Selective effects of forest fires on the structural domains of soil humic acids as shown by dipolar dephasing <Superscript>13</Superscript>C NMR and graphical-statistical analysis of pyrolysis compounds

Purpose

Data management strategies of pyrolysis results and NMR acquisition modes were examined in humic acids (HAs) from control soils and fire-affected soils. The information supplied by dipolar dephasing (DD) ¹³C NMR spectroscopy and Curie-point pyrolysis were used to assess chemical structures hardly recognizable and measurable, or of unclear interpretation, when using ¹³C NMR under standard acquisition pulses (cross-polarization/magic angle spinning, CPMAS).

Materials and methods

The HAs were isolated from two forest soils under Pinus halepensis and Pinus sylvestris in control and burned sites affected by medium or severe-intensity wildfires. For NMR analyses, during DD acquisition conditions, a 180° ¹³C pulse was inserted to minimize phase shifts. Curie-Point pyrolysis was carried out at 510 °C for 5 s, and the pyrolysis fragments were analyzed by GC/MS. The total abundances of the major pyrolysis products were compared by an update of the classical Van Krevelen’s graphical-statistical approach, i.e., as surface density values in the space defined by the compound-specific H/C and O/C atomic ratios.

Results and discussion

The DD ¹³C NMR experiments displayed significant differences in the HA spectral profiles as regards to the standard CPMAS ¹³C NMR acquisition conditions, mainly in the chemical shift region of alkyl structures as well as for tannin- or carbohydrate-like O-alkyl structures. In fact, the comparison between DD and CPMAS solid-state NMR suggested shortening of alkyl chains and generation of carbohydrate-derived, unsaturated structures—viz. furans—which adds to the aromatic domain. Pyrolytic results showed fire-induced specific changes in HAs chemical structure and its molecular diversity. The changes were evident in the location and sizes of the different clusters of pyrolysis compounds defined by their atomic ratios.

Conclusions

The DD ¹³C NMR provided specific information on the fate of aliphatic structures and the origin of unsaturated HA structures, which could be helpful in differentiating “inherited” from “pyrogenic” aromatic structures. This is further confirmed by the analysis of the molecular assemblages of pyrolytic products, which showed accumulation of condensed polyaromatic domains in the HAs after the high-intensity fire, accompanied by a recalcitrant alkyl hydrocarbon domain. Medium-intensity fire led to aromaticity increase due to a selective accumulation of lignin-derived phenols concomitant to the depletion of aliphatic hydrocarbon constituents.

     

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.

     

The properties and functions of biochars in forest ecosystems

Purpose

The production of large quantities of biochar from natural fires has been a part of human history for millennia, causing CO₂ emissions to the atmosphere and exerting long-term effects on soil processes. Despite its potential importance and recent work reflecting the wide interest in biochar, a general review of our deep understanding of biochar functions within forest soils is currently lacking. Gaps in research knowledge in this field are identified in this paper.

Materials and methods

This paper summarizes recent research to provide a better understanding of the concentrations, distribution, and characteristics of biochar produced from forest wildfire and its influences on soil processes. Perspectives and recommendations for future research on biochar in post-fire forest soils are also discussed.

Results and discussion

The concentration, distribution, and characteristics of biochar produced from forest wildfire largely depend on forest landscapes, regional climates, and mostly its feedstock and fire history, like, its duration and severity. The influences of biochar on soil processes, particularly carbon and nitrogen transformations and cycling, like, nitrification and nitrous oxide emissions reduction (Clough and Condron, J Environ Qual 39:1218–1223, 2010), are also determined mainly by the fire temperature and raw materials. Mechanisms can be attributed to the adsorption of organic compounds and nutrients or changed microenvironment, termed as charsphere, by biochar. We also identify the microbial mechanisms involved in the biochar-containing soils.

     

Fluorescence properties of humic acid interaction products with s-triazine and bipyridilium herbicides and their Cu complexes: a multivariate approach

Purpose

Excitation–emission matrices spectroscopy (EEMS) of soil humic acids (HAs) contains large amount of information on their properties, as well as on the dynamics related to their intra- and inter-molecular interactions. The objective of this research was (i) to show that EEMS represents a useful tool to investigate the molecular and mechanistic aspects of HA adsorbing capacity towards atrazine (A) and paraquat (P) with or without Cu²⁺ ions and (ii) to evaluate if additional information on these mechanisms can be obtained by combining EEMS with principal component analysis (PCA).

Materials and methods

HAs have been isolated from soil samples collected in a citrus field at three locations, within the plant rows (HAa); between the rows (HAb) and in a adjacent, control soil (HAc). Interaction products were obtained between each HA and A and P, with or without Cu ions. Elemental analysis and Fourier-transorm Infrared Spectroscopy were applied to support fluorescence data. Fluorescence spectra were recorded on aqueous solutions, and fluorescence intensity (FI) values were normalized using a quinine sulphate standard. PCA analysis was performed using the software STATGRAPHICS Centurion XV.I.

Results and discussion

The EEM spectra of the three unreacted HAs are characterized by the presence of two fluorophores α and β, in the region of longer wavelengths pairs (EEWP). The EEM spectra of the HA-A interaction products are featured by the same two fluorophores, with FI values decreased of about 50%, whereas those of the HA-P interaction products show a unique peak, γ, at intermediate EEWP. Finally, the EEM spectra of HA-A-Cu²⁺ show, with respect to HA-A samples, a blue shift of the α peak with an additional decrease of FI values (about 60%) and the disappearance of the peaks β, whereas those of HA-P-Cu²⁺ feature, in comparison with those of HA-P, a small red-shift of the peak γ. PCA data suggest that Cu ions do not affect the interaction mechanism between HA and P, whereas it appears to exercise a strong influence on interaction between HA and A.

Conclusions

The results obtained indicate that EEMS allows direct measurements of the adsorbing capacity of HA towards atrazine and paraquat. Additional information obtained by PCA analysis show that Cu ions behave like a good antagonist in preventing the formation of ionic bonds between HA and atrazine, whereas Cu is not able to affect the prevalent mechanism of HA interaction with paraquat, which is a charge-transfer bond.

     

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.

     

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.

     

The role of silvicultural systems and forest types in preventing soil erosion processes in mountain forests: a methodological approach using cesium-137 measurements

Purpose

Forests play a key role in providing protection against soil erosion. Particularly, the role of vertical forest structure in increasing rainfall interception capacity is crucial for mitigating raindrop impact and reducing splash and rill erosion. For this reason, studies on the relationships between forest structures, the past management, and the observed rates of soil loss are needed. In the last few decades, importance was given to the use of cesium-137 (¹³⁷Cs) as radioactive tracer to estimate soil erosion rates. The ¹³⁷Cs technique is linked to the global fallout of bomb-derived radiocesium which occurred during a period extending from the mid 1950s to the late 1970s.

Materials and methods

The ¹³⁷Cs technique, providing long-term retrospective estimates, could be related to forest treatments applied during the last decades in different sites, also considering the tree species composition. This approach could be useful to compare the effect of different canopy cover and biomass on soil erosion rates related to different tree species. In the work proposed here, a study area dominated by pine and beech high forests located in the Aspromonte Mountains (Calabria, Italy) was selected. The measurements, related to forest structural traits, focusing on canopy cover and biomass, and also on management approaches and forest types, are compared with rates of soil erosion provided by ¹³⁷Cs.

Results and discussion

The overall results suggest that the minimum values of soil loss are documented in areas with higher canopy cover and biomass evidencing the protective effect provided by forests against soil erosion. Also, techniques based on the use of tracers like ¹³⁷Cs proved to be helpful to select the best forest management options useful to optimize the protective role of forests, with the aim to reduce erosion processes in a long-term perspective.

Conclusions

The experiment indicates that care must be taken when new silviculture treatments are planned. These findings are in agreement with what documented by other authors in similar environments but need further studies to confirm the effectiveness of using ¹³⁷Cs in different forest ecosystems.

     

Phosphate adsorption on uncoated and humic acid-coated iron oxides

Purpose

The phosphate adsorption on natural adsorbents is of particular importance in regulating the transport and bioavailability of phosphates in environmental system. In soils, oxides are often associated with organic matter and form mineral-organic complexes. The aim of the present paper was to investigate the mechanisms of phosphate adsorption on these complexes.

Materials and methods

Phosphate adsorption on uncoated and humic acid (HA)-coated iron oxide complexes was investigated at different ionic strengths and pH by isotherm experiments and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy.

Results and discussion

Results showed that HA-coated iron oxide complexes caused a decrease in the specific surface area (SSA) and the isoelectric point (IEP) of oxides. Phosphate adsorption on iron oxides was insensitive to changes of ionic strength, while it increased on the complexes with increasing ionic strength. The presence of HA decreased the maximum adsorption and the affinity of phosphate on the complexes. The zeta potential of phosphate-bound iron oxides linearly reduced with the increment of phosphate surface coverage, while the zeta potential of complexes with adsorbed phosphate kept at the same level. ATR-FTIR analysis suggested the formation of phosphate-metal complexation. The presence of HA promotes the formation of the monodentate phosphate complexes at pH 4.5 and significantly influenced phosphate species at pH 8.5.

Conclusions

The amount of phosphate adsorbed was reduced, and the phosphate speciation was also influenced when phosphate was adsorbed on HA-coated iron oxide complexes compared with phosphate adsorption on pure goethite and hematite.

     

Ecosystem carbon stock across a chronosequence of spruce plantations established on cutovers of a high-elevation region

Purpose

This study quantified the above- and belowground carbon (C) stocks across a chronosequence of spruce (Picea asperata) plantations established on cutovers and explored the turning point after which the increase in biomass C slowed or biomass C decreased for guiding forest management.

Materials and methods

We assessed above- and belowground plant biomass stocks at 11 sites in three regions, representing 12- to 46-year-old spruce plantations established on clear-cut areas in the eastern Tibetan Plateau, China. Biomass and C stocks of trees, understory vegetation, and forest floor litter were determined from plot-level inventories and destructive sampling. Fine root (<2 mm) biomass and mineral soil organic C (SOC) stock were estimated from soil cores. Tree biomass was quantified using allometric equations based on diameter at breast height (DBH) and height (H).

Results and discussion

Plant biomass C stocks in spruce plantations rapidly increased from 12 to 20 years at a rate of 7.8 Mg C ha^?1 year^?1, but decreased from 25 to 46 years at a rate of 0.79 Mg C ha^?1 year^?1. SOC stocks in spruce plantations gradually decreased from 12 to 46 years at a rate of 4.4 Mg C ha^?1 year^?1. Total C stock in the ecosystem remained unchanged for the first 20 years after the planting of spruce on cutovers, because the buildup of C stock in spruce biomass during the first 20 years was offset by the decrease in SOC. From 21 to 46 years after the reforestation, ecosystem C stock even decreased at a rate of 5.2 Mg C ha^?1 year^?1. The contribution of the understory vegetation, forest floor litter, and fine root to ecosystem C stock was low (<5.0 %) in the spruce plantations.

Conclusions

Ecosystem C stock in the spruce forest established on the cutover in the eastern Tibetan Plateau was related to stand age. During the first 20 years, this ecosystem was C neutral. However, aged (20–46 years) spruce plantation ecosystem can be a C source if no management was implemented to revitalize tree growth, promote understory vegetation, and enhance SOC accumulation.

     

Humic substances and aggregate stability in rhizospheric and non-rhizospheric soil

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.

     

Quantifying soil macropore networks in different forest communities using industrial computed tomography in a mountainous area of North China

Purpose

Macropores have important effects on the movement of soil water, air, and chemical substances. However, the quantitative relationship between complex 3D soil macropore networks and forest communities remains unclear in the northern mountainous area in China. The objectives of this study were to (1) use industrial computed tomography (CT) scanning and image analysis to quantitatively analyze macropore networks in intact soil columns and (2) identify characteristics of soil macropore networks in different forest communities.

Materials and methods

Intact soil columns (100-mm diameter, 300 mm long) were taken from six local forest communities with three replicates for a total of 18 samples. Industrial X-ray CT was used to scan soil samples; then, the scanned images were used to obtain the 3D images of rock fragments and macropore structures. Next, the macropore structure was quantified, including volume, diameter, surface area, length, angle, tortuosity, and number of macropores. This technique provided an accurate method to quantify the structure of macropores.

Results and discussion

The analysis and results revealed that different forest communities influence soil macropore 3D structure significantly and in different ways. Macropores in mixed Pinus tabulaeformis, Castanea mollissima, and Ulmus pumila forest had the largest diameter, surface area, network density, and length density of macropores as well as the smallest mean tortuosity of soil macropores. This is caused by the fact that mixed forest soils had more complex root systems, better soil structure, and more biotic activity. Within the soils of a single forest community, macropore porosity, network density, surface area density, and length of macropores decreased with increased soil depth, because more roots and more biological activity were present in the surface soil.

Conclusions

Advanced industrial CT technology can allow an accurate quantification of soil macropore structure. This is important because this type of structure has significant effects on soil water, air, and chemical transport. The results suggest that mixed forest is the best afforestation model in the northern mountainous area in China because of its ability to improve soil structure.

     

Nitrogen addition impacts on the emissions of greenhouse gases depending on the forest type: a case study in Changbai Mountain,Northeast China

Purpose

Anthropogenic-induced greenhouse gas (GHG) emission rates derived from the soil are influenced by long-term nitrogen (N) deposition and N fertilization. However, our understanding of the interplay between increased N load and GHG emissions among soil aggregates is incomplete.

Materials and methods

Here, we conducted an incubation experiment to explore the effects of soil aggregate size and N addition on GHG emissions. The soil aggregate samples (0–10 cm) were collected from two 6-year N addition experiment sites with different vegetation types (mixed Korean pine forest vs. broad-leaved forest) in Northeast China. Carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄) production were quantified from the soil samples in the laboratory using gas chromatography with 24-h intervals during the incubation (at 20 °C for 168 h with 80 % field water capacity).

Results and discussion

The results showed that the GHG emission/uptake rates were significantly higher in the micro-aggregates than in the macro-aggregates due to the higher concentration of soil bio-chemical properties (DOC, MBC, NO₃ ^?, NH₄ ⁺, SOC and TN) in smaller aggregates. For the N addition treatments, the emission/uptake rates of GHG decreased after N addition across aggregate sizes especially in mixed Korean pine forest where CO₂ emission was decreased about 30 %. Similar patterns in GHG emission/uptake rates expressed by per soil organic matter basis were observed in response to N addition treatments, indicating that N addition might decrease the decomposability of SOM in mixed Korean pine forest. The global warming potential (GWP) which was mainly contributed by CO₂ emission (>98 %) decreased in mixed Korean pine forest after N addition but no changes in broad-leaved forest.

Conclusions

These findings suggest that soil aggregate size is an important factor controlling GHG emissions through mediating the content of substrate resources in temperate forest ecosystems. The inhibitory effect of N addition on the GHG emission/uptake rates depends on the forest type.

     

Characterization and spatial distribution of particulate and soluble carbon and nitrogen from wildfire-impacted sediments

Purpose

The heavily forested Cache la Poudre (CLP) watershed in northern Colorado, USA, was impacted by the High Park wildfire in 2012. The wildfire burned land and vegetation immediately adjacent to the CLP River where blackened, ashy sediment samples were collected from five sites upstream of the City of Fort Collins drinking water intake to evaluate the spatial distribution and characteristics of burned sediments, along with quantifying and characterizing soluble compounds following a leaching experiment.

Materials and methods

At each site, samples were collected from three locations: (1) the edge of the bank adjacent to the water edge (downbank), (2) 1 m upslope of location 1 (midbank), and (3) 2 m upslope of location 1 (upperbank). All solid sediment samples were analyzed for elemental composition, and a subset of solid sediment samples were analyzed with ¹³C solid-state nuclear magnetic resonance spectroscopy. Sediments were mixed with the background CLP River water collected from upstream of the wildfire and allowed to leach for 6 and 24 h to determine the quantity and quality of water-soluble constituents. Filtered samples were analyzed for dissolved organic carbon (DOC), iron, manganese, and inorganic nutrient concentrations, by optical properties, and for disinfection byproduct (DBP) formation.

Results and discussion

Percent carbon and nitrogen content of the solid sediments were good predictors of leachate DOC concentration. The mean fluorescence index was higher for wildfire-impacted sediment leachates (1.50) compared to the background CLP River water (1.37), which may be due to changes in DOM molecular weight and oxidation of organic matter. All sediment leachates showed consistently higher haloacetonitrile and chloropicrin yields (DBP concentration/DOC concentration) compared to background CLP River water, whereas carbonaceous DBPs did not.

Conclusions

The collected sediments showed that burned material accumulated downstream near the river and was composed of inputs from burned soil and biomass along with the mobilization of unburned terrestrial material. The leachates of these sediments have different characteristics compared to the background CLP River water, indicating that DOM leached from sediments following a wildfire may increase aquatic DOC concentrations and N-DBP formation.

     

Impact of fire on soil gross nitrogen transformations in forest ecosystems

Purpose

Forests play a key role in the global carbon (C) and nitrogen (N) cycling. Fire is a global phenomenon occurring in many forest ecosystems, which has several environmental and ecological effects. The objective of this review was to improve our understanding of the effect of fire on soil gross N transformations in forest ecosystems.

Methods and results

We have reviewed the published studies using ¹⁵N pool dilution technique with analytical data analysis method to study the effect of fires on gross N transformations in forest ecosystems. Wildfires increased gross N mineralization rates in the short term and the effect disappeared from 3 years after the fire, while the effect of prescribed fires disappeared from 2 years after the burning. Both wildfires and prescribed fires reduced gross nitrification in the short term, while their effects varied from 6 months following the burning.

Conclusions

The different responses of gross N transformations to the fires in forest ecosystems depended on many factors including forest types, the intensity and frequency of fires, the time elapsed between the fires and sampling events, incubation conditions (field or laboratory incubation), climatic conditions and so on. In view of many factors influencing the effect of fires on gross N transformations, more comprehensive studies with physical, chemical, microbial and ecological characterization are needed to improve our knowledge about the effect of fires on soil gross N transformations and then N cycling in forest ecosystems.

     

A simple and economic method for simultaneous determination of 11 antibiotics in manure by solid-phase extraction and high-performance liquid chromatography

Purpose

A simple and highly efficient economic method for the analysis of 11 antibacterial drugs including two tetracyclines, three quinolones, four sulfonamides, chloramphenicol and tylosin, in livestock manure, was developed using solid-phase extraction (SPE) and high-performance liquid chromatography (HPLC).

Materials and methods

The analytes were successively extracted by EDTA-McIlvaine solution and organic solvent mixture. The extracts were degreased with n-hexane and cleaned through SPE on a hydrophile-lipophile balance (HLB) cartridge. All compounds were determined on a C18 reverse phase column with gradient elution.

Results and discussion

Recoveries calculated from spiked samples of animal manures ranged from 62.65 to 99.16 % for 11 antibiotics with relative standard deviations of less than 10.0 %. Limits of detection ranged from 0.1 to 1.9 μg kg^?1, and limits of quantification ranged from 0.3 to 5.9 μg kg^?1.

Conclusions

The results show that SPE-HPLC is an inexpensive and practical method for rapid detection of multiple antibiotics in animal manure.

     

Variations in forest aboveground biomass in Miyun Reservoir of Beijing over the past two decades

Purpose

In recent years, climate change, particularly rising carbon dioxide (CO₂) concentration and global warming, has attracted much attention around the world. Forest ecosystems still play a crucial role in global carbon (C) fixation. Quantifying forest aboveground biomass (AGB) and its temporal variations is essential for understanding external impacts (e.g., urbanization, environmental change) as well as assessing the potential of forest ecosystems assimilating the atmospheric CO₂.

Materials and methods

In this study, we established regression models for AGB estimation in the Miyun Reservoir region, based on relationships between Landsat-derived variables and ground truth AGB values, which were obtained from both plot measurements and estimations using Light Detection and Ranging (LiDAR) dataset. The models were applied to calibrated Landsat images acquired in 1990, 2000, and 2010 to track the forest AGB temporal variations and the corresponding spatial distributions for each period. The AGB estimations using LiDAR showed high consistency with values based on the plot measurements, while the established models presented an acceptable accuracy.

Results and discussion

The AGB density in the Miyun Reservoir experienced an overall increase since 1990 and was averaged at 52.20 and 32.12 t ha^?1, for stand forest and shrub in 2010, respectively. Total AGB in 2010 was estimated to be 4.5 × 10⁷ t, which increased by 8% when compared with the level in 1990. Our results are in the similar range of AGB density reported by other studies carried out in Northern China. Ecological programs including Three-North Shelter Forest Project and Returning Farming to Forest promoted the forest expansion and development during this period, while local farming activities exerted certain negative effects on the surrounding forest systems.

Conclusions

Absolute AGB density values indicated that regions with less external interventions present more consistent biomass accumulation. However, the coarse spatial resolution and 10-year interval of the datasets limited detailed analysis of impacts from urbanization of Beijing City. Future studies incorporating sophisticated ecosystem research methods are expected to uncover the mechanisms and key drivers for the observed variations in the AGB in the Miyun Reservoir region of Beijing, China.

     

Using delayed luminescence to characterize humic acids from lake sediments

Purpose

Using the delayed luminescence (DL) method, the photoluminescent properties of humic acids (HA) extracted from lake sediments were analyzed. Delayed luminescence is a promising technique for characterizing HA and other forms of organic matter of various origins. But information is insufficient regarding both its optimal operating conditions and also those environmental factors that control the chemical constitution of samples, which affects patterns of delayed luminescence.

Materials and methods

The research material was HA from lake sediments. Humic acid extractions were carried out using the method developed by the International Humic Substances Society. Studies on excitation and recording of DL intensity of HA solutions were carried out with the use of a specially designed device for continuous recording of photo-induced luminescence. The DL was excited with a monochromatic light at multiple wavelengths.

Results and discussion

The DL intensity depended on the wavelength of the exciting light. The highest DL intensity was obtained with excitation by blue light, the lowest DL intensity was observed with excitation by red light. Statistically significant differences among DL intensity were observed in the examined humic acids. These differences may be evidence of the variable quantitative and qualitative contributions of photoluminophores to the structure of the studied HA molecules as well as their different photochemical reactivities. For the blue- and green-excited DL intensity, statistically significant positive correlations were obtained with the elemental atomic ratio O:H and CQ coefficient. For the red-excited DL intensity, statistically significant positive correlations were obtained with the C and H content and H:C, C:N atomic ratios, while negative correlations were obtained with the free radical concentration.

Conclusions

Delayed luminescence depended on the wavelength of the exciting light. The DL intensity excited by blue and green light was different than the red light-excited DL intensity. The DL depended on the primary composition of sediments and the properties and structure of HA too. The grouping of HA showed that the HA formed in organic matter-poor, silicate-rich sediments had greater DL emission than did those extracted from sediments that were enriched in organic matter.

     

Total petroleum hydrocarbons and heavy metals in road-deposited sediments in Tijuana,Mexico

Purpose

A study was carried out to evaluate the concentration of heavy metals (Pb, Cu, Cr, Cd, and Hg) and total petroleum hydrocarbons (TPH) in road-deposited sediments (RDS) from Tijuana, Mexico, and identify their possible sources.

Materials and methods

Thirty RDS samples were randomly collected during the dry season using a brush and dustpan and classified according to construction material, traffic intensity, and land use. Soil samples were collected from a nonurban area and their concentrations were used as background values. For TPH, the samples were quantified gravimetrically after Soxhlet extraction, whereas heavy metals were extracted by acid digestion and their concentrations were measured by atomic absorption spectrometry.

Results and discussion

The mean TPH concentrations for RDS were 4208 mg kg^?1 and ranged from 1186 to 9982 mg kg^?1. For heavy metals, mean concentrations were 31.8, 50.2, 17.1, 0.1, and 0.1 mg kg^?1 for Pb, Cu, Cr, Cd, and Hg, respectively. The Igeo results showed that RDS from Tijuana are moderately to strongly polluted with Pb and Cu and moderately polluted with Cr. Principal component analysis (PCA) showed that Pb, Cu, and Cr could have their origin in tire wear, brake pads, bearings, and bushings.

Conclusions

The findings of this study revealed that RDS from Tijuana are polluted with TPH and heavy metals and that their principal sources are anthropogenic activities.