The C/N ratio and phenolic groups of exogenous dissolved organic matter together as an indicator for evaluating the stability of mineral-organic associations in red soil

Purpose

Mineral-organic associations (MOAs) are the basic structural units of soil aggregates and are important reservoirs of nutrients for plants and soil microorganisms, determining the soil structure and fertility. However, the influence of exogenous dissolved organic matter (DOM) chemistry on the stability of MOAs is rarely reported.

Materials and methods

We first characterized different exogenous DOM through elemental analysis and spectroscopy analysis technologies. Then, a chamber incubation experiment was conducted with DOM addition concentration at 3 g C kg^?1 red soil. Principal component analysis, redundancy analysis, and the partial least squares path model were used to better understand the effect of exogenous DOM chemistry on the stability of MOAs.

Results and discussion

The addition of DOM into the red soil significantly increased not only the organic carbon both in the bulk soil and the soil heavy fraction, but also the soil combined humus and the soil mineral-organic compound quantity. Moreover, the rice straw-derived DOM had the best effect on improving the soil mineral-organic compound quantity/degree (additional), followed by the animal-derived DOM, while the fulvic acid increased it the least. The ratios of elements (C/N ratio, O/C ratio, and H/C ratio), aromaticity (SUVA₂₅₄), and phenolic C content of exogenous DOM had positively significant contributions to the stability of MOAs.

Conclusions

The rice straw-derived DOM had the greatest enhancement on the stability of the MOAs for its higher C/N ratio and phenolic groups content, so the exogenous DOM characteristics could be as an indicator in predicting the stability of the MOAs and evaluating the soil fertility.

     

Dissolved organic matter enhances the sorption of atrazine by soil

The influence of dissolved organic matter (DOM) on the sorption of atrazine (2-chloro-4-ethylamino-6-isopylamino-1,3,5-triazine) by ten soils was investigated. Batch sorption isotherm techniques were used to evaluate the important physiochemical properties of soil determining the sorption of atrazine in the presence of DOM. The sorption of atrazine as a representative of nonionic organic contaminants (NOCs) by soil with and without DOM could be well described by the Linear and Freundlich models. The n values of the Freundlich model were generally near to 1, indicating that linear partitioning was the major mechanism of atrazine sorption by soil samples. The apparent distribution coefficient, value, for atrazine sorption in the presence of DOM initially increased and decreased thereafter as the DOM concentration increased in the equilibrium solution. DOM at relatively lower concentrations significantly enhanced the sorption of atrazine by soil, while it inhibited the atrazine sorption at higher concentrations. For all the soil samples, the maximum of was 1.1~3.1 times higher than its corresponding K _d value for the control (without DOM). The maximum enhancement of the distribution coefficient () in the presence of DOM was negatively correlated with the content of soil organic carbon (SOC) and positively correlated with the clay content. The critical concentration of DOM, below which DOM would enhance atrazine sorption, was negatively correlated with SOC. The influence of DOM on atrazine sorption could be approximately considered as the net effect of the cumulative sorption and association of atrazine with DOM in solution. Results of this study provide an insight into the retention and mobility of a NOC in the soil environment.     

Efficiency of diethylaminoethyl cellulose in the isolation of dissolved organic matter from forest soil solutions

Abstract

Diethylaminoethyl cellulose (DEAE cellulose), a weak anion exchange resin, has been used to isolate dissolved organic matter (DOM) from soil solutions collected from three different soil types, to investigate the amount of DOM isolated from soil solutions of various origin, and the extent to which inorganic ions are isolated together with DOM. The concentration of DOM in the various soil solutions ranged from 2.5 to 32.8 mg#lbL^‐1 DOC. More than 80% of dissolved organic carbon (DOC) was usually isolated with DEAE cellulose. High concentrations of aluminum (Al) and sulfate (SO₄ ^2‐) in the soil solutions have reduced DOC recovery. More than 90% of potassium (K⁺), calcium (Ca²⁺), and magnesium (Mg²⁺), were removed during the isolation procedure, but 10 to 20% of Al and 30 to 40% of iron (Fe) were isolated together with the DOC, probably due to strong complexation to DOM. The advantages of using DEAE cellulose were that the use of strong acids and bases was limited and that pH adjustments of the sample, leading to chemical modification of DOM, was not required.     

Soil organic matter composition and soil lightness

Relationships between soil lightness, soil organic matter (SOM) composition, content of organic C, CaCO₃, and texture were studied using 42 top‐soil horizons from different soil types located in southern Germany. SOM composition was determined by CPMAS ¹³C NMR spectroscopy, soil color was measured by diffuse‐reflectance spectrophotometry and given in the CIE L*a*b* color coordination system (Commission Internationale de l'Eclairage, 1978). Multiple‐regression analysis showed, that soil lightness of top‐soil horizons is principally determined by OC concentration, but CaCO₃ and soil texture are also major variables. Soil lightness decreased with increasing OC content. Carbonate content had an important effect on soil lightness even at low concentrations due to its lightening property. Regressions between soil lightness and organic C content were strongly linear, when the soils were differentiated according to texture and CaCO₃ content. The aryl‐C content was the only SOM component which correlated significantly with soil lightness (r_S = –0.87). In the linear regressions carried out on the different soil groups, soil aryl‐C content was a more significant predictor for soil lightness than total OC content.     

Tracing the source and fate of dissolved organic matter in soil after incorporation of a C labelled residue: A batch incubation study

While dissolved organic matter (DOM) in soil solution is a small but reactive fraction of soil organic matter, its source and dynamics are unclear. A laboratory incubation experiment was set up with an agricultural topsoil amended with ¹³C labelled maize straw. The dissolved organic carbon (DOC) concentration in soil solution increased sharply from 25 to 186 mg C L⁻¹ 4 h after maize amendment, but rapidly decreased to 42 mg C L⁻¹ and reached control values at and beyond 2 months. About 65% of DOM was straw derived after 4 h, decreasing to 29% after one day and only 1.3% after 240 days. A significant priming effect of the straw on the release of autochthonous DOM was found. The DOM fractionation with DAX-8 resin revealed that 98% of the straw derived DOM was hydrophilic in the initial pulse while this hydrophilic fraction was 20-30% in control samples. This was in line with the specific UV absorbance of the DOM which was significantly lower in the samples amended with maize residues than in the control samples. The δ¹³C of the respired CO₂ matched that of DOC in the first day after amendment but exceeded it in following days. The straw derived C fractions in respired CO₂ and in microbial biomass were similar between 57 and 240 days after amendment but were 3-10 fold above those in the DOM. This suggests that the solubilisation of C from the straw is in steady state with the DOM degradation or that part of the straw is directly mineralised without going into solution. This study shows that residue application releases a pulse of hydrophilic DOM that temporarily (<3 days) dominates the soil DOM pool and the degradable C. However, beyond that pulse the majority of DOM is derived from soil organic matter and its isotope signature differs from microbial biomass and respired C, casting doubt that the DOM pool in the soil solution is the major bioaccessible C pool in soil.     

Zinc Sorption in Selected Soils

Abstract

The adsorption of nutrient elements is one of the most important solid‐ and liquid‐phase interactions determining the retention and release of applied plant nutrients and the efficiency of fertilization. The study showed that the soils with high cation exchange capacity (CEC), CaCO₃, organic matter contents, and heavy texture adsorbed more zinc (Zn). The alkaline soils from Pakistan adsorbed more Zn than English acidic soils. Langmuir and Freundlich isotherm fit was excellent, and r² values for the Langmuir isotherm were highly significant (r²=0.84 to 0.99). The Langmuir b values, representing the adsorptive capacity of a soil, increased as the texture fineness increased in the soil, with increases in the concentration of adsorptive material (such as organic matter and CaCO₃) and with increases in CEC and pH. The alkaline soils from Pakistan had higher bonding energy constant and higher log Kf values than the acidic English soils. Sequential extraction of Zn in these soils showed that most of the Zn was held in CaCO₃ pool in the alkaline soils, whereas in acidic soils adsorbed Zn was in exchangeable form.     

Adsorption of “real” dissolved organic matter on the clay and fine silt fractions of a forested Stagnic Gleysol

Dissolved organic matter (DOM) in soils is partially adsorbed when passing through a soil profile. In most adsorption studies, water soluble organic matter extracted by water or dilute salt solutions is used instead of real DOM gained in situ by lysimeters or ceramic suction cups. We investigated the adsorption of DOM gained in situ from three compartments (forest floor leachate and soil solution from 20 cm (Bg horizon) and 60 cm depth (2Bg horizon)) on the corresponding clay and fine silt fractions (< 6.3 μm, separated together from the bulk soil) of the horizons Ah, Bg, and 2Bg of a forested Stagnic Gleysol by batch experiments. An aliquot of each clay and fine silt fraction was treated with H₂O₂ to destroy soil organic matter. Before and after the experiments, the solutions were characterized by ultra‐violet and fluorescence spectroscopy and analyzed for sulfate, chloride, nitrate, and fluoride. The highest affinity for DOM was found for the Ah samples, and the affinity decreased in the sequence Ah > Bg > 2Bg. Dissolved organic matter in the 2Bg horizon can be regarded as slightly reactive, because adsorption was low. Desorption of DOM from the subsoil samples was reflected more realistically with a non‐linear regression approach than with initial mass isotherms. The results show that the extent of DOM adsorption especially in subsoils is controlled by the composition and by the origin of the DOM used as adsorptive rather than by the mineralogical composition of the soil or by contents of soil organic matter. We recommend to use DOM gained in situ when investigating the fate of DOM in subsoils.     

Molecular weight and humification index as predictors of adsorption for plant- and manure-derived dissolved organic matter to goethite

Sorptive retention of organic matter is important in maintaining the fertility and quality of soils in agricultural ecosystems. However, few sorption studies have been conducted that use dissolved organic matter (DOM) characteristic of agricultural amendments. We investigated the sorption to goethite (α‐FeOOH) of DOM extracted from: (i) above‐ground biomass of wheat straw (Triticum aestivum L.), maize residue (Zea mays L.), soybean residue (Glycine max (L.) Merr.), and hairy vetch residue (Vivia billosa L.); (ii) below‐ground biomass from maize, soybean, canola (Brassica napus L.), and green bean (Phaseolus vulgaris L.); and (iii) beef, dairy, poultry, and pig animal manures. The apparent molecular weight (MW_AP) of the DOM was measured by high performance‐size exclusion chromatography and ranged from 312 to 1074 g mol⁻¹. The carboxyl‐group content of the DOM measured by potentiometric titration ranged from 4.84 to 21.38 mmol₍₋₎ g⁻¹ carbon. The humification index (HIX) determined by fluorescence spectrometry varied from 1.15 to 4.33. Sorption was directly related to both MW_AP and HIX values of the DOM. Molecular weight analysis of the solution prior to and after sorption indicated that the DOM molecules > 1800 g mol⁻¹ were preferentially sorbed, resulting in fractionation of the DOM upon reaction with goethite. The multiple regression equation, based only on MW_AP and HIX parameters, explained 76% of the variance in amount of DOM sorbed. The results indicate that MW_AP and HIX are important factors in controlling the sorption of DOM to mineral surfaces. Amendment with materials that release DOM of higher molecular weight and greater humification will result in enhanced initial sorption of DOM to soil solids, thereby contributing to accumulation of a larger soil organic C pool.     

Characteristics of hydrophobic and hydrophilic acid fractions in drainage waters of undisturbed soil lysimeters

Purpose

Dissolved organic matter (DOM), a heterogeneous mixture of low concentrations of organic matter draining from soils, plays a significant role in soil C cycling and in nutrient and pollutant transport. DOM from undisturbed soil profiles has rarely been studied. Hydrophobic acids (Ho) and hydrophilic acids (Hi), the major components of DOM, were recovered, using XAD-8 and XAD-4 resins in series, from waters draining in winter and in spring periods from well-drained and poorly drained Irish grassland soil profiles in lysimeters.

Materials and methods

Waters were collected from 45 soil undisturbed lysimeters at the Teagasc Research Centre, Johnstown Castle, Wexford, Ireland. Four Irish representative soils had been collected as undisturbed 1.0-m-deep monoliths, transported to the experiment site and arranged randomly in an experimental facility. Water collections were carried out in winter and spring periods. The DOM was isolated and fractionated using an XAD-8 and XAD-4 resins in-tandem procedure, and hydrophobic acids (Ho) and hydrophilic acids (Hi) were isolated.

Results and discussion

The amounts of DOM recovered in the winter period were much greater than those in the spring period, and the soil types had only minor influences on the DOM concentrations recovered. The Ho and Hi fraction contents ranged from 62 to 90 and 10 to 28%, respectively, of the total DOM content extracted. The Hi acids were most enriched in ¹³C, and considered to reflect greater microbial inputs. The neutral sugar (NS) contents for the Ho and Hi fractions were in the range of 15 to 52 μg mg^?1, with the Hi fraction most enriched. The amino acids (AAs) for the Ho and Hi fractions varied from 0.6 to 2.4%, and the total AAs and NS of the Ho acids were well correlated. The DOM fractions from the drainage waters contained much less AAs and NS than the corresponding fractions in the parent soils. The solid- and liquid-state NMR data indicated organic structures with low aromaticity, significant amounts of carbohydrate and with lesser amounts of peptide structures, and with long-chain methylene (CH₂)_n and methine (-CH-) groups.

Conclusions

The application of a variety of wet chemistry and of spectroscopy procedures has given a more in-depth awareness of the compositions of the DOM in the drainage waters from four different soils in 1.0-m-deep lysimeter arrangements. Based on wet chemistry analyses, and FTIR and liquid- and solid-state NMR spectrometry, it is clear that there are some differences between the compositions of the DOM fractions recovered. Alkyl functionalities dominated the structures. These included significant amounts or O-alkyl (predominantly carbohydrate), and with lesser (and variable amounts in the different fractions) aromatic structures (to which aromatic amino acid components were considered to be significant contributors), and with no evidence for lignin-derived structures The results suggest that, during residence in the soil solution, microbiological processes transform the SOM components released into products that are greatly different from their materials of origin in the SOM.

     

Electron transfer capacity of soil dissolved organic matter and its potential impact on soil respiration

Purpose

Soil dissolved organic matter (DOM) as the labile fraction of soil organic carbon (SOC) is able to facilitate biogeochemical redox reactions effecting soil respiration and carbon sequestration. In this study, we took soil samples from 20 sites differing in land use (forest and agriculture) to investigate the electron transfer capacity of soil DOM and its potential relationship with soil respiration.

Materials and methods

DOM was extracted from 20 soil samples representing different land uses: forest (nos. 1–12) and agriculture (nos. 13–20) in Guangdong Province, China. Chronoamperometry was employed to quantify the electron transfer capacity (ETC) of the DOM, including electron acceptor capacity (EAC) and electron donor capacity (EDC), by applying fixed positive or negative potentials to a working electrode in a conventional three-electrode cell. The reversibility of electron accepting from or donating to DOM was measured by applying switchable potentials to the working electrode in the electrochemical system with the multiple-step potential technique. Carbon dioxide produced by soil respiration was measured with a gas chromatograph.

Results and discussion

Forest soil DOM samples showed higher ETC and electron reversible rate (ERR) than agricultural soil DOM samples, which may be indicative of higher humification rate and microbial activity in forest soils. The average soil respiration of forest soil (nos. 1–12) and agricultural soil (nos. 13–10) was 26.34 and 18.58 mg C g^?1 SOC, respectively. Both EDC and EAC of soil DOM had close relationship with soil respiration (p?<?0.01). The results implied that soil respiration might be accelerated by the electroactive moieties contained in soil DOM, which serve as electron shuttles and facilitate electron transfer reactions in soil respiration and SOC mineralization.

Conclusions

DOM of forest soils showed higher ETC and ERR than DOM of agricultural soils. As soil represents one of the largest reservoirs of organic carbon, soil respiration affects C cycle and subsequently CO₂ concentration in the atmosphere. As one of the important characteristics of soil DOM related to soil respiration, ETC has a significant impact on greenhouse gas emission and soil carbon sequestration but has not been paid attention to.     

Extraction of water‐soluble organic matter from mineral horizons of forest soils

Dissolved organic matter (DOM) is involved in many important biogeochemical processes in soil. As its collection is laborious, very often water‐soluble organic matter (WSOM) obtained by extracting organic or mineral soil horizons with a dilute salt solution has been used as a substitute of DOM. We extracted WSOM (measured as water‐soluble organic C, WSOC) from seven mineral horizons of three forest soils from North‐Rhine Westphalia, Germany, with demineralized H₂O, 0.01 M CaCl₂, and 0.5 M K₂SO₄. We investigated the quantitative and qualitative effects of the extractants on WSOM and compared it with DOM collected with ceramic suction cups from the same horizons. The amounts of WSOC extracted differed significantly between both the extractants and the horizons. With two exceptions, K₂SO₄ extracted the largest amounts of WSOC (up to 126 mg C kg^–1) followed by H₂O followed by CaCl₂. The H₂O extracts revealed by far the highest molar UV absorptivities at 254 nm (up to 5834 L mol^–1 cm^–1) compared to the salt solutions which is attributed to solubilization of highly aromatic compounds. The amounts of WSOC extracted did not depend on the amounts of Fe and Al oxides as well as on soil organic C and pH. Water‐soluble organic matter extracted by K₂SO₄ bore the largest similarity to DOM due to relatively analogue molar absorptivities. Therefore, we recommend to use this extractant when trying to obtain a substitute for DOM, but as WSOM extraction is a rate‐limited process, the suitability of extraction procedures to obtain a surrogate of DOM remains ambiguous.     

Kinetics of the reaction between the hydroxyl radical and organic matter standards from the International Humic Substance Society

Purpose

The objective of this study was to evaluate the effect of the physicochemical properties of five dissolved organic matter (DOM) isolates on their reactivity with the hydroxyl radical (HO·) in water.

Materials and methods

Five DOM isolates were purchased from the International Humic Substance Society (IHSS). Weight average molecular weight (M _W) of these samples was quantified using size exclusion chromatography based on polyethylene glycols as reference standards. Functional group and elemental composition of the DOM samples were available from the IHSS website. Room temperature rate constants were measured using electron pulse radiolysis.

Results and discussion

Five IHSS standards were examined in this study: two soil organic and three aquatic organic matters. The composition varied from samples that had primarily aliphatic carbon (Pony Lake fulvic acid) to mostly aromatic carbon moieties (Elliot Soil humic acid). The M _W values of the five samples ranged from 2,400 to 4,100 Da, with an average value of 3,060 Da. Second-order reaction rate constants between DOM and HO· (k _DOM-HO·) were measured using thiocyanate competition kinetics, giving values ranging from 1.21 to 10.36?×?10⁸ M_C ^?1?s^?1. The k _DOM-HO· values were not found to correlate with either M _W or the aliphatic-aromatic carbon ratio, which is consistent with previous reports looking at natural organic matter (NOM), but is different to reports on size-fractionated (ultrafiltration through 15–1 kDa membranes) effluent organic matter (EfOM). We attribute this difference to the larger molecular weight distributions in size-fractionated EfOM compared to NOM.

Conclusions

The k _DOM-HO· values in this study ranged over a factor of 10, suggesting that hydroxyl radical reactivity does depend on the sample composition; however, no major correlation was found between the measured reactivity and bulk physicochemical properties of DOM.     

Factors controlling the partitioning of pyrene to dissolved organic matter extracted from different soils

The mobility of hydrophobic organic compounds (HOCs) in soils can be influenced by the presence of dissolved organic matter (DOM). While numerous studies have determined interactions of HOCs with humic and fulvic acids, only few data exist on the partitioning of HOCs to natural, non‐fractionated DOM as it occurs in soil solutions. In this study, DOM was extracted from 17 soil samples with a broad range of chemical and physical properties, originating from different land uses. The partition coefficients of pyrene to DOM were determined in all soil extracts and for two commercial humic acids using the fluorescence quenching method. For the soil extracts, log K_DOC values ranged from 3.2 to 4.5 litres kg^?1. For the Aldrich and Fluka humic acids, log K_DOC was 4.98 and 4.96 litres kg^?1, respectively, thus indicating that they are not representative for soil DOM. After excluding these two values, the statistical analysis of the data showed a significant negative correlation between log K_DOC and pH. This was also shown for one sample where the pH was adjusted to values ranging from 3 to 9. A multiple regression analysis suggested that ultraviolet absorbance at 280 nm (an indicator for aromaticity) and the E4:E6 ratio (an indicator for molecular weight) had additional effects on log K_DOC. The results indicate that the partitioning of pyrene to DOM is reduced at alkaline pH, probably due to the increased polarity of the organic macromolecules resulting from the deprotonation of functional groups. Only within a narrow pH range was the K_DOC of pyrene mainly related to the aromaticity of DOM.     

Impact of dissolved organic matter on Zn extractability and transfer in calcareous soil with maize straw amendment

Purpose

Crop straw return into arable land is a common method of disposing of excess straw in China and can improve soil dissolved organic matter (DOM) that is known to modify soil zinc (Zn) extractability and mobility.

Materials and methods

We conducted a soil box (internal dimensions, 160?×?140?×?80 mm³) experiment to evaluate the response of Zn extractability and transfer by diffusion to DOM after maize straw amendment (St, 0 and 15 g kg^?1) in calcareous soil treated with ZnSO₄·7H₂O (Zn, 0 and 20 mg kg^?1). Soil treated with St₀Zn₀ (control), St₁₅Zn₀, St₀Zn₂₀, or St₁₅Zn₂₀ was isolated in the 10-mm center of the box, and untreated soil was placed in compartments at either side.

Results and discussion

Results revealed that addition of St₀Zn₂₀ or St₁₅Zn₂₀ increased the concentration of Zn extracted with diethylenetriaminepentaacetic acid (DTPA-extractable Zn) in the central layer compared with control or addition of St₁₅Zn₀. Over the course of 45 days, transfer of DTPA-extractable Zn into the adjacent untreated soil was detected at 15–20 mm in soil with St₁₅Zn₂₀ but at 10–15 mm with St₀Zn₂₀ and only 0–5 mm with St₁₅Zn₀. Additionally, a higher amount of DTPA-extractable Zn transfer into the adjacent untreated soil also occurred in St₁₅Zn₂₀. This increased DTPA-extractable Zn transfer may be associated with the formation of Zn-fulvic acid complexes with the provision of DOM derived from straw.

Conclusions

Soluble Zn combined with straw return may be a promising strategy for improving both Zn mobility and extractability in calcareous soil.

     

Washing agents from sewage sludge: efficiency of Cd removal from highly contaminated soils and effect on soil organic balance

Purpose

To compare Cd removal from different soils with three washing agents recovered from sewage sludge (dissolved organic matter (DOM), soluble humic-like substances (HLS), soluble humic substances (SHS)). Also, to investigate how washing with these agents changes soil organic-matter composition (OM).

Materials and methods

Sandy clay loam (S1) and clay (S2) highly contaminated with Cd (300 mg kg^?1) were washed with DOM, HLS, or SHS solutions at various pHs, and with various washing times and washing modes (single or double). Cd distribution and OM composition were determined (including content of humic substances (HS), fulvic fraction (FF), labile humic acids (L-HA), and stable humic acids (S-HA)).

Results and discussion

Cd removal proceeded with pseudo-second-order kinetics. Equilibrium was reached in 30 min (S1) and 60 min (S2). DOM, HLS, and SHS removed 75–82% of Cd from S1, and 80–87% from S2. The most mobile fraction of Cd was removed after one wash. S2 retained more OM, including HS, than S1. Although washing did not change the HA/FF ratio in most variants, washing with DOM and HLS increased the percentage of L-HA in both soils. Washing with SHS increased S-HA content in both soils, but the percent content of S-HA was similar to that in the unwashed soil.

Conclusions

DOM, HLS, and SHS derived from sewage sludge can effectively remediate clay and sandy clay soils highly contaminated with Cd. Washing with an SHS solution can increase the content of the most stable carbon forms (HA), which is beneficial for carbon sequestration in remediated soils.

     

The Weathering of Mineral Soil by Natural Soil Solutions

Chemical weathering is an important neutralisation process and sourceof cations in forest soil. The presence of dissolved organic matter in the soil solution can have a considerable influence on weathering release. The aim of this study is to compare the weathering potentialof natural soil solutions, collected from Norway spruce, Scots pine and birch sites, to release Al, Ca, Mg, K, Na, and Si from the fine fraction in the C horizon of a podzol. Residual organic matter in the mineral soil was removed with H₂O₂. The <0.06 mm fraction of the mineral soil was suspended in soil solution, collected from the three sites, for 11 days with continuous agitation. Ultrapure water was used as a control. The pH of the suspensions was maintained at 5.4 by bubbling with CO₂. The initial mean DOC concentrations in the soil solutions were 65, 56 and 40 mg L^-1 for the spruce, pine and birch sites, respectively. The presence of DOM in the soil solution did not significantly enhance the capacity to weather mineral soil material, and no systematic differences were found between the three sites. However, Al release from the mineral soil was slightly higher in the soil solutions containing DOM compared to the control solution with no DOM. The proportions of DOM fractions capable of enhancing weathering were comparable with those reported in earlier studies. The weathering of metals was found to be primarily due to pH-driven processes. The lack of considerable weathering enhancement by DOM could be due to the fact that the cation-binding sites of the organic ligands were already saturated by e.g. Al and Fe in the soil solution derived from these podzolic, Al- and Fe-rich soils.     

Labile ester sulphate in organic matter extracted from podzolic soils

Summary We studied the effect of soil pretreatment, molecular-weight fractionation, and K₂SO₄ addition on the concentration and biochemical stability of ester sulphate in soil organic matter. A labile ester sulphate fraction (8.1 g S g^–1 soil) was detected in the organic matter extracted from a sulphate-rich podzolic sandy loam. This fraction was susceptible to loss during soil pretreatment with water and KCl solution and subsequent extraction of organic matter from the soil. The low-sulphate loam was low in labile ester sulphate (0.6 g S g^–1 soil) and the pretreatments had little effect. The addition of K₂SO₄ to the organic matter extracted from the low-sulphate soil resulted in the formation of appreciable amounts of labile ester sulphate. Newly formed ester sulphate tends to be biochemically less stable than indigenous ester sulphate in soil humic polymers and the ester sulphate associated with the low molecular-weight fractoin of soil organic matter appears to be more susceptible to loss by enzymatic hydroylsis. The results were interpreted in terms of steric effect. Ester sulphate groups bound to external surfaces of soil humic polymers may be easily accessible to sulphatase enzyme and thus readily mineralizable during incubation or extraction of soil organic matter at low soluble-sulphate levels. Sulphate groups on inner surfaces of the organic polymers are shielded from the enzyme due to size exclusion and hence more stable.     

Forms and buffering potential of aluminum in tropical and subtropical acid soils cultivated with Pinus taeda L

Purpose

Several interactions between Al and the solid phase of soil influence Al buffering in soil solution. This work evaluated soils cultivated with Pinus taeda L. to determine Al forms in organic and mineral horizons using various extraction methods and to relate acidity with clay mineralogy.

Materials and methods

Organic and mineral horizons of 10 soil profiles (up to 2.1 m deep) in southern Brazil were sampled. Organic horizons were separated into fresh, aged, and fermented/humified litter. The following Al extraction methods were utilized: 0.5 mol L^?1 pH 2.8 CuCl₂–Al complexed in organic matter; 1.0 mol L^?1 KCl–exchangeable Al; water–Al soluble in soil solution; HF concentrated?+?HNO₃ concentrated?+?H₂O₂ 30% (v/v)–total Al. Six sequential extractions were carried out to isolate different forms of amorphous minerals that can buffer Al on soil solution: 0.05 and 0.1 mol L^?1 sodium pyrophosphate; 0.1 and 0.2 mol L^?1 ammonium oxalate; 0.25 and 0.5 mol L^?1 NaOH. Samples of clay were also analyzed by XRD.

Results and discussion

There was a clear effect of litter age on increasing total Al concentration. In the aged litter and fermented and/or humified litter, levels of total Al were 1.4 to 3.8 and 1.5 to 7.8 times greater than in fresh litter, respectively. The CuCl₂ method had higher Al extraction capacity than the KCl method for litter. The lowest Al–pyrophosphate values were observed in the Oxisol, which also had a predominance of gibbsite and the lowest levels of Al–KCl and Al–CuCl₂. There was an inverse relationship between degree of soil weathering and soluble and exchangeable Al in soils. Available Al increased with higher Si proportion in minerals of the clay fraction (2:1?>?1:1?>?0:1).

Conclusions

The worst scenario was soils with the combination of high soluble and exchangeable Al levels and high concentrations of amorphous forms of Al minerals. The best predictors of Al accumulation in the youngest litter horizon were extractions of amorphous minerals with pyrophosphate and NaOH. These extractors are normally used to predict the level of Al buffering in soils. Organic matter had less influence on Al dynamics in soils.

     

源于松针的可溶性有机物对土壤多环芳烃吸附和解吸行为的影响研究

Study of the relationship between plant litter-derived dissolved organic matter（DOM） and organic pollutant transport in soil is important for understanding the role of forest litter carbon cycling in influencing pollutant behaviour and fate in forest soil.With the aim of providing insight into the capacity of plant litter-derived DOM to influence sorption and desorption of selected polycyclic aromatic hydrocarbons（PAHs） on soil, batch experiments were carried out with application of a sorption-desorption model incorporating DOM effects. Freshly fallen pine（Pinus elliottii） needles were used as the source of organic matter. Input of the pine needle litter-derived DOM was found to significantly decrease desorption hysteresis as well as soil adsorption capacity of phenanthrene（PHE） and fluoranthene（FLA）. Addition of 1 728 mg L-1dissolved organic carbon（DOC） lowered the organic carbon-normalized sorption distribution coefficient of PHE from 7 776 to 2 541 L kg-1C and of FLA from 11 503 to 4 368 L kg-1C. Decreases of the apparent sorption-desorption distribution coefficients of PHE and FLA with increased DOC concentration indicated that DOM favored desorption of PAHs from soil. Increases in the fraction of apparently dissolved PAHs were attributable to the dissolved PAH-DOM complexes, accounting for the dissolved proportions of 39% to 69% for PHE and 26% to 72% for FLA in the sorption and desorption processes as the concentration of the added DOM solution rose from 0 to 1 728 mg L-1. Our results suggest that pine needle litterderived DOM can have a substantial effect of inhibiting PAHs sorption and promoting PAHs desorption, thus leading to enhanced leaching in soil, which should be taken into account in risk assessment of PAHs accumulated in forest soil.     

Liming effects on soil pH and crop yield depend on lime material type,application method and rate,and crop species: a global meta-analysis

Purpose

The aim of this meta-analysis was to investigate the interactive effects of environmental and managerial factors on soil pH and crop yield related to liming across different cropping systems on a global scale.

Materials and methods

This study examined the effects of liming rate, lime application method, and liming material type on various soil chemical properties and crop yield based on data collected from 175 published studies worldwide since 1980.

Results and discussion

The most important variables that drive changes in soil pH and crop yield were liming rate and crop species, respectively. Soil conditions, such as initial soil organic matter and soil pH, were more important for increasing soil pH in field-based experiments, while lime material type and application method were more important for improving crop yield. To effectively neutralize soil acidity, the optimum liming duration, rate, and material type were?<?3 years, 3–6 Mg ha^?1, and Ca (OH)₂, respectively. Averaged across different crop species, the application of CaO, CaCO₃, Ca (OH)₂, and CaMg (CO₃)₂ increased yield by 13.2, 34.3, 29.2, and 66.5%, respectively.

Conclusions

This meta-analysis will help design liming management strategies to ameliorate soil acidity and thus improve crop yield in agroecosystems.