Humic acid composition and humification processes in wetland soils of a Mediterranean semiarid wetland

Purpose

The present study focuses on a compositional characterization of the humic acid (HA) fraction of several wetland soils using solid-state ¹³C NMR spectroscopy. The data were analysed using the molecular mixing model (MMM), based on an empirical approach by Nelson and Baldock. The compositional data from HAs obtained with this model were used to obtain a wider assessment of the process of humification from comparison of total soil wetland organic matter composition and HA composition.

Materials and methods

Twenty samples of humic acids (HAs) isolated from a Mediterranean semiarid wetland (‘Tablas de Daimiel’, central Spain) were studied using elemental analysis and cross polarization magic angle spinning (CPMAS)¹³C nuclear magnetic resonance (NMR) spectroscopy. The NMR data were analysed with the molecular mixing model (MMM) considering up to six generic components (carbohydrate, protein, lignin, lipid, char and ‘carbonyl’). HAs are considered a conceptual mixture of these model components, and the MMM determines the proportions of the characteristic biomolecules contributing to HA composition.

Results and discussion

The composition of the HAs under study depends on local factors such as site vegetation and occurrence of fire. Correlations between the proportions of the six generic components and further comparison with those determined for the unfractionated OM (whole sample, WS), gave information on HA origin and humification mechanisms. In particular, the proportions of char and carbohydrate (R ² 0.637) and contents of lignin and protein (R ² 0.471) in the HAs were negatively correlated (P < 0.05). Significant correlations (R ² 0.439) also existed for char contents in whole sample (WS) compared to HA, and for carbohydrates in WS compared to HA (R ² 0.558). Char proportion grew in HA with respect to the WS, and carbohydrates dropped to a half on average in HA compared to WS.

Conclusions

Two different humification mechanisms could be identified for no-fire and fire areas. In the former, HA-char was preserved selectively from char in the sample, whereas in the latter, char was newly formed by fire effect.

     

Changes in forest soil organic matter quality affected by windstorm and wildfire

Purpose

Windthrows and fires are major natural disturbances in forest ecosystems, which can affect organic matter in the surface and the mineral layer of forest soil. The main aim of this study was to evaluate the changes occurring in the structure and properties of humic acid (HA) in the lands where windthrows and wildfires occurred.

Materials and methods

In November 2004, the forest in the area of 12,000 ha in the Tatra National Park, Slovakia, was seriously damaged by northern wind gusts exceeding 200 km/h. In July 2005, a wildfire broke out in a 220 ha of wind-damaged area. The HAs have been isolated from four research plots: (a) the area where the fallen trees were removed (EXT); (b) an area after windstorm covered by wood from struck trees (NEX), left for spontaneous succession; (c) an area after extracted timber, damaged by the surface wildfire (FIR); and (d) a reference intact spruce forest area (REF). Changes in the chemical structure of the HAs isolated from the research plots were determined on the basis of elemental analysis and UV-Vis, EPR, IR, and ¹³C NMR spectroscopy.

Results and discussion

All used analytical methods showed a decrease in the humification degree of the humic acids extracted from the soils where the spruce forest has been affected by a wildfire and a windthrow. In the case of the control sample HA (REF), the calculated atomic H/C and O/C ratios and the degree of aromaticity (α) calculated from the ¹³C NMR spectra were higher, indicating higher aromaticity of HA from the REF area. The more complex and developed structure of REF HA was confirmed by the higher value of E^1%₆ and the lower E4/E6 ratio obtained using UV-Vis spectroscopy. Also, the higher g-parameters determined from the EPR spectra of the stable radicals present in HA confirmed the lower aromaticity on the plots that have been subjected to the calamities. The ¹³C NMR spectra and the elemental analysis show that the structure of the HA extracted from the NEX plot is the closest to the REF.

Conclusions

The results of the systematic research showed significant changes in the structure of HA taken from spruce forest soils that were subjected to windstorms and fires. An enrichment of the HAs in aliphatic carbon and so a lower humification degree of the organic matter in the areas calamity-affected were observed. The results clearly indicate that the HAs extracted from the disturbed plots of the spruce forest are not as stable as those extracted taken from the control plot.

     

Effects of biochar and polyacrylamide on decomposition of soil organic matter and <Superscript>14</Superscript>C-labeled alfalfa residues

Purpose

Various soil conditioners, such as biochar (BC) and anionic polyacrylamide (PAM), improve soil fertility and susceptibility to erosion, and may alter microbial accessibility and decomposition of soil organic matter (SOM) and plant residues. To date, no attempts have been made to study the effects of BC in combination with PAM on the decomposition of soil SOM and plant residues. The objective of this study was to evaluate the effects of BC, PAM, and their combination on the decomposition of SOM and alfalfa residues.

Materials and methods

An 80-day incubation experiment was carried out to investigate the effects of oak wood biochar (BC; 10 Mg ha^?1), PAM (80 kg ha^?1), and their combination (BC?+?PAM) on decomposition of SOM and ¹⁴C-labeled alfalfa (Medicago sativa L.) residues by measuring CO₂ efflux, microbial biomass, and specific respiration activity.

Results and discussion

No conditioner exerted a significant effect on SOM decomposition over the 80 days of incubation. PAM increased cumulative CO₂ efflux at 55–80 days of incubation on average of 6.7 % compared to the soil with plant residue. This was confirmed by the increased MBN and MB¹⁴C at 80 days of incubation in PAM-treated soil with plant residue compared to the control. In contrast, BC and BC?+?PAM decreased plant residue decomposition compared to that in PAM-treated soil and the respective control soil during the 80 days. BC and BC?+?PAM decreased MBC in soil at 2 days of incubation indicated that BC suppressed soil microorganisms and, therefore, decreased the decomposition of plant residue.

Conclusions

The addition of oak wood BC alone or in combination with PAM to soil decreased the decomposition of plant residue.

     

A new 3D conceptual structure modeling of biochars by molecular mechanic and molecular dynamic simulation

Purpose

The combination of analytical chemistry and simulation methods provides more complete information about biochars.

Materials and methods

The biochars prepared by pyrolysis of the crop straw at 300 and 500 °C were investigated by elemental analysis, pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) and solid-state ¹³C nuclear magnetic resonance (NMR) spectrometry to build the 2D structural models of biochars. The most stable and balanced 3D conformations were gained by optimizing in the optimized potential for liquid simulation (OPLS) force field of molecular mechanic and molecular dynamic simulation of HyperChem® software.

Results and discussion

There were few O-containing and N-containing compounds in biochars. According to the results of Py-GC-MS, 41 and 28 pyrolysis products were identified for the building of the conceptual models of 300 and 500 °C biochars. Solid-state ¹³C NMR data also demonstrated that there were very high values of unsaturated C in biochars. The agreement between elemental concentration and chemical functional groups of two simulated models and experimental biochars was successfully achieved. Quantitative structure activity relationship (QSAR) properties were calculated and indicated the correlation of molecular structures with properties such as surface area, volume, polarizability, refractivity, and hydration energy.

Conclusions

The conceptual structural models of corn straw biochars produced at 300 and 500 °C were C₇₈H₆₈N₂O₂₅ and C₅₉H₂₉NO₁₀, respectively. The simulation results showed that the 3D structure of the 300 °C biochar with ?4 charges and the nonprotonated 3D structure of the 500 °C biochar were the most stable. Deprotonation reaction is an endothermic process.

     

Changes in humic fraction characteristics and humus-enzyme complexes formation in semiarid degraded soils restored with fresh and composted urban wastes. A 5-year field experiment

Purpose

The aim of this study was to evaluate in the medium term (5 years) the effect of two organic amendments, which were spiked to a degraded soil as a strategy for bioremediation, on the amount and characteristics of soil humic acids (HAs) and their ability to associate with certain extracellular enzymes.

Materials and methods

Soil samples were collected in an experimental field where 5 years earlier, a mixture of the organic fraction of household waste and sewage sludge (2:1 ratio), both composted (composted residue, CR) and non-composted (fresh residue, FR), had been added in triplicate at rates equivalent to 1 % (D1) and 3 % of organic carbon (D2) to 30-m² plots as a strategy for degraded soil restoration. Humic substances (HSs) and HAs were extracted from the collected soil samples and submitted to chemical, biochemical, spectroscopic (FTIR), and chemical-structural (CPMAS ¹³C NMR) analyses.

Results and discussion

After 5 years, the amended soils showed significantly higher HS and HA content than did the control soil, and the differences with respect to the control were greater with compost addition than with FR addition. The HA from the amended soils had higher H, N, and S contents than the HA from the non-amended soil in addition to a lower oxygen content and lower O/C ratio values. Furthermore, the FTIR spectra of the HA from the amended soils showed a higher absorption intensity in bands corresponding to aliphatic and amide-carboxylic groups and polysaccharide structures and a lower absorption intensity in bands corresponding to carbonyls and carboxylic groups than the HA from the control. These results were confirmed by ¹³C-NMR spectra, which showed a clear increase of aliphatic compounds in the HA from the amended soils with respect to the HA from the control. HA spectra were not greatly influenced by the maturity of the amendment or by the application dose.

Conclusions

In general, the addition of organic amendments increased the quantity of enzymes immobilized in the humic colloid. Furthermore, the addition of the composted residues favored to a greater extent the immobilization of the abovementioned enzymes, which represent a biological reservoir in the soil. This is of great importance since these enzymes possess functional capacity even when the soils are under conditions that are stressful or unfavorable for microbial life. An increase in the quantity of immobilized enzymes such as that observed in amended soils supposes an important improvement in soil quality.

     

Evaluation of different fractions of the organic matter of peat on tetracycline retention in environmental conditions: in vitro studies

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.

     

Interactive effects of biochar and polyacrylamide on decomposition of maize rhizodeposits: implications from <Superscript>14</Superscript>C labeling and microbial metabolic quotient

Purpose

The applications of biochar (BC) and polyacrylamide (PAM) may have interactive effects on carbon (C) dynamics and sequestration for improving the soil quality and achieving sustainable agriculture. Relative to BC and PAM, rhizodeposits act as C and energy source for microorganisms and may change the mineralization dynamics of soil organic matter (SOM). No attempt has been made to assess the effects of BC, anionic PAM, or their combination on the decomposition of different aged ¹⁴C-labeled rhizodeposits. The objective of this study was to investigate the effects of the treatments mentioned above on the decomposition of different aged ¹⁴C-labeled maize rhizodeposits.

Materials and methods

biochar (BC) at 10 Mg ha^?1 or anionic PAM at 80 kg ha^?1 or their combination (BC + PAM) was applied to soils with/without 2-, 4-, 8-, and 16-day-aged ¹⁴C-labeled maize rhizodeposits. After that, the soil was incubated at 22 °C for 46 days.

Results and discussion

After 2 days of incubation, the total CO₂ efflux rates from the soil with rhizodeposits were 1.4–1.8 times higher than those from the soil without rhizodeposits. The cumulative ¹⁴CO₂ efflux (32 % of the ¹⁴C input) was maximal for the soil containing 2-day-aged ¹⁴C-labeled rhizodeposits. Consequently, 2-day-aged rhizodeposits were more easily and rapidly decomposed than the older rhizodeposits. However, no differences in the total respired ¹⁴CO₂ from rhizodeposits were observed at the end of the incubation. Incorporation of ¹⁴C into microbial biomass and 66–85 % of the ¹⁴C input remained in the soil after 46 days indicated that neither the age of ¹⁴C-labeled rhizodeposits nor BC, PAM, or BC + PAM changed microbial utilization of rhizodeposits.

Conclusions

Applying BC or BC + PAM to soil exerted only minor effects on the decomposition of rhizodeposits. The contribution of rhizodeposits to CO₂ efflux from soil and MBC depends on their age as young rhizodeposits contain more labile C, which is easily available for microbial uptake and utilization.

     

Variability of the quality and quantity of organic matter in soil affected by multiple wildfires

Purpose

Fire in mountainous areas can lead to increased variability of their soil organic matter (SOM) due to spatial inhomogeneity and pre-fire fuel distribution. Here, we elucidated if this was the case in our study area and how this affected the reliability of solid-state ¹³C NMR spectroscopy applied for the study of the medium-term impact of fire on SOM

Materials and methods

The study occurred in the Sierra de Aznalcóllar, Southern Spain, which experienced their last intense fire 7 years before sampling. In a first approach (method 1), the corners and the center of a randomly chosen square with a side length of 15 m were sampled and analyzed separately. For comparison, composite samples (method 2) were obtained from three soils. We characterized material from unburnt, burnt, and double burnt regions. Data describing the physical and chemical properties of the soils together with the NMR spectroscopic characterization were analyzed using ANOVA.

Results and discussion

Both sampling methods yielded comparable results with comparable standard errors. No major differences between the fire-affected and unburnt soils were observed with respect to physical and chemical properties and C and N contents, but solid-state ¹³C NMR spectroscopy indicated a small but significant elevation of aromaticity in the soils with fire history.

Conclusions

The analysis showed that sampling with reduced replicates (method 1) can still lead to representative NMR data. The more complex sampling of comparing three composite samples (method 2) did not decrease the standard error. Our results also indicate that in the study area typical properties of the soil and its SOM induced by former burnings will not persist beyond a few decades.

     

Changes in CH<Subscript>4</Subscript> production during different stages of litter decomposition under inundation and N addition

Purpose

Our aim was to examine linkages between mass loss, chemical transformation and CH₄ production during decomposition of leaf litters submerged under water. We hypothesised that (i) labile leaf litters would fuel a rapid, high rate of methane (CH₄) production and that recalcitrant litters would fuel long-lasting but lower emissions, (ii) leaf litters experiencing a greater alteration to chemical properties would stimulate increased CH₄ production and (iii) nitrogen (N) addition would increase CH₄ emissions.

Materials and methods

Litters from six plant species were collected from a riparian ecosystem adjacent to Wyaralong Dam, located in Queensland, Australia, i.e., Lophostemon confertus, Cynodon dactylon, Heteropogon contortus, Chamaecrista rotundifolia, Chrysocephalum apiculatum and Imperata cylindrica. We evaluated the rate of mass loss and CH₄ emissions for 122 days of incubation in inundated microcosms with and without N addition. We quantified the chemical changes in the decomposing litters with ¹³ C-cross polarization and magic angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectrum.

Results and discussion

The inundation treatment of plant litters significantly affected decomposition rates. All litters decomposed in either inundated or aerobic microcosms were quite distinct with regard to the NMR spectra of their initial litters. N addition altered the NMR spectra under both inundation and aerobic conditions. The N treatment only marginally influenced the decomposition rates of I. cylindrica and C. apiculatum litters. The diurnal patterns of CH₄ production in the H. contortus, C. rotundifolia and C. apiculatum litters under inundation incubation could be expressed as one-humped curves, with the peak value dependent on litter species and N treatment. N addition stimulated CH₄ emission by C. rotundifolia and C. apiculatum litters and inhibited CH₄ emission from microcosms containing the litters of the three gramineous species, i.e., I. cylindrica, C. dactylon and H. contortus.

Conclusions

Our results provide evidence that labile leaf litters could fuel a rapid, high rate of CH₄ production and that recalcitrant litters fuelled a lower CH₄ emission. We did not find that leaf litters with altered chemical properties stimulated increased CH₄ production. We also found that N addition was able to increase CH₄ emissions, but this effect was dependent on the litter species.

     

Soil nitrogen availability alters rhizodeposition carbon flux into the soil microbial community

Purpose

Soil microorganisms are important in the cycling of plant nutrients. Soil microbial biomass, community structure, and activity are mainly affected by carbon substrate and nutrient availability. The objective was to test if both the overall soil microbial community structure and the community-utilizing plant-derived carbon entering the soil as rhizodeposition were affected by soil carbon (C) and nitrogen (N) availability.

Materials and methods

A ¹³C-CO₂ steady-state labeling experiment was conducted in a ryegrass system. Four soil treatments were established: control, amendment with carboxymethyl cellulose (CMC), amendment with ammonium nitrate (NF), combined CMC and NF. Soil phospholipid fatty acid (PLFA) and ¹³C labeling PLFA were extracted and detected by isotope ratio mass spectrometer.

Results and discussion

The combined CMC and NF treatment with appropriate C/N ratio (20) significantly enhanced soil microbial biomass C and N, but resulted in lower soil inorganic N concentrations. There was no significant difference in soil PLFA profile pattern between different treatments. In contrast, most of the ¹³C was distributed into PLFAs 18:2ω6,9c, 18:1ω7c, and 18:1ω9c, indicative of fungi and gram-negative bacteria. The inorganic-only treatment was distinct in ¹³C PLFA pattern from the other treatments in the first period of labeling. Factor loadings of individual PLFAs confirmed that gram-positive bacteria had relatively greater plant-derived C contents in the inorganic-only treatment, but fungi were more enriched in the other treatments.

Conclusions

Amendments with CMC can improve N transformation processes, and the ryegrass rhizodeposition carbon flux into the soil microbial community is strongly modified by soil N availability.

     

Soil pyrogenic organic matter characterisation by spectroscopic analysis: a study on combustion and pyrolysis residues

Purpose

Pyrogenic organic matter (PyOM) in the soil of a maritime pine forest in Central Italy, formed during a fire of high severity, was characterised by Fourier transform infrared (FT-IR) and ¹³C nuclear magnetic resonance (NMR). Furthermore, soil samples from burnt and unburnt sampling sites and natural charcoal collected from the ground were characterised after progressive heating under air and nitrogen atmosphere. The aim was to better understand the role fire plays on PyOM formation and oxidation.

Materials and methods

The top 10 cm of mineral soil and the above-lying charcoal particles were collected soon after the fire. Sampling was also performed on an adjacent unburnt portion of the forest. The bulk soil organic matter (SOM), its extractable fraction and charcoal particles were investigated by FT-IR and ¹³C NMR spectroscopies. They also underwent thermogravimetric analysis under air or N₂, stopping the thermal reactions at the end of the first exothermic reaction in the range 350–500 °C.

Results and discussion

The NMR investigation clearly revealed a significant enrichment in aromatic and alkyl C in the burnt soil compared to the unburnt one. Several clues led to hypothesise that SOM was not exposed to extreme heating during the fire, notwithstanding the high fire severity estimated by a vegetation-based visual scale. In the thermal treatment mimicking fire, charcoal lost much of its mass and carbon content. However, at 500 °C, it still maintained a significant recalcitrant fraction. Nitrogen concentration in the bulk soil increased after heating, particularly under air condition. This phenomenon could be due to the formation of heterocyclic nitrogen compounds in the charred material.

Conclusions

In the study area, PyOM is rich in aliphatic compounds presumably because of the understory sclerophyllous vegetation typically found in Mediterranean environments. A large fraction of the charcoal released to the soil during the fire is sensitive to oxidation by subsequent fires. On the other hand, charcoal preserves a significant fraction of C, the most recalcitrant one, with expected long residence time in soil. PyOM formed under high oxygen availability is richer in N than that formed in inert atmosphere, which might make PyOM more susceptible to biochemical degradation.

     

Increases in soil CO<Subscript>2</Subscript> and N<Subscript>2</Subscript>O emissions with warming depend on plant species in restored alpine meadows of Wugong Mountain,China

Purpose

Ecosystem restorations can impact carbon dioxide (CO₂) and nitrous oxide (N₂O) emissions which are important greenhouse gasses. Alpine meadows are degraded worldwide, but restorations are increasing. Because their soils represent large carbon (C) and nitrogen (N) pools, they may produce significant amounts of CO₂ and N₂O depending on the plant species used in restorations. In addition, warming and N deposition may impact soil CO₂ and N₂O emissions from restored meadows.

Materials and methods

We collected soils from degraded meadows and plots restored using three different plant species at Wugong Mountain (Jiangxi, China). We measured CO₂ and N₂O emissions when soils were incubated at different temperatures (15, 25 or 35 °C) and levels of N addition (control vs. 4 g m^?2) to understand their responses to warming and N deposition.

Results and discussion

Dissolved organic C was higher in restored plots (especially with Fimbristylis dichotoma) compared to non-restored bare soils, and their soil inorganic N was lower. CO₂ emission rates were increased by vegetation restorations, decreased by N deposition, and increased by warming. CO₂ emission rates were similar for the three grass species at 15 and 25 °C, but they were lower with Miscanthus floridulus at 35 °C. Soils from F. dichotoma and Carex chinensis plots had higher N₂O emissions than degraded or M. floridulus plots, especially at 25 °C.

Conclusions

These results show that the effects of restorations on soil greenhouse gas emissions depended on plant species. In addition, these differences varied with temperature suggesting that future climate should be considered when choosing plant species in restorations to predict soil CO₂ and N₂O emissions and global warming potential.

     

Nitrogen transformation rates and N<Subscript>2</Subscript>O producing pathways in two pasture soils

Purpose

Better understanding of N transformations and the regulation of N₂O-related N transformation processes in pasture soil contributes significantly to N fertilizer management and development of targeted mitigation strategies.

Materials and methods

¹⁵N tracer technique combined with acetylene (C₂H₂) method was used to measure gross N transformation rates and to distinguish pathways of N₂O production in two Australian pasture soils. The soils were collected from Glenormiston (GN) and Terang (TR), Victoria, Australia, and incubated at a soil moisture content of 60% water-filled pore space (WFPS) and at temperature of 20 °C.

Results and discussion

Two tested pasture soils were characterized by high mineralization and immobilization turnover. The average gross N nitrification rate (n_tot) was 7.28 mg N kg^?1 day^?1 in TR soil () and 5.79 mg N kg^?1 day^?1 in GN soil. Heterotrophic nitrification rates (n_h), which accounting for 50.8 and 41.9% of n_tot, and 23.4 and 30.1% of N₂O emissions in GN and TR soils, respectively, played a role similar with autotrophic nitrification in total nitrification and N₂O emission. Denitrification rates in two pasture soils were as low as 0.003–0.004 mg N kg^?1 day^?1 under selected conditions but contributed more than 30% of N₂O emissions.

Conclusions

Results demonstrated that two tested pasture soils were characterized by fast N transformation rates of mineralization, immobilization, and nitrification. Heterotrophic nitrification could be an important NO₃^?–N production transformation process in studied pasture soils. Except for autotrophic nitrification, roles of heterotrophic nitrification and denitrification in N₂O emission in two pasture soils should be considered when developing mitigation strategies.

     

Biotite weathering by <Emphasis Type="Italic">Aspergillus niger</Emphasis> and its potential utilisation

Purpose

Biotite, as a type of associated mineral, is normally applied as a filling material for buildings, or is discarded as tailings. However, as a potassium-bearing phyllosilicate mineral, biotite can be easily weathered by fungi, which leads to its internal potassium being released for agricultural production (1), and the mineral residues being weathered by the fungus may be applied for adsorption of heavy metal ions (2).

Materials and methods

This work investigates the weathering of biotite by Aspergillus niger through the analysis of the differences in ion dissolution from biotite, producing of organic acids, the change of mineral morphology and composition by inductively coupled plasma optical emission spectrometry (ICP-OES), high-performance liquid chromatography (HPLC), scanning electron microscopy (SEM), and X-ray diffraction (XRD). Besides, the mineral residues were applied for adsorption of heavy metal ions.

Results and discussion

Results showed that the mycelia envelope the mineral and form fungal–mineral aggregates. The fungus can secrete a variety of organic acids including citric acid and oxalic acid; these attacked the surface and cleavage of biotite to release ions (Al³⁺, Fe³⁺, Mg²⁺, and K⁺). During incubation with A. niger, biotite weathered as shown by the relative decrease in biotite content and increase in interlayer spacing. Moreover, a certain concentration of phytic acid and tween-80 could promote the release of K⁺, and the fermentation liquid of rice bran has the same effect. Biotite residues showed a good adsorption for Cd²⁺, Pb²⁺, Zn²⁺, and Cu²⁺.

Conclusions

The results indicate that biotite can be biotransformed and release K⁺, of which the production can be acted as heavy metal ion adsorbent. It provides a reference for application of biotite in agriculture and control of heavy metal ion pollution in soil.

     

Quantification of vertical solid matter transfers in soils during pedogenesis by a multi-tracer approach

Purpose

Vertical transfer of solid matter in soils (bioturbation and translocation) is responsible for changes in soil properties over time through the redistribution of most of the soil constituents with depth. Such transfers are, however, still poorly quantified.

Materials and methods

In this study, we examine matter transfer in four eutric Luvisols through an isotopic approach based on ¹³⁷Cs, ²¹⁰Pb_(xs), and meteoric ¹⁰Be. These isotopes differ with respect to chemical behavior, input histories, and half-lives, which allows us to explore a large time range. Their vertical distributions were modeled by a diffusion-advection equation with depth-dependent parameters. We estimated a set of advection and diffusion coefficients able to simulate all isotope depth distributions and validated the resulting model by comparing the depth distribution of organic carbon (including ^12/13C and ¹⁴C isotopes) and of the 0–2-μm particles with the data.

Results and discussion

We showed that (i) the model satisfactorily reproduces the organic carbon, ¹³C, and ¹⁴C depth distributions, indicating that organic carbon content and age can be explained by transport without invoking depth-dependent decay rates; (ii) translocation partly explains the 0–2-μm particle accumulation in the Bt horizon; and (iii) estimates of diffusion coefficients that quantify the soil mixing rate by bioturbation are significantly higher for the studied plots than those obtained by ecological studies.

Conclusions

This study presents a model capable of satisfactorily reproducing the isotopic profiles of several tracers and simulating the distribution of organic carbon and the translocation of 0–2-μm particles.

     

Enhanced <Emphasis Type="Italic">Scirpus triqueter</Emphasis> phytoremediation of pyrene and lead co-contaminated soil with alkyl polyglucoside and nitrilotriacetic acid combined application

Purpose

This study aimed to evaluate the effect of combination of alkyl polyglucoside (APG) and nitrilotriacetic acid (NTA) on improving the efficiency of phytoremediation for pyrene and lead (Pb) co-contaminated soil by Scirpus triqueter.

Materials and methods

Seedlings of S. triqueter with a similar size and biomass (3 g/pot) were grown on 2-month aged soil contaminated with 184.5 mg kg^?1of pyrene and 454.3 mg kg^?1 of Pb at pH?=?8.3. After growth for 10 days, different doses of APG and NTA were added into the soil. After 60 days, the height of plants, Pb concentrations in plants, and pyrene amounts in soil were determined.

Results and discussion

Combined application of NTA and APG with lower dosage (1 + 1 g kg^?1 soil and 1 + 2 g kg^?1 soil) had no notable negative influence on the growth of S. triqueter. Moreover, significant synergy on Pb accumulation in S. triqueter was achieved with APG and NTA combined application. Besides, the dissipation of pyrene from soil after 60-day planting was increased in APG and NTA treatments when compared with the control treatments. Application of APG alone or combined with NTA had greater effect on enhancing dissipation of pyrene from soil than NTA alone.

Conclusions

This study demonstrated that the remediation of Pb and pyrene co-contaminated soil by S. triqueter can be enhanced by combined application of APG and NTA. Long-term evaluation of this strategy is needed in co-contaminated field sites.

     

Soil porosity in physically separated fractions and its role in SOC protection

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.

     

Sorption of copper(II) from synthetic oil sands process-affected water (OSPW) by pine sawdust biochars: effects of pyrolysis temperature and steam activation

Purpose

Remediate metal contamination is a fundamental step prior to reclaim oil sands tailing ponds, and copper (Cu(II)) is the most abundant metal in the tailings water or oil sands process-affected water (OSPW). Biochars produced at four pyrolysis conditions were evaluated for sorption of Cu(II) in synthetic OSPW to explore different biochar potentials in removing Cu(II) from the contaminated water.

Materials and methods

Pine sawdust biochars pyrolyzed at 300 and 550 °C with and without steam activation were investigated by batch sorption experiments. Isotherm and kinetic studies were conducted to compare the sorption capacities of the four biochars and to examine potential mechanisms involved.

Results and discussion

For all the biochars, Langmuir and pseudo-second order models were the best-fit for isotherm and kinetic studies, respectively. According to the Langmuir parameters, the maximum adsorption capacities of the biochars produced at 550 °C were around 2.5 mg Cu(II)?g^?1, which were 30-folds higher than those produced at 300 °C. However, steam activation did not cause any significant difference in the biochars’ sorption performance. The kinetic study suggested that chemisorption involving valence forces was the limiting factor of the sorption. In addition, ion exchange and precipitation were likely the primary mechanisms for Cu(II) sorption which outweigh complexation with functional groups on the biochars’ surface.

Conclusions

Pine sawdust biochar produced at 550 °C without steam activation could be utilized as a sustainable and cost-effective material to remove Cu(II) from the OSPW.

     

Structure-function relationship of vermicompost humic fractions for use in agriculture

Purpose

The use of humic substances (HS) in agriculture is beneficial and has positive environmental impacts. However, to optimize the use of HS possible links between their structural characteristics and bioactivity must be shown. The goal of this study is to evaluate the bioactivity of different humic fractions extracted from vermicompost (VC) in rice plants and to shed light to possible structure-function relationships.

Materials and methods

Humic-like fractions were obtained from cattle manure vermicompost processed by African nightcrawlers (Eudrilus eugeniae spp.). Humic-like acid fraction using only water as extractor (HLAw), HLA fraction extracted following the International Humic Substances Society (IHSS) recommended method, and the solid residue (humified residual (HR)) after extraction of HLA were characterized using complementary chemical, physic, and spectroscopic technics (elemental composition, UV-Vis and Fourier transform infrared spectroscopy (FTIR) spectroscopies, ¹³C-CP MAS NMR, and MEV). Biological activity of the three HS was conducted in growth chambers and measured in roots using WinRhizo Arabidopsis software. Principal component analysis (PCA) was used to find a grouping pattern between the structural variables evaluated and the obtained root parameters.

Results and discussion

Differences were found in elemental composition among HS with larger C/N ratio in HR than in HLA and HLAw. HLA and HLAw FTIR spectra showed carboxyl band at 1714.66 cm^?1 better resolved than in HR. Bands at 1642 cm^?1 (amide I) and 1510 cm^?1 (lignin), were better resolved in HLA. ¹³C-NMR showed the following order of aromaticity: HLA > HLAw > HR. For HLAw bioactivity, the structures C_Alkyl-H,R, C_C=O, and C_COO-H,R correlated with the number and growth of smaller root. The aromatic C_Ar-H,R, C_Ar-O,N, and aliphatic C_Alkyl-O,N, C_Alkyl-O, and C_Alkyl-di-O structures in HLA, correlated with larger roots growth. HR also stimulated root growth and development in rice plants.

Conclusions

Aliphatic and oxygenated structures in HLAw showed a relation with induction of initial root emissions, whereas the presence of aromatic compounds in HLA was related with root growth stimulation activity. Higher concentration of HLAw was necessary to produce an equivalent stimulus compared with HLA; it could indicate that, although both fractions showed similar types of structures in their composition, differences in the predominant structures may be determining different effects on the root.

     

Contrasting response of two grassland soils to N addition and moisture levels: N<Subscript>2</Subscript>O emission and functional gene abundance

Purpose

Nitrification and denitrification processes dominate nitrous oxide (N₂O) emission in grassland ecosystems, but their relative contribution as well as the abiotic factors are still not well understood.

Materials and methods

Two grassland soils from Duolun in Inner Mongolia, China, and Canterbury in New Zealand were used to quantitatively compare N₂O production and the abundance of bacterial and archaeal amoA, denitrifying nirK and nirS genes in response to N additions (0 and 100 μg NH₄ ⁺–N g^?1 dry soil) and two soil moisture levels (40 and 80 % water holding capacity) using microcosms.

Results and discussion

Soil moisture rather than N availability significantly increased the nitrification rate in the Duolun soil but not in the Canterbury soil. Moreover, N addition promoted denitrification enzyme activities in the Canterbury soil but not in the Duolun soil. The abundance of bacterial and archaeal amoA genes significantly increased as soil moisture increased in the Duolun soil, whereas in the Canterbury soil, only the abundance of bacterial amoA gene increased. The increase in N₂O flux induced by N addition was significantly greater in the Duolun soil than in the Canterbury soil, suggesting that nitrification may have a dominant role in N₂O emission for the Duolun soil, while denitrification for the Canterbury soil.

Conclusions

Microbial processes controlling N₂O emission differed in grassland soils, thus providing important baseline data in terms of global change.