Effect of Long-Term Effluent Recharge on Phosphate Sorption by Soils in a Wastewater Reclamation Plant

The Soreq recharge basins, used for wastewater reclamation employing the Soil-Aquifer Treatment (SAT) system, have been recharged, on average, by about 1,800 m depth of secondary effluent during their operation period of ～25 years. An estimated amount of ～6 kg P m^?2 was added to the soil/sediment column during this period. The objective of this study was to compare phosphorous sorption characteristics of representative pristine soils in the Soreq recharge site to those of the basin soils sampled after a long period of effluent recharge. Batch isotherm experiments were conducted: samples of one g of soil were equilibrated with 25 mL of 0.02 M NaCl solution containing 0–3.2 mM of phosphate for 7 days at 25± 1^°C and P sorption was measured. Long-term effluent recharge significantly decreased the maximum P sorption capacity of the top sandy soil (0.15–0.3 m) and only very slightly decreased maximum P isotherm capacity of the deep clayey-sand soil (10–10.5 m). The retention of P in the basin sandy soil primarily involved sorption and surface precipitation reactions on soil carbonates. In the basin clayey-sand soil, P was retained by its sorption on surfaces of Fe, Al, Mn oxide/hydroxides and clay minerals. Long-term effluent recharge increased EPC₀, (the equilibrium P concentration in solution at which there is no sorption or desorption to or from the soil under the given conditions), of the basin soils compared to the pristine soils. Due to loading of the top horizons with P by prolonged recharge and reduced P concentration in the effluent, EPC₀ of the basin sandy soil is now equal to the average P concentration of the recharged effluents. If effluent P concentration will decrease further, the top sandy soil will become a source of P to the reclaimed water, rather than a sink. The clayey-sand layers and lenses in the vadose zone of the SAT system of the Soreq site offer a large capacity for P adsorption. With gradual leaching of carbonate minerals and synthesis of secondary clay minerals, driven by long-term effluent recharge, P retention mechanisms in the basin soil may be changed, but this process would be extremely slow.     

Competitive sorption between imidacloprid and imidacloprid-urea on soil clay minerals and humic acids

Soil organic matter and clay minerals are responsible for the adsorption of many pesticides. Adsorption and competitive sorption of imidacloprid on clay minerals and humic acids (HA) were determined using the batch equilibration method. The sorption coefficient of imidacloprid on humic acids was significantly higher than that on Ca-clay minerals, indicating that soil organic matter content was a more important property in influencing the adsorption of imidacloprid. Competitive sorption was investigated between imidacloprid and its main metabolite imidacloprid-urea on HA and Ca-clay minerals. The results showed that the sorption capacity of imidacloprid on clay minerals and HA was reduced in the presence of the metabolite, implying that imidacloprid-urea could occupy or block adsorption sites of imidacloprid on soil, potentially affecting the fate, transport, and bioavailability of imidacloprid in the environment. The interactions between a Ca-clay or HA-clay mixture and adsorption of imidacloprid and imidacloprid-urea were studied using IR differential spectra on thin films made of the adsorbent. The possible mechanisms were discussed from the shift of characteristic IR absorption bands of imidacloprid and imidacloprid-urea after sorption.     

Sorption and transport of sulfonamides in soils amended with wheat straw-derived biochar: effects of water pH,coexistence copper ion,and dissolved organic matter

Purpose

Sulfonamides are widely used for the prevention and treatment of bacterial infections, hard-degraded contaminants distributed in the environment if they are discharged into the soil and water. Biochar could probably influence the geochemical behavior of ionized antibiotics in the soils.

Materials and methods

To determine the sorption/desorption of three representative sulfonamides (SAs) in soils amended with biochar, we investigated the effects of water pH, Cu²⁺, and dissolved humic acid on the sorption of sulfamethoxazole (SMX), sulfamethazine (SMZ), and sulfadiazine (SD) onto two different soil samples (S1 pH?=?5.13 and S2 pH?=?7.33) amended with wheat straw-derived biochar (size 0.5～0.6 mm).

Results and discussion

Batch experiments showed that the sorption/desorption isotherms of SAs on soil with/without biochar followed the Freundlich model. The biochar had a strong adsorption potential for SMX, SMZ, and SD both in S1 and S2 at low water pH. Except for SMX, the presence of Cu²⁺ inhibited the sorption of SMZ and SD through competing hydrophobic adsorption region in soils. HA suppressed the sorption of three sulfonamides in soil S2 by electrostatic repulsion under alkaline condition. The soil leaching column experiments showed the SA transport in soils, and S1 and S2 amended with biochar (0.5 and 1.0 wt%) brought about 12–20 % increase in SMX, SMZ, and SD retention compared to the untreated soil.

Conclusions

The results indicated that the presence of biochar effectively mitigated the mobility of ionized antibiotics such as SMX, SMZ, and SD in soils, which helps us reconsider the potential risk of antibiotics in the environment.

     

致谢审稿专家

Two agricultural soils were collected from Dahu and Pinchen counties and swine manure compost (SMC) from Pingtung County in Taiwan, China to investigate the sorption and dissipation of three tetracyclines (TCs), i.e., oxytetracycline (OTC), tetracycline (TC) and chlortetracycline (CTC), in compost, soils and soil/compost mixtures with different organic carbon (OC) contents. There were seven treatments in total. TCs were most strongly adsorbed to SMC in all treatments due to the high OC content. When SMC was present in the soils, the sorption of TCs was significantly enhanced, which might be attributed to the increased OC content and CEC. The adsorption of TCs showed non-linear adsorption isotherms and fitted well to the Freundlich model. After 49 d of incubation at 25 oC in soils and soil/compost mixtures in the dark, TCs elapsed in all substrates, with the time required for 50% degradation (DT50) between 20 and 41 d, and the time for 90% degradation (DT90) between 68 and 137 d. Soil amended with compost enhanced the stability of TCs and reduced their mobility. The dissipation of TCs in a soil environment was slow, indicating that these compounds might be persistent in soil.     

红壤基质组分对磷吸持指数的影响

在红壤旱地肥料长期定位试验(始于1988年)中,选取了无机肥试验区的NPK、NP、NK、PK,有机无机配施试验区的CK、CK+猪厩肥(BM)及CK+花生秸秆(SR)等7个施肥处理土壤,测定了土壤磷吸持指数(Phosphate sorption index,PSI),分析了PSI与红壤最大吸磷量(Xm)的相关关系,讨论了土壤pH、有机质、黏粒、铁铝氧化物及无机磷酸盐等基质组分对PSI的影响。结果表明:长期施磷或配施有机肥均可显著降低红壤PSI值,随着土壤pH的升高、有机质及铁结合态磷酸盐(Fe-P)含量的增加,红壤PSI显著降低;土壤游离铁铝氧化物及黏粒含量越高,PSI也越大。PSI与Xm呈显著线性相关关系(Xm=0.5PSI+412.8,n=15,r=0.967**,p<0.01),因此,可以用PSI替代Xm来表征土壤固磷能力,亦可由PSI的大小来推断土壤磷的供磷能力。     

Pesticide sorption and diffusion in natural clay loam aggregates

Pesticide sorption in soils is controlled by time-dependent processes such as diffusion into soil aggregates and microscopic sorbent particles. This study examines the rate-controlling step for time-dependent sorption in clay loam aggregates. Aggregates (5 mm) were stabilized with alginate, and adsorption of azoxystrobin, chlorotoluron, and cyanazine was measured in batch systems equilibrated for periods between 1 h and 7 days. Stepwise desorption was measured at 1- or 3-day intervals following 1 or 7 days of adsorption. Time-dependent adsorption was also measured on dispersed soil. Results were interpreted using process-based modeling. Adsorption on dispersed soil was described by intraparticle sorption and diffusion. Adsorption in the aggregates was much less than in suspension, suggesting that part of the sorption capacity of the dispersed soil was not available within the aggregates (approximately 50%). Adsorption and desorption were reversible and could be described by pore diffusion into the aggregate with effective diffusion coefficients between 0.5 x 10(-10) and 1 x 10(-10) m2 s(-1), a factor 3-6 slower than estimated theoretically. Intraparticle diffusion did not seem to contribute to sorption in the aggregates at this time scale. Apparent hysteresis was explained by nonattainment of equilibrium during the adsorption and desorption steps.     

土壤不同粒径有机无机复合体对丁草胺的吸附特性

为了解土壤不同粒径组分对农药吸附-解吸行为的影响和吸附贡献率,以及不同粒径组分中有机无机组分的结合方式和复合程度如何影响有机质对农药的吸附,选取我国6个省区的7种理化性质差别较大的土壤,并采用物理方法提取该7种土壤的三个粒径有机无机复合体(黏粒:0.002mm;粉粒:0.02~0.002 mm;砂粒:0.05~0.02 mm)为研究材料,采用批量平衡法研究丁草胺在不同土壤和不同粒径有机无机复合体固/液界面的分配规律。同时,定量计算土壤各粒径组分对丁草胺的吸附贡献率,并从有机无机复合体角度探讨不同粒径组分中总有机碳(TOC)对丁草胺的吸附特性。结果表明:土壤黏粒组分对丁草胺具有最大的吸附量和较小的解吸率,而砂粒组分对丁草胺则具有较小的吸附量和最大的解吸率。土壤黏粒、粉粒和砂粒组分对丁草胺的吸附贡献率分别为36.7%~72.4%、21.7%~50.5%和10%。TOC是影响各粒径组分对丁草胺吸附的主要原因,但其影响程度受各粒径组分中TOC的理化性质以及其与无机矿物的复合程度控制。     

Potential contributions of smectite clays and organic matter to pesticide retention in soils.

Soil organic matter (SOM) is often considered the dominant sorptive phase for organic contaminants and pesticides in soil-water systems. This is evidenced by the widespread use of organic-matter-normalized sorption coefficients (K(OM)) to predict soil-water distribution of pesticides, an approach that ignores the potential contribution of soil minerals to sorption. To gain additional perspective on the potential contributions of clays and SOM to pesticide retention in soils, we measured sorption of seven pesticides by a K-saturated reference smectite clay (SWy-2) and SOM (represented by a muck soil). In addition, we measured the adsorption of atrazine by five different K-saturated smectites and Ca-saturated SWy-2. On a unit mass basis, the K-SWy-2 clay was a more effective sorbent than SOM for 4,6-dinitro-o-cresol (DNOC), dichlobenil, and carbaryl of the seven pesticides evaluated, of which, DNOC was sorbed to the greatest extent. Atrazine was sorbed to a similar extent by K-SWy-2 and SOM. Parathion, diuron, and biphenyl were sorbed to a greater extent by SOM than by K-SWy-2. Atrazine was adsorbed by Ca-SWy-2 to a much lesser extent than by K-SWy-2. This appears to be related to the larger hydration sphere of Ca(2+) (compared to that of K(+)) which shrinks the effective size of the adsorption domains between exchangeable cations, and which expands the clay layers beyond the apparently optimal spacing of approximately 12.2 A for sorption of aromatic pesticide structures. Although a simple relation between atrazine adsorption by different K-smectites and charge properties of clay was not observed, the highest charge clay was the least effective sorbent; a higher charge density would result in a loss of adsorption domains. These results indicate that for certain pesticides, expandable soil clays have the potential to be an equal or dominant sorptive phase when compared to SOM for pesticide retention in soil.     

Evaluation of arsenite sorption in Spanish soils

Abstract

Arsenite sorption was studied at different temperatures (30,40, and 50°C) to investigate processes that remove arsenite from soil solution (adsorption or precipitation), and if adsorption was taking place, how many sites were involved in this process, and their nature. Adsorption was the only reaction reducing levels of soluble arsenite in the two alkaline soils used in this experiment, Jijona and Agost; however, arsenite precipitation occurred in an acidic substrate (pH 4.0), Galicia soil. Iron (Fe) oxides and clay minerals were the soil components controlling arsenite sorption in the Jijona soil (pH 7.9 and high levels of Fe oxides). Calcite and clay minerals were the inorganic constituents involved in arsenite sorption in the Agost soil (pH 8.0 and high levels of CaCO₃). Arsenite sorption was an endothermic and non‐spontaneous process. The fact that the higher the temperature, the higher the arsenite sorption, however, was likely due to an increase in the diffusion rate.     

Adsorption of atrazine, hydroxyatrazine, deethylatrazine, and deisopropylatrazine onto Fe(III) polyhydroxy cations intercalated vermiculite and montmorillonite

This paper describes the modification of the clay minerals vermiculite (VT) and montmorillonite (MT) by intercalating Fe(III) polymers of different [OH(-)]:[Fe(III)] ratios with the aim of removing atrazine (AT) and its metabolites deethylatrazine (DEA), deisopropylatrazine (DIA), and hydroxyatrazine (ATOH) from aqueous solution. An enhancement of adsorption capacity was observed for both intercalated clay minerals in comparison to the potassium-saturated materials (KVT or KMT). The results showed that different [OH(-)]:[Fe(III)] molar ratios had a small influence on the adsorption capacity, as well as in the basal spacing, BET surface area, and porosity. For the lowest initial concentrations of AT, DIA, and ATOH (0.050 mg L(-)(1)) studied, the modified VT adsorbed almost 80% of AT and DIA, while ATOH was removed at concentration levels below the detection limit of the technique, implying in at least 99.5% of sorption. Weak interaction between intercalated VT and DEA was observed, although a significant adsorption enhancement occurred in comparison to KVT. Within a 24 h interval, desorption of AT and DIA in aqueous medium reached levels close to 20% of the amount initially adsorbed, while for ATOH only 3% of the adsorbed compound was desorbed. The adsorption capacity of the Fe(III)-intercalated VT decreased after the first adsorption/desorption cycle, implying that the material is not suitable for reutilization. The intercalated MT was a powerful sorbent for AT, DEA, DIA, and ATOH, removing all of these chemicals from solution almost quantitatively (sorption greater than 99.5%), even at initial concentrations as high as 1.0 mg L(-)(1). Additionally, desorption of AT, ATOH, and DIA in water was not measurable up to the tube corresponding to the initial concentration of 1.0 mg L(-)(1), suggesting strong irreversible binding of these compounds to the intercalated MT materials. Desorption of DEA from the intercalated MT was between 5 and 30%. Unlike what was observed for VT, the intercalated MT materials were recyclable, keeping an excellent performance when reutilized.     

Phosphate sorption by red mediterranean soils from Greece

Abstract

In nineteen surface horizons of red Mediterranean soils from various locations of Greece, phosphorus (P) sorption experiments were conducted and the sorption characteristics were studied in relation to soil properties. Phosphate sorption data were fitted both to the Langmuir and Freundlich equations. From these equations, the following P sorption parameters were determined from the Freundlich equation, X = ACⁿ, the parameters A (the phosphate sorbed at C = 1 mg P/L), n (the P sorption intensity), the P sorption index (PS = X/log C) and maximum P sorption (Xmfr). From the Langmuir equation, C/X = 1/KXm + C/Xm, the parameters K (showing the bonding energy), maximum P sorption (Xmla), the quantity of P adsorbed at a standard concentration of 0.2 mg P/L (P0.2), and P maximum buffering capacity (PMBC). The Freundlich parameter A was strongly correlated to the clay and sesquioxides ("free”; iron and aluminum oxides and amorphous iron oxides) content. Seventy‐four percent of the variance of this parameter was explained by clay and “free”; iron (Fe) content. The Freundlich parameter n was significantly correlated with pH and amorphous iron oxides content, while 52% of its variance was explained by amorphous Fe and dithionite extrac‐table aluminum (Al). The P sorption maxima calculated from the Freundlich equation were in general lower than those calculated by the Langmuir equation. Both these parameters were strongly correlated with clay and more slightly with sesquioxides content. About 50% of their variance was explained by clay content of the soils. The P sorption index was strongly correlated with the clay content and less strongly with dithionite‐extractable Fe and Al. The P‐buffering capacity calculated from the data of Langmuir equation was also strongly correlated with these two parameters. In addition, clay content and dithionite‐extractable Fe and Al were well correlated to the amounts of P required to obtain an equilibrium concentration of 0.2 mg P/L while 61% of the variation of this parameter was explained by the clay and the dithionite‐extractable Fe content. From these findings, it seems that for the red Mediterranean soils from Greece, P sorption is affected by clay content and iron and aluminum oxide contents.     

Competitive sorption of cadmium and lead in acid soils of Central Spain

The bioavailability and ultimate fate of heavy metals in the environment are controlled by chemical sorption. To assess competitive sorption of Pb and Cd, batch equilibrium experiments (generating sorption isotherms) and kinetics sorption studies were performed using single and binary metal solutions in surface samples of four soils from central Spain. For comparisons between soils, as well as, single and binary metal solutions, soil chemical processes were characterized using the Langmuir equation, ionic strength, and an empirical power function for kinetic sorption. In addition, soil pH and clay mineralogy were used to explain observed sorption processes. Sorption isotherms were well described by the Langmuir equation and the sorption kinetics were well described by an empirical power function within the reaction times in this study. Soils with higher pH and clay content (characterized by having smectite) had the greatest sorption capacity as estimated by the maximum sorption parameter (Q) of the Langmuir equation. All soils exhibited greater sorption capacity for Pb than Cd and the presence of both metals reduced the tendency for either to be sorbed although Cd sorption was affected to a greater extent than that of Pb. The Langmuir binding strength parameter (k) was always greater for Pb than for Cd. However, these k values tended to increase as a result of the simultaneous presence of both metals that may indicate competition for sorption sites promoting the retention of both metals on more specific sorption sites. The kinetic experiments showed that Pb sorption is initially faster than Cd sorption from both single and binary solutions although the simultaneous presence of both metals affected the sorption of Cd at short times while only a minor effect was observed on Pb. The estimated exponents of the kinetic function were in all cases smaller for Pb than for Cd, likely due to diffusion processes into micropores or interlayer space of the clay minerals which occurs more readily for Cd than Pb. Finally, the overall sorption processes of Pb and Cd in the smectitic soil with the highest sorption capacity of the studied soils are slower than in the rest of the soils with a clay mineralogy dominated by kaolinite and illite, exhibiting these soils similar sorption rates. These results demonstrate a significant interaction between Pb and Cd sorption when both metals are present that depends on important soil properties such as the clay mineralogy.     

Sorption of lead,copper, zinc,and cadmium by soils: effect of nitriloacetic acid on metal retention

Abstract

Laboratory experiments were carried out to evaluate lead (Pb), copper (Cu), zinc (Zn), and cadmium (Cd) sorption‐desorption by three soils of contrasting characteristics. Talamanca (silt loam, montmorillonite, Calcic Haploxeralfs), Mazowe (clay, kaolinite, Rhodic Kandiustalf), and Realejos (sandy silt loam, allophane, Typic Hapludands). A second objective was to study the effect of nitriloacetic acid (NTA) on the sorption process. The Talamanca soil, which had a native pH of 6.4 and presented the highest effective cation exchange capacity (ECEC), sorbed more of each of the metal tested than did the other two soils. When the other two soils were compared metal sorption was also related to pH and ECEC. The very low sorption capacity showed by Realejos may be attributed to the low net surface negative charge density of this soil, arising from its allophanic nature. A common feature of the three soils was the relative strong sorption of both Pb and Cu relative to Cd and Zn with Pb showing the highest sorption levels. The selectivity sequences of metals retention were Pb>Cu>Zn>Cd for Talamanca soil, Pb>Cu>Zn≈Cd for Mazowe, and Pb>Cu>Cd>Zn for Realejos. Metal desorption values were low. The order of metal desorption (Cd≈Zn>Cu>Pb) was the same for the three soils studied. Quantitative differences observed in the extractability of the sorbed metals between the soils (Realejos>Mazowe>Talamanca) indicated that soil properties which enhanced metal sorption contributed at the same time to slow down the backward reaction. The addition of NTA to the soil suspension significantly depressed metal sorption by the three soils investigated. Compared with the free ligand system Pb, Cu, Zn, and Cd sorption in the presence of NTA decreased roughly 50%.     

Adsorption and Desorption of Copper in Pasture Soils

Abstract

Copper (Cu) is bound strongly to organic matter, oxides of iron (Fe) and manganese (Mn), and clay minerals in soils. To investigate the relative contribution of different soil components in the sorption of Cu, sorption was measured after the removal of various other soil components; organic matter and aluminum (Al) and Fe oxides are important in Cu adsorption. Both adsorption and desorption of Cu at various pH values were also measured by using diverse pasture soils. The differences in the sorption of Cu between the soils are attributed to the differences in the chemical characteristics of the soils. Copper sorption, as measured by the Freundlich equation sorption constants [potassium (K) and nitrogen (N)], was strongly correlated with soil properties, such as silt content, organic carbon, and soil pH. The relative importance of organic matter and oxides on Cu adsorption decreased and increased, respectively, with increasing solution Cu concentrations. In all soils, Cu sorption increased with increasing pH, but the solution Cu concentration decreased with increasing soil pH. The cumulative amounts of native and added soil Cu desorbed from two contrasting soils (Manawatu and Ngamoka) during desorption periods showed that the differences in the desorbability of Cu were a result of differences in the physico‐chemical properties of the soil matrix. This finding suggests that soil organic matter complexes of Cu added through fertilizer, resulted in decreased desorption. The proportions of added Cu desorbed during 10 desorption periods were low, ranging from 2.5% in the 24‐h to 6% in the 2‐h desorption periods. The desorption of Cu decreased with increasing soil pH. The irreversible retention of Cu might be the result of complex formation with Cu at high pH.     

The adsorption characteristics of soils and removal of cadmium and nickel from wastewaters

The interactions between the adsorption characteristics of 27 experimental soils and the sorption of Cd and Ni from the municipal wastewaters were investigated in this study. The removal of these elements from soil solution was followed for 50 days. All the adsorption characteristics, except cation exchange capacity and organic matter, were significantly correlated to the sorption of Cd after one day shaking. After 7 days of shaking, none of the soil adsorption characteristics except free CaCO₃ was significantly correlated to Cd removal from wastewater. The soil saturated paste pH and suspension pH were strongly correlated to Cd sorption throughout this experiment. The behavior of Ni in soils was different from that of Cd. Surface area, total Fe, and total Al were significantly correlated to Ni sorption. The correlation between Ni removal and pH was the strongest than any other parameter studied. After 7 days shaking, clay content and total Ca were not significantly correlated to Ni sorption. The cation exchange capacity of the soils was not significantly correlated to Cd or Ni sorption in this experiment. It seems that in the experimental soils, concentration of Cd and Ni were probably not controlled by adsorption process. The precipitation process was probably playing a major role in the removal of these elements from the municipal wastewaters. As observed in this experiment, the cation exchange capacity of experimental soils was a poor parameter to define sorption capacity of these soils for Cd and Ni. The guidelines for determining the soil sludge load, which are mainly based on the cation exchange capacity of soils, should be revised.     

批实验中土壤对地乐酚吸附的差异

吸附是决定除草剂地乐酚在土壤中迁移的重要机制之一。通常用简单快捷的批实验来衡量土壤对除草剂的吸附。由大量的批实验确定地乐酚在不同土壤样本中的吸附参数,并对各土壤特性与吸附参数的相关性作统计分析。结果表明土壤有机C含量,粘粒含量及阳离子代换量与吸附参数显著正相关,土壤pH值与吸附参数显著负相关。方差分析表明地乐酚在土壤中的吸附表现出很强的空间差异,在不同地点的地乐酚吸附参数无显著区别,而在不同的深度区别显著。超过85%的地乐酚吸附参数的空间差异可由土壤有机C含量的空间差异来解释。     

Sorption and desorption characteristics of sulfamethazine in three different soils before and after removal of organic matter

Organic matter (OM) is the most critical factor in controlling the sorption-desorption of SMZ in soil, however, few studies have explored the effects of OM removal on these important behaviors among different soils. Batch experiments were conducted to investigate the sorption and desorption characteristics of SMZ in three different soils: fluvo-aquic soil (FS), paddy soil (PS), and red soil (RS). The SMZ sorption in the evaluated soils was dominated by physisorption. The SMZ sorption capacities of FS and PS, which had a relatively higher OM content than RS, were higher than that of RS. The SMZ sorption in FS was dominated by linear partitioning. In contrast, the SMZ sorption in PS and RS was mainly nonlinear surface adsorption. After OM removal, the SMZ sorption capacity was significantly reduced in FS but increased in PS and RS. Furthermore, OM removal restrained the sorption intensity of SMZ in soils. Relatively higher OM and clay contents inhibited the SMZ desorption in FS and PS. The strong negative desorption hysteresis of SMZ in the three soils indicated that SMZ was able to move into the soil solution, thereby posing a risk to humans. Taken together, the findings of this study showed that OM indeed plays an important role during SMZ sorption-desorption in soil.     

Effect of earthworm activity (Aporrectodea giardi) on atrazine adsorption and biodegradation

We investigated the influence of earthworm (Aporrectodea giardi) activity on soil properties and on atrazine (AT) adsorption and biodegradation by comparing a coarse‐textured smectite‐free wetland soil (Brittany, France) with the earthworm casts derived from the top horizon of this soil. Casts are characterized by lower pH, are enriched in organic carbon (OC) and clay content, have a larger cation exchange capacity, and a greater exchangeable Ca content. The clay mineralogy of the soil studied and casts is characterized by a muscovite–kaolinite–chlorite association. In addition, the clay fraction of the soil contains lepidocrocite (γ‐FeOOH), which was not found in the casts. Atrazine adsorption isotherms were reasonably well described by the Freundlich equation and were all non‐linear. The mean amounts of adsorbed AT for starting concentrations of 3–30 mg litre^?1 ranged from 8 to 34%, being largest in earthworm casts. Soil AT adsorption capacity was well correlated with OC content. Non‐decomposed organic matter present in the coarse size fractions and specific compounds present in earthworm casts (proteins, mono‐ and polysaccharides, polyphenols, sugars, lignin) and microbial and fungal biomass contribute to AT adsorption. Weak electrostatic (physical) sorption of AT on organic compounds and on mineral surfaces prevails. For casts, the formation of additional hydrophobic interactions between AT and SOM is proposed. We also studied AT biodegradation by the model bacterium Pseudomonas sp. strain ADP in the presence of soils or earthworm casts. An enhancement of the AT disappearance rate was observed in the presence of all the solid matrices tested compared with that obtained in an aqueous medium. The biodegradation rate was shown to be dependent not only on the OC content of the solid matrix, but mainly on its composition and structure.     

沙粒中添加Ca2+、粘土以及富粘土对水溶性有机碳呼吸作用及其吸附的影响研究

Clay addition to light-textured soils is used to ameliorate water repellency and to increase nutrient retention. However, clay addition may also increase the potential to bind organic matter and thus C sequestration. Divalent calcium ions （Ca2＋） play an important role in binding of organic matter to clay because they provide the bridge between the clay particles and organic matter which are both negatively charged. In the first experiment, quartz sand was mixed with clay isolated from a Vertosol at rates of 0, 50 and 300 g kg-1, finely ground mature wheat residues （20 g kg-1） and powdered CaSO4 at 0, 5 and 10 g kg-1. Soil respiration was measured over 28 d. Compared to the sand alone, addition of isolated clay at 300 g kg-1 increased cumulative respiration with a stronger increase than that at 50 g kg-1. Addition of CaSO4 increased electrical conductivity, decreased sodium adsorption ratio and reduced cumulative respiration. The latter can be explained by enhanced sorption of organic matter to clay via Ca2＋ bridges. In a second experiment, isolated clay or subsoil of the Vertosol without or with powdered CaSO4 at 10 g kg-1 were used for a batch sorption with water-extractable organic C （WEOC） from wheat straw followed by desorption with water. Addition of 10 g kg-1 CaSO4 increased sorption and decreased desorption of WEOC in both subsoil and isolated clay. In the third experiment, subsoil of the Vertosol was used for a batch sorption in which WEOC was added repeatedly. Repeated addition of WEOC increased the concentration of sorbed C but decreased the sorbed proportion of the added WEOC. This indicates that sorption of WEOC may be underestimated if it is added only once in batch sorption experaments.     

Poorly crystalline iron and aluminium oxides contribute to the carbon saturation and sorption of dissolved organic carbon in the soil

Soil carbon (C) saturation implies an upper limit to a soil's capacity to store C depending on the contents of silt + clay and poorly crystalline Fe and Al oxides. We hypothesized that the poorly crystalline Fe and Al oxides in silt + clay fraction increased the C saturation and thus reduced the capacity of the soil to sorb additional C input. To test the hypothesis, we studied the sorption of dissolved organic carbon (DOC) on silt + clay fractions (<53 µm) of highly weathered oxic soils, collected from three different land uses (i.e., improved pasture, cropping and forest). Soils with high carbon saturation desorbed 38% more C than soils with low C saturation upon addition of DOC, whereas adsorption of DOC was only observed at higher concentration (>15 g kg^?1). While high Al oxide concentration significantly increased both the saturation and desorption of DOC, the high Fe oxide concentration significantly increased the desorption of DOC, supporting the proposition that both oxides have influence on the DOC sorption in soil. Our findings provide a new insight into the chemical control of stabilization and destabilization of DOC in soil.