Purpose

Due to the modernization of the agro-industrial sector, compounds with different toxicity and effects on human health and animal have been used and consequently affecting the environment. Among them, tetracycline (TC) stands out as one of the antibiotics most commonly used worldwide. This study evaluated the TC interaction with different fractions of peat in natura and humic substances, humic acid, fulvic acid, and humin.

Materials and methods

The different fractions of the organic matter were characterized by organic matter content, elemental analysis, spectroscopic analysis (E₄/E₆), and nuclear magnetic resonance of carbon 13 (NMR ¹³C), and the interaction between TC and different fractions of organic matter was made by fluorescence spectrometry. We used the tangential ultra-filtration system for determining the complexation capability of humic substances (HSs), fulvic acids (FA), humic acids (HA), and humin (HUM) from peat with TC. Finally, we evaluated sorption kinetic experiments between TC and peat in natura.

Results and discussion

The peat samples, humic substances, FAs, HAs, and HUM were characterized by organic matter (OM), atomic ratio (H/C and C/O) calculated from elemental analysis data, functional groups quantified by NMR ¹³C data, and E₄/E₆ ratio, and the results show significant differences in the structural characteristics of the fractions of OM influenced by the type of microorganisms and environmental factors associated with this decomposition. Data analysis revealed the strongest interaction between HUM and TC (59.19 mg g^?1), followed by interaction between HS and TC (43.36 mg g^?1 HS). In the sorption studies, these conditions showed the best model to describe the system under consideration using the Freundlich model.

Conclusions

The results showed that the different fractions of the OM extracted from peat show different contributions that affect the bioavailability of contaminants to the environment.

Purpose

Soil organic carbon (SOC) and its labile fractions are strong determinants of physical, chemical and biological properties. The objective of the present work was to evaluate the effects of organic amendments (technosol made of wastes and biochar) and Brassica juncea L. on the soil C fractions in a reclaimed mine soil.

Materials and methods

The studied soil was from a former copper mine that was subsequently partially reclaimed with vegetation and wastes. A greenhouse experiment was carried out to amend the mine soil with different proportions of technosol and biochar mixture and planting B. juncea. B. juncea plants can tolerate high levels of metals and can produce a large amount of biomass in relatively short periods of time.

Results and discussion

The results showed that with the addition of biochar and wastes, soil pH increased from 2.7 to 6.18, SOC from undetectable to 105 g kg^?1 and soil total nitrogen (TN) from undetectable to 11.4 g kg^?1. Amending with wastes and biochar also increased dissolved organic carbon (DOC) from undetectable to 5.82 g kg^?1, carbon in the free organic matter (FOM) from undetectable to 30.42 g kg^?1, FAP (carbon in fulvic acids removed with phosphoric acid) from undetectable to 24.14 g kg^?1 and also increased the humification ratio, the humification index, the polymerisation rate and the organic carbon in the humified fractions (humic acids, fulvic acids and humin). Soils amended and vegetated with B. juncea showed lower FOM values and higher humification index values than the soils amended only with biochar and wastes.

Conclusions

This study concludes that the combined addition of wastes and biochar has a greater potential for both increasing and improving organic carbon fractions in mine soils. The authors recommend the application of biochar and technosol made of wastes as a soil amendment combined with B. juncea on soils that are deficient in organic matter, since they increased all of the SOC fractions in the studied copper mine soil.

Purpose

The traditional method to determine humic content (humic and fulvic acids) in commercial fertilizers, biostimulants, and organic materials is based on the oxidation of the organic carbon contained in the basic-soluble but acid-insoluble fraction (humic acids) and the basic-acid soluble fraction (fulvic acids) of their alkaline water extracts. This methodology, merely operational, makes it impossible to distinguish if the quantified carbon corresponds to substances with “humic” chemical nature or to non-humic organic matter but with similar solubility properties to those of humic matter. The aim of this work is to develop a new methodology that not only quantifies the humic content in commercial products (and raw materials) but also assesses the humic quality of the quantified organic matter.

Materials and methods

To this end, humic and fulvic (-like) fractions have been isolated/purified from several humic and non-humic materials and characterized by means of elemental analysis and UV-visible, fluorescence, and infrared spectroscopies, and these data have been used to perform a discriminant analysis (DA).

Results and discussion

The model obtained from the DA is able to discriminate humic and fulvic fractions from apparently humic or fulvic ones and provides discriminant classification functions that have proven to successfully predict the “humic quality” of the fractions isolated from commercial products, after their elemental and spectroscopic characterization.

Conclusions

Therefore, the combination of the fractionation, characterization, and evaluation by the DA is proposed as an effective methodology for quantifying and assessing the quality of the humic content claimed in the labels of commercial products.

Purpose

The application of bio-fertilizers is one of the management practices that can help to maintain or increase the content of organic matter (OM) and improve soil fertility in arable soils. While some results have been obtained in relation to the influence of bio-fertilizers on organic matter content, less in known about the fractional composition of humus.

Materials and methods

The aim of this study was to determine the effects of the bio-fertilizer UGmax on soil total organic carbon (TOC), dissolved organic carbon (DOC), and the fractional composition of organic matter (C of humic acids (CHAs), C of fulvic acids (CFAs), and C in humins) in the humus horizon of an arable field. Measurements were taken in 2005 before the application of UGmax and in 2008, 3 years after its application, which was done in 2005, 2006, and 2007. Forty soil samples were taken in 2005 (the control year without UGmax), while 20 samples were taken after UGmax treatment and 20 from the control in 2008. Samples were always collected after the plants were harvested.

Results and discussion

After the 3-year period of the experiment, the TOC content was 6.3 % higher in plots on which UGmax was applied in comparison to the control, while the DOC content was 0.19 percentage points lower after 3 years of bio-fertilizer use as compared to the initial year of the experiment. The contribution of DOC to TOC decreased significantly after the application of UGmax in comparison with the control. The content of CFAs and its contribution in the TOC pools in soil without UGmax was higher at the end of the experiment compared to the beginning, while there was an inverse relationship in the soil with the bio-fertilizer. In comparison with the control, organic matter in the soil treated with UGmax had a higher content of C of humic acids, C in humins, and higher CHAs/CFAs ratio.

Conclusions

We conclude that the use of a bio-fertilizer that increases the stable fractions of organic matter provides evidence of an increase in the soil OM stability. In turn, the contribution of the organic matter fractions that are more resistant to decomposition is crucial for increasing soil carbon sequestration.

Purpose

The mineralization/immobilization of nutrients from the crop residues is correlated with the quality of the plant material and carbon compartments in the recalcitrant and labile soil fractions. The objective of this study was to correlate the quality and quantity of crop residues incubated in the soil with carbon compartments and CO₂-C emission, using multivariate analysis.

Materials and methods

The experiment was conducted in factorial 4?+?2?+?5 with three replicates, referring to three types of residues (control, sugarcane, Brachiaria, and soybean), and two contributions of the crop residues in constant rate, CR (10 Mg ha^?1 residue), and agronomic rate, AR (20, 8, and 5 Mg ha^?1 residue, respectively, for sugarcane, soybean, and Brachiaria), evaluated five times (1, 3, 6, 12, and 48 days after incubation). At each time, we determined the CO₂-C emission, nitrogen and organic carbon in the soil, and the residues. In addition, the microbial biomass and water-soluble, labile, and humic substance carbons fractionated into fulvic acids, humic acids and humin were quantified.

Results and discussion

Higher CO₂-C emissions occurred in the soil with added residue ranging from 0.5 to 1.1 g CO₂-C m^?2 h^?1 in the first 6 days of incubation, and there was a positive correlation with the less labile organic soil fractions as well as residue type. In the final period, after 12 days of soil incubation, there was a higher relation of CO₂-C emission with carbon humin. The sugarcane and soybean residue (20 Mg ha^?1) promoted higher CO₂-C emission and the reduction of carbon residue. The addition of residue contributed to an 82.32 % increase in the emission of CO₂-C, being more significant in the residue with higher nitrogen availability.

Conclusions

This study shows that the quality and quantity of residue added to soil affects the carbon sequestration and CO₂-C emission. In the first 6 days of incubation, there was a higher CO₂-C emission ratio which correlates with the less stable soil carbon compartments as well as residue. In the final period of incubation, there is no effect of quality and quantity of residue added to soil on the CO₂-C emission.

Purpose

Processes that lead to soil organic carbon (SOC) protection depend on both soil porosity and structure organization, as well as chemical and biological properties. In particular, the soil micro-nano porosity (<30 μm) regulates microorganism accessibility to the soil pore system and offers surfaces for organic carbon adsorption and intercalation into soil minerals. The aim of this work was to investigate how pore size distribution can selectively protect specific carbon pools in different aggregate size fractions, by considering the effects of long-term application of farmyard manure (FYM) and mineral (Min) fertilization.

Materials and methods

Macroaggregates (250–2000 μm), microaggregates (53–250 μm), and silt–clay (<53 μm) fractions of three different soils (clayey, peaty, and sandy) were separated by wet sieving technique and then subjected to chemical and physical analysis. Sample porosity and pore size distribution were analyzed using mercury intrusion porosimetry (MIP), while SOC chemical structure was characterized by means of nuclear magnetic resonance (¹³C cross-polarization–magic angle spinning nuclear magnetic resonance (CP MAS ¹³C NMR)) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopies.

Results and discussion

Results showed that FYM increased organic (OC) and humic carbon (HC) content compared to the Min fertilization and unfertilized soils. However, it caused a gradual decrease in O,N-alkyl C, and alkyl C of humic C from macroaggregate to silt–clay fractions, suggesting an advanced state of humic component degradation as revealed by CP MAS ¹³C NMR, DRIFT analyses. MIP analysis showed a clear increase of micropores (5–30 μm) and cryptopores (0.0035–0.1 μm) from macroaggregate to silt–clay fractions, while minor differences were observed among the treatments. The application of principal component analysis to mineral soil fractions identified the formation of three main clusters, where (i) macroaggregates of clayey soil were mainly associated to cryptopores and OC and (ii) microaggregates and silt–clay fraction were mainly isolated by carbonyl C, ultramicropores, and total porosity. The third cluster was associated with medium and fine sand of the sand soil fraction as coupled with O,N-alkyl C, anomeric C, mesopores, and HC/OC ratio.

Conclusions

Overall, this study indicates that pore size distribution may be a valuable indicator of soil capacity to sequester carbon, due to its direct influence on SOC linkages with soil aggregates and the positive effects against SOC decomposition phenomena. In this context, micropore- to nanopore-dominated structures (e.g., clayey soil) were able to protect OC compounds by interacting with mineral surfaces and intercalation with phyllosilicates, while meso/macropore-dominated structures (i.e., sandy soil) exhibited their low ability to protect the organic components.

Purpose

Returning straw to soil improved soil carbon sequestration capacity and increase soil organic matter. However, in different soil depth, especially in subsoil, there were few studies on the effects of straw decomposition on soil carbon sequestration and the properties of humic substances. Therefore, an in-situ incubation study, with six different straw rates and three different soil depths, was carried out to explore the effects of straw decomposition on soil organic carbon and humic substance composition at different soil depths.

Materials and methods

The experiment was composed of six straw rates: 0, 0.44, 0.88, 1.32, 2.64, and 5.28% of soil dry mass. The maize straw was proportionately mixed with soil and put into nylon bags. Then, the nylon bags were buried in soil at three depths (15, 30, and 45 cm) and the straw decomposition trial lasted for 17 consecutive months in-situ. Soil samples were collected after completion of the field trial. Humic substances were quantitatively and qualitatively analyzed using the modification method of humus composition and the methods specified by the International Humus Association. Fourier transform infrared spectroscopy and fluorescence spectroscopy were used in this study.

Results and discussion

Results indicated that CO₂ concentration increased with increase in soil depth. Compared with the “zero” straw control, soil organic carbon contents in the treatments amended with 1.32, 2.64, and 5.28% maize straw increased significantly, and most accumulations were at 30–45 cm depths. FTIR and fluorescence emission spectra analyses indicated that the addition of straw enhanced the aliphatic structure and decreased the aromaticity of humic acid (HA), that was to say that HA molecular structure approaches to the development of simplification and younger. The maximum change in HA molecular structure was under the 5.28% treatment in the 30–45 cm depth.

Conclusions

Returning maize straw to the subsoil layers is more conducive to the accumulation of soil organic carbon and improvement of the quality and activity of HA and the organic carbon in the subsoil can be renewed.

Purpose

Heavy metal contamination is a priority issue affecting millions of hectares of soil throughout the world. One of the most promising, environmentally friendly, and cost-effective approaches to restore polluted soils could be applying organic amendments. We investigated the remediation potential of three types of humic products with regard to their effect on the bioavailability of Pb and Zn, content of nutrients, and the ability to mitigate acute phytotoxicity in contaminated soil.

Materials and methods

Spodosol samples were spiked with Pb (550 mg kg^?1) and Zn (880 mg kg^?1). Then, two different commercial humic products (from peat and lignosulfonate) and natural humic acids (from brown oxidized coal) were added in two doses to reach an equal content of carbon: a 10% increment and a 30% increment of the initial total organic carbon in the soil. After 30 days, the content of metals and nutrients (S, K, Na, Ca, Mn, P) was determined by the sequential extraction (i?H₂O, ii?NH₄COOH pH 4.8, iii–CH₃COOH). The effect of humic products on heavy metals bioavailability was evaluated using the calculated partition indexes. Seed germination and root elongation of Sinapis alba were also determined. Chemical and biochemical variables were aggregated by the principal component analysis.

Results and discussion

Humic products reduced the amount of bioavailable fractions of Pb and Zn in soils. The partition index, which quantitatively describes bioavailable fractions of the Zn and Pb in the soil, was 28–49% lower than in the spiked (Pb+Zn) control. The inhibition of root elongation and seed germination of mustard by Zn and Pb was significantly mitigated by humic products; in the soil test, the root length and seed germination were up to 36–87% higher than those of the Pb+Zn control and did not differ from those in the non-amended treatments. This effect may have been associated with the structural differences (H/C and O/C ratio) and content of nutrients (Na and K) in humic products.

Conclusions

Commercial humic products used in poor multi-contaminated soils can maintain plant growth by improving nutrient status due to heavy metals immobilization and can be a promising approach to remediate the soil contaminated with heavy metals at extremely high concentrations.

Purpose

Application of functional organisms in soil organic amendments has the potential to accelerate organic matter decomposition and stimulate C cycling. In this study, a short-term (a year) field experiment was conducted to investigate the collaborative effects of earthworms and phosphate-solubilizing bacteria on C accumulation in pig manure-amended soil.

Materials and methods

A field experiment was conducted with six treatments established. The first three treatments, including control (CK), pig manure (Pm), and pig manure?+?slurry (Pm?+?S), were set up to evaluate the influences of pig manure on soil C accumulation. The other three treatments, including manure?+?slurry?+?earthworms (Te), manure?+?slurry?+?phosphate-solubilizing bacteria (Tb), and manure?+?slurry?+?earthworms?+?bacteria (T(e?+?b)), were set up to investigate the collaborative effects of functional organisms on soil C cycling. The Pm?+?S treatment was chosen as the control (T) for this purpose.

Results and discussion

The results showed that the soil C pools did not increase significantly under the manure treatment. In contrast, an integrated application of manure, slurry, earthworms, and bacteria significantly increased the various C fractions, such as SOC and humin, indicating a rapid and positive effect of earthworms and bacteria on C accumulation. Besides, C sequestration by the integrated application was as high as 1.35 Mg C ha^?1 soil, half of which was stabilized.

Conclusions

The T(e?+?b) was an efficient strategy to sequestrate and stabilize SOC in arid hillside soils. The bacteria increased the labile OC, especially microbial biomass C, while the earthworms were apparently essential for the increase in stable OC.

Purpose

The objective of this study was to determine the impact of restoration processes on the selected soil properties and organic matter transformation of mountain fens under the Caltho-Alnetum community in the Babiogórski National Park in Outer Flysch Carpathians.

Materials and methods

Restoration processes were conducted on three degraded mountain fens in the Babiogórski National Park in Outer Flysch Carpathians, Poland. The degradation degree of soils was the criterion for the selection of habitats for further studies. To determine the influence of restoration processes on mountain fen soil properties and organic matter transformation, samples were collected in 2011 and 2013. The soil samples were assayed for pH, base cation concentration, hydrolytic acidity, organic carbon and total nitrogen content, total exchangeable base cation concentration, cation exchange capacity, and base saturation. Organic matter fractions were extracted by IHSS method. Quantitative and qualitative study of organic matter was based on fraction composition analysis and the ratio of humic acid carbon to fulvic acid carbon. The research results were statistically verified.

Results and discussion

Based on morphological and chemical properties, the studied mountain fen soils can be classified as Sapric Dranic Eutric Histosols and Sapric Dranic Dystric Histosols according to WRB guidelines (2015). Before restoration processes, the mountain fen soils subjected to a different water regime showed various contents of total nitrogen and organic carbon. The decreasing of the groundwater level was reflected in pH, calcium ion content, exchangeable base cation concentration, and base saturation. The increase of the groundwater level had influence on chemical properties of mountain fen soils such as pH, total exchangeable base cation concentration, hydrolytic acidity, cation exchange capacity, and base saturation. Three-year restoration processes did not cause significant changes in the composition of humic substance fractions.

Conclusions

Mountain fens under Caltho-Alnetum community are priority habitats in Babiogórski National Park in Outer Flysch Carpathians, Poland. These habitats responded to restoration processes in varying degrees depending on the extent of their degradation. The least degraded mountain fen was characterized by a short response time on the restoration processes. The reaction of higher degraded habitats was weaker.

Purpose

The aim of the study was to present variability of content and quality of soil organic matter on the landslide surface. Attempts were made to demonstrate the progress of the process of soil cover restoration 7 years after the landslides and biochemical activity of soil associated with the restoration of soil cover.

Materials and methods

The landslide area was located in southern Poland, in the Sucha Forest District. The soil properties were studied on a regular grid of points, which covered the entire area of the landslide. In soil samples, particle size, soil aggregates content, pH, total carbon and nitrogen content, microbial biomass carbon and nitrogen and the activity of dehydrogenases were determined. Additionally, the fractions of soil organic matter were used in the study as an indicator of soil quality due to the many important interactions of these components in the soil system.

Results and discussion

This study identified the landslide area as characterised by a stronger diversification of physical, chemical, and biological properties. The upper part of the landslide (in the area referred to as the landslide niche) is strongly eroded and characterised by the least advanced soil cover recovery. Additionally, low soil organic matter content was observed in the upper part of the landslide, which restricted biological activity of the studied soils. Soil microbial biomass carbon increased with restoration of landslide soils.

Conclusions

The soil organic matter plays a key role for the initial stage of soil formation on a landslide. The amount of soil organic matter on the studied landslide had a positive effect on the microbial biomasses C and N, dehydrogenases activity. Estimating the soil organic matter fraction can be utilised as an indicator of changes in soil.

Purpose

Aquatic macrophytes are an important source of autochthonous dissolved organic carbon in aquatic ecosystems. Yield and mass loss of aquatic humic substances released from macrophytes decomposition could be affected by the plant species and oxygen availability. Our aim was to describe the kinetics of dissolved fulvic and humic acids formed from decomposition of four aquatic macrophytes under aerobic and anaerobic conditions.

Materials and methods

Samples of Eichhornia azurea (Sw.) Kunth, Egeria najas Planch, Oxycaryum cubense (Poepp. and Kunth), and Salvinia molesta (Mitchell) were incubated under aerobic and anaerobic conditions. On sampling days, the remaining particulate detritus were weighted and were measured for the pH, the electrical conductivity, and the organic carbon in the dissolved fraction. Humic substances were extracted from the dissolved fraction, separated into fulvic and humic acids, and then quantified. The mass loss of particulate and dissolved fractions were fitted to first order kinetic models.

Results and discussion

Aerobic environment favored mineralization of aquatic macrophyte detritus and humification of organic dissolved carbon. Incubations under aerobic conditions formed 3.6 times more humic acid than incubations under anaerobic conditions. However, incubations in an anaerobic environment formed 1.84 times more fulvic acid. The dissolved humic compounds presented low mineralization rates probably due to the presence of the macrophyte detritus in the incubation representing a more attractive source of resource for microorganisms.

Conclusions

In many cases, the mineralization of HS was not noticed, leading to an increase in humic and fulvic acid concentration in the water. O. cubense detritus presented the highest carbon concentration, were related to refractory features, and generated the highest amounts of dissolved HA (mainly under aerobic condition). Egeria najas detritus presented the lowest carbon concentration, were related to labile features, and generated the highest amounts of dissolved FA (mainly under anaerobic condition). Besides that, high humic substance concentrations in the dissolved organic carbon were related to low mineralization of this fraction.

Purpose

This study was to investigate the changes of heavy metals in the soils amended with different municipal sewage sludge hydrochars.

Materials and methods

Sewage sludge hydrochars prepared at either 190 or 260 °C, for 1, 6, 12, 18, or 24 h, respectively, were added to soil samples and then incubated for 60 days. Water-extractable organic carbon (WEOC) and CO₂ evolution were determined during the incubation. The total quantities of heavy metals and their different fractions were analyzed by inductively coupled plasma spectrometry (ICP).

Results and discussion

Hydrochar-amended soils had much higher water-extractable carbon and more CO₂ evolution than control soil, indicating that the added hydrochars contained a significant amount of WEOC and could be decomposed during the incubation. Hydrochar addition immediately and significantly increased the total heavy metals of the soil. Moreover, both oxidizable and residual fractions of all heavy metals were significantly higher in all the hydrochar-added soils than those in control soil. Both oxidable and residual fractions of heavy metals decreased in the hydrochar-amended soils during 60-day incubation. In contrary, both acid soluble and reducible fractions of heavy metals increased in the hydrochar-amended soils during incubation. It is thus obvious that the heavy metals in both oxidable and residual fractions may be released during hydrochar decomposition and then be adsorbed by soil matrix such as carbonates, iron oxides, and clays.

Conclusions

Municipal sewage sludge can be readily carbonized into hydrochar. However, it is watchful of applying the hydrochar into soil since hydrochar addition increases in both total and bioavailable heavy metals in soil. More work is particularly required to investigate the long-term impacts on soil and environment.

Purpose

The paper describes rhizospheric (Rs) and non-rhizospheric (nRs) soil to demonstrate the zone of the plant root impact on physical and chemical properties of the soil. The effects of the process accompanying the transformations of organic matter into humic substances in the rhizosphere of “common dandelion” Taraxacum officinale have been determined, and the properties of humic acids (HAs) were described. The importance of iron and clay minerals for the formation of a stable and water-resistant soil structure has been emphasized.

Materials and methods

The laboratory analysis involved determination of basic physical and chemical soil properties: texture, pH, cation exchange capacity (CEC), electrical conductivity, and content of total organic carbon (TOC) and dissolved organic carbon (DOC) and quality of humic substances: optical properties of HAs and its separation into hydrophilic (HIL) and hydrophobic (HOB) fractions, speciation of iron, glomalin operationally described as an easily extractable glomalin-related soil protein (EE-GRSP), and soil aggregate stability (SAS) of six size classes of soil aggregates.

Results and discussion

The Rs was reported with a higher TOC and DOC content (measured in the CaCl₂ extracts), however not significantly. The HAs isolated from Rs revealed a significantly higher content of humic substances at its initial decomposition stage, as compared with nRs. A significantly higher concentration of EE-GRSP was noted in the aggregates of the rhizospheric zone (mean 1.14 g kg^?1) than in the aggregates collected from root-free soil (mean 0.94 g kg^?1). There was noted the highest mean share of 1–3 mm soil aggregates in Rs as well as in nRs, respectively 44.4 and 38.3%. The soil material both in Rs and in nRs contained high amounts of exchangeable Ca²⁺, and smectite is the predominant clay mineral. It was favorable for the accumulation of organic carbon and for the formation of good soil physical condition (tilth). Higher but insignificant SAS values were observed for Rs (mean SAS?=?95.6%) than for nRs (mean SAS?=?93.9%).

Conclusions

The studies confirm the role of common dandelion roots in the process of organic carbon accumulation in rhizospheric zone and a favorable effect on the mechanism of the formation of water-resistant aggregates. Higher values of SAS for the Rs were affected by the content of TOC, DOC, exchangeable Ca²⁺ and the concentration of EE-GRSP, and, less considerably, the content of Fe and clay minerals.

Purpose

The objectives of the study were (1) to quantify the long-term effects of nitrogen-phosphorus fertilizer (NP) and a combination of nitrogen-phosphorus with organic manure (NPM) on total soil organic carbon (SOC) and total soil inorganic carbon (SIC), (2) to identify the changes of SOC and SIC in soil particle-size fractions, and (3) to investigate the relationship between SOC and SIC.

Materials and methods

Two long-term field experiments (sites A and B) were performed in 1984 (site A) and 1979 (site B) in the North China Plain. The soil samples were collected in 2006 and separated for clay, silt and sand size particle fractions and then determined for SOC and SIC.

Results and discussion

The long-term fertilization significantly increased SOC in 0–20 cm soil layer by 9–68% but significantly decreased or had no effect on SIC. In total, soil carbon storage was little affected by NP, but significantly increased by NPM application (p < 0.05). Fertilization affected both SOC and SIC in sand- and silt-sized particles but not in clay-size fraction. Both NP and NPM increased SOC in sand- and silt-sized particles by 8.7–123.9% in the 0–20 cm layer but decreased SIC up to 80.4% in the 40–60 cm layer. The SOC concentration in the particle-size fractions was negatively correlated with SIC concentration, which may imply an antagonistic interaction between organic and inorganic carbon levels.

Conclusions

These results illustrate the importance of soil inorganic carbon pool in evaluating soil total carbon pool in semi-arid farmlands. Previous assessments of the effects of fertilizers on the soil carbon pool, using only SOC determinations, require re-evaluation with the inclusion of SIC determinations.

Purpose

The study aimed to describe the carbohydrates and amino compounds content in soil, the light fraction (LF), the >53 μm particulate organic matter (POM), and the mobile humic acid (MHA) fraction and to find out whether the carbohydrates and amino compounds can be used to explain the origin of SOM fractions.

Materials and methods

Soil samples were collected from two agricultural fields managed under organic farming in southern Italy. The LF, the POM, and the MHA were sequentially extracted from each soil sample then characterized. Seven neutral sugars and 19 amino compounds (amino acids and amino sugars) were determined in each soil sample and its correspondent fractions.

Results and discussion

The MHA contained less carbohydrate than the LF or the POM but its carbohydrates, although dominated by arabinose, were relatively with larger microbial contribution as revealed by the mannose/xylose ratio. The amino compounds were generally less in the LF or the POM than in the MHA, while the fungal (aspartic and serine) and bacterial (alanine and glycine) amino acids were larger in the MHA than in the LF or the POM, underlining the microbial contribution to the MHA. Results from both sites indicated that total carbohydrates content decreased moving from the LF (younger fraction) to the MHA (older fraction), which seems to follow a decomposition continuum of organic matter in the soil-plant system.

Conclusions

The study showed that the MHA is a labile humified fraction of soil C due to its content of carbohydrates and concluded that the content of carbohydrates and amino compounds in the LF, the POM and the MHA can depict the nature of these fractions and their cycling pattern and response to land management.

Purpose

The scope of this article was to investigate the spatial and temporal variability of enantiomeric fractions (EFs) of persistent organic pollutants (POPs) in soil compared to the uncertainty of the analytical data.

Materials and methods

Soil samples were taken with high spatial resolution at two sites in Czech Republic in 2008 to investigate variability on a small spatial scale. In addition, composite soil samples were taken from ten sites in 2005 and 2008 to investigate temporal variations. All samples were analysed for a suite of soil properties as well as concentrations and EFs of polychlorinated biphenyl (PCB) -95, PCB-132 and PCB-149; α-hexachlorocyclohexane (HCH); o,p′-dichlorodiphenyltrichloroethane (DDT); and o,p′-dichlorodiphenyldichloroethane (DDD).

Results and discussion

Median EFs of PCB-95 and PCB-149, α-HCH, o,p′-DDT and o,p′-DDD did not change significantly on the sites sampled in 2005 and again in 2008, while PCB-132 changed from EF?=?0.38 to EF?=?0.53. The sampling methodology is therefore very important, and composite samples will not be the best option if enantioselective degradation processes are investigated. Non-racemic EFs of POPs in the subsampled sites in 2008 were correlated to soil parameters, such as total organic carbon (TOC), total nitrogen and humic acids. These parameters are site specific and might vary on a small scale. This can explain why certain soil parameters are reported as significantly correlated with non-racemic EFs of chiral POPs in some studies, but not always in other similar studies.

Conclusions

While composite samples may still represent the overall prevailing EF range, they are not ideally suited to study enantiomeric degradation processes, which are taking place at a relative small scale, depending on the heterogeneity of soil parameters such as TOC, total organic nitrogen (TON) and humic acids.

Purpose

Soil aggregates play an important role in promoting soil fertility, as well as increasing the sink capacity and stability of soil carbon. In this study, we consider the following research questions:1. Under field conditions, do different dosages of biochar increase the soil aggregation after 3 years of application?2. How does the application of biochar affect the concentration and distribution of soil total organic carbon (TOC) and total nitrogen (TN) in different sizes of aggregates?3. Can the application of biochar alter the composition of organic carbon in soil aggregates?

Materials and methods

Different amounts of biochar (up to 90 t ha^?1) were applied to a calcareous soil in a field experiment in 2009 along with the application of chemical fertilizer annually and the returning of winter wheat and summer maize straws. After 3 years, 0–20-cm soil samples were taken to measure the size distribution of soil water-stable aggregates by wet sieving, the concentrations of TOC and TN in whole aggregates and light or heavy fractions by elemental analysis equipment, and composition of TOC by Fourier transform infrared (FTIR) and pyrolysis-gas chromatography/mass spectrometer (Py–GC/MS).

Results and discussion

(1) The 3 years of biochar application had no significant effects on degree of soil aggregation but reduced the breakage of large soil aggregates (>1000 μm); (2) biochar significantly increased the contents of TOC and TN in soil macro-aggregates (>250 μm), as well as their ratios to total soil amount. Biochar also significantly increased the contents of TOC and TN in light fractions as well as the C/N ratio, which made the soil organic matter more active. The biochar dosage showed a significant positive correlation with organic carbon, total nitrogen, and C/N ratio in light fraction components of aggregates (>250 μm). Biochar mainly affected the organic matter in the heavy fraction components of macro-aggregates; (3) from the Py–GC/MS results, biochar increased the CO₂ content originated from active organic carbon.

Conclusions

Long-term application of biochar improved the stability of soil aggregates, increased the contents of TOC and TN as well as organic carbon and total nitrogen in macro-aggregates, and usually increased the contents of CO₂ originated from active organic carbon in light fractions. The findings were helpful in evaluating the effects of biochar on soil aggregation and organic matter stability.

Purpose

Both overharvesting and climate changes have greatly altered forest composition in northeastern China; however, forest-specific effects on soil organic carbon (SOC), N, and compositional features in different soil fractions have not yet been defined.

Materials and methods

By sampling from broad-leaved Korean pine forest (the climax vegetation) and aspen–birch forest (the secondary forest), five soil fractions were separated by a physicochemical soil fractionation method, and Fourier transform infrared spectroscopy, X-ray diffraction analysis, and X-ray photoelectron spectrometry were used for functional groups, mineral diffraction, and elemental composition determination together with SOC and N measurements.

Results and discussion

Aspen–birch forests tended to sequestrate more SOC in the slow fractions (sand and aggregate [SA] and easily oxidized fractions) and more N in the sensitive fractions (particulate and soluble fractions), indicating that in aspen–birch forests, high SOC sequestration (1.26-fold) coincided with the active and rapid N supply. Much higher percentages (13.1–40.5 %) of O–H and N–H stretching, O–H bending, and C=O, COO–, and C–H stretching, and also the much lower quartz grain size and mineral diffraction peaks in SA and acid-insoluble fraction (over 85 % of total soil mass), in aspen–birch forests were possibly associated with the 1.17- to 1.53-fold higher SOC compared to broad-leaved Korean pine forest. However, elemental composition on soil particles might marginally contribute to the SOC and N forest-dependent differences.

Conclusions

Considering the increase of aspen–birch forests and the decrease of broad-leaved Korean pine forests in historical and future scenarios in northeastern China, more SOC has been and also will sequestrate in intact soils and stable soil fractions, with more N in sensitive fractions, and these should be highlighted in evaluating forest C and N dynamics during forest successions in this region.

Purpose

Plantation is an important strategy for forest restoration and carbon (C) storage. Plantations with different tree species could significantly affect soil properties, including soil pH, soil nutrient content, soil microbial activities, and soil dissolved organic C. Changes in these abiotic and biotic factors could regulate mineralization of soil organic C (SOC). However, it remains unclear to what extent these factors affect the mineralization of SOC under different tree species plantations.

Materials and methods

Soil was collected at 0–10 cm depth from plantations with Pinus elliottii Engelm. var. elliottii, Araucaria cunninghamii, and Agathis australis, respectively, in southeast Queensland, Australia. Soil samples were assayed for soil organic C; organic N and mineralization of SOC; soil particle size; total C, N, and P; and pH. In addition, a 42-day laboratory incubation with substrate additions was done to examine the influence of different substrates and their combinations on bio-available organic C.

Results and discussion

Our results suggested that SOC mineralization was mainly determined by soil pH and soil C content among plantations with different tree species, whereas SOC mineralization was not correlated with soil N and P contents. These results were further confirmed by the substrate addition experiments. SOC mineralization of soils from slash pine showed greater response to C (glucose) addition than soils from other two plantations, which suggested significant differences in SOC mineralization among plantations with different tree species. However, neither N addition nor P addition had significant effects on SOC mineralization.

Conclusions

Our results indicated that plantations with different tree species substantially affect the mineralization and stability of soil organic C pool mainly by soil pH and soil C content.