the topographic variation. The carbon and nitrogen origins were evaluated in the extract up to 100 cm of depth. Soil samples from the 0-5, 5-10, and 10-20 cm layers were collected for chemical characterization and fractioning (granulometric and chemical) of the SOM. The soil density in the 0-5 and 5-10 cm layers was determined to calculated the carbon stocks. The isotopic composition showed predominance of 13C. The highest organic carbon and particulate carbon contents were found in the convex pedoform. The distribution of humic fractions showed that the larger part of the humidified carbon was in the humin fraction. The humin and fulvic acid fractions were higher in the convex pedoform. The carbon stocks were, in general, higher in the convex pedoform, decreasing as the soil depth increased; and nitrogen stocks presented no differences. The higher carbon and nitrogen contents were found in the convex pedoform and in the lower region of the concave pedoform.