Organic matter chlorination rates in different boreal soils: the role of soil organic matter content

Transformation of chloride (Cl(-)) to organic chlorine (Cl(org)) occurs naturally in soil but it is poorly understood how and why transformation rates vary among environments. There are still few measurements of chlorination rates in soils, even though formation of Cl(org) has been known for two decades. In the present study, we compare organic matter (OM) chlorination rates, measured by (36)Cl tracer experiments, in soils from eleven different locations (coniferous forest soils, pasture soils and agricultural soils) and discuss how various environmental factors effect chlorination. Chlorination rates were highest in the forest soils and strong correlations were seen with environmental variables such as soil OM content and Cl(-) concentration. Data presented support the hypothesis that OM levels give the framework for the soil chlorine cycling and that chlorination in more organic soils over time leads to a larger Cl(org) pool and in turn to a high internal supply of Cl(-) upon dechlorination. This provides unexpected indications that pore water Cl(-) levels may be controlled by supply from dechlorination processes and can explain why soil Cl(-) locally can be more closely related to soil OM content and the amount organically bound chlorine than to Cl(-) deposition.     

Electrochemistry of free chlorine and monochloramine and its relevance to the presence of Pb in drinking water

The commonly used disinfectants in drinking water are free chlorine (in the form of HOCl/OCl-) and monochloramine (NH2Cl). While free chlorine reacts with natural organic matter in water to produce chlorinated hydrocarbon byproducts, there is also concern that NH2Cl may react with Pbto produce soluble Pb(II) products--leading to elevated Pb levels in drinking water. In this study, electrochemical methods are used to compare the thermodynamics and kinetics of the reduction of these two disinfectants. The standard reduction potential for NH2Cl/Cl- was estimated to be +1.45 V in acidic media and +0.74 V in alkaline media versus NHE using thermodynamic cycles. The kinetics of electroreduction of the two disinfectants was studied using an Au rotating disk electrode. The exchange current densities estimated from Koutecky-Levich plots were 8.2 x 10(-5) and 4.1 x 10(-5) A/cm2, and by low overpotential experiments were 7.5 +/- 0.3 x 10(-5) and 3.7 +/- 0.4 x 10(-5) A/cm2 for free chlorine and NH2Cl, respectively. The rate constantforthe electrochemical reduction of free chlorine at equilibrium is approximately twice as large as that for the reduction of NH2Cl. Equilibrium potential measurements show that free chlorine will oxidize Pb to PbO2 above pH 1.7, whereas NH2Cl will oxidize Pb to PbO2 only above about pH 9.5, if the total dissolved inorganic carbon (DIC) is 18 ppm. Hence, NH2Cl is not capable of producing a passivating PbO2 layer on Pb, and could lead to elevated levels of dissolved Pb in drinking water.     

Zinc-sulfur and cadmium-sulfur association in metalliferous peats: evidence from spectroscopy,distribution coefficients,and phytoavailability

Peat deposits can concentrate chalcophilic metals such as Zn and Cd by biogeochemical processes; as a result, there is a possibility that the solubility, mobility, and bioavailability of these metals could increase when such deposits are drained and cropped, initiating oxidation of organic matter and sulfides under aerobic conditions. We use spectroscopic, chemical, and bioassay approaches to characterize high Zn (88-15,800 mg kg(-1)), Cd (0.55-83.0 mg kg(-1)), and S (3.52-9.54 g kg(-1)) peat soils collected from locations in New York State and Ontario that overlie Silurian-age metal-enriched dolomite bedrock. Total and KNO3-extractable trace metals were determined by ICP emission spectrometry, and labile Cd and Zn were measured in the KNO3 extracts by anodic stripping voltammetry. A greenhouse bioassay with maize and canola was conducted to determine the bioavailability and toxicity of the soil Zn and Cd. The electronic oxidation states of sulfur in the peat soils were determined by X-ray absorption near edge spectroscopy (XANES) and Zn and S distribution in soil particles by energy-dispersive X-ray absorption (EDX) spectroscopy. Sulfur-XANES analyses show that a high percentage (35-45%) of the total soil S exists in the most reduced electronic oxidation states (such as sulfides and thiols), while <5% exists in the most oxidized forms (such as sulfate). EDX analyses indicate a microscopic elemental association between Zn and S in these soils. Despite the EDX evidence of close association between Zn and S in soil particles, conventional X-ray diffraction analyses of the bulk soils did not detect a mineral phase of sphalerite (ZnS) in any of the soils. The distribution coefficients (Kd) for Zn and Cd increased with soil pH and indicated stronger Cd retention than Zn in the peats. The results of the bioassaytests showed that most of the high-Zn soils were very phytotoxic, with plant shoot Zn levels exceeding 400 mg kg(-1). Conversely, Cd concentrations in the plant shoots were generally below 2 mg kg(-1), showing a tendency toward low Cd phytoavailability relative to Zn. The information gained from the spectroscopic analyses (S-XANES and EDX) was used to explain the macroscopic observations (Cd and Zn Kd values and phytoavailability data) in these peat soils; we conclude that sulfur biogeochemical cycling may play an important role in Zn and Cd retention in these organic soils.     

Vertical and lateral redistribution of POPs in soils developed along a hydrological gradient

Study of the dispersion of persistent organic pollutants (POPs) throughout the environment is necessitated by their toxicological properties and propensity to accumulate in biota. In this study, we use data from the analysis of three (210)Pb dated soil profiles collected along a 30-m hydrological gradient to demonstrate postdepositional mobility of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) and polychlorinated biphenyls (PCBs). We found that (i) humus originating from litter exposed to surface fallout during the 1960-1970s contains the highest concentrations of PCDD/Fs and PCBs in the O-horizon of the soils; (ii) accumulation rates of PCDD/Fs and PCBs in the O-horizon (～5.0 and ～210 μg m(-2) yr(-1)) constituted only 9.1% and 3.5%, respectively, of the measured annual input, demonstrating that a minor fraction of the deposited material is retained within the O-horizon; (iii) POP inventories in the upper 0.5-0.9 m of the mineral soil constituted a considerable part (40-70%) of the total pool stored in the soil, implying significant vertical translocation of atmospheric derived POPs; and (iv) increasing downslope inventories of POPs suggest a lateral downward transport of POPs from uphill soils. The findings challenge the commonly accepted view that POPs fallout is effectively retained within O-horizons.     

Spatial and vertical distribution of short chain chlorinated paraffins in soils from wastewater irrigated farmlands

Chlorinated paraffins (CPs) are one of the most complex groups of halogenated contaminants in the environment. However, studies of short chain CPs (SCCPs) in China are very scarce. In this study, the concentrations and distribution of SCCPs in farm soils from a wastewater irrigated area in China were investigated. SCCPs were detected in all topsoil samples, with the sum of the concentrations (ΣSCCPs) in the range of 159.9-1450 ng/g (dry weight, dw). A noticeable spatial trend and specific congener distribution were observed in the wastewater irrigated farmland. Soil vertical profiles showed that ΣSCCP concentrations below the plowed layer decreased exponentially and had a significant positive relationship (R(2) > 0.83) with total organic carbon in soil cores. Furthermore, soil vertical distributions indicated that lower chlorinated (Cl(5-6)) and shorter chain (C(10-12)) congeners are more prone to migrate to deeper soil layers compared to highly chlorinated and longer chain congeners. This work demonstrated that effluents from sewage treatment plants (STPs) could be a significant source of SCCPs to the ambient environment and wastewater irrigation can lead to higher accumulation of SCCPs in farm soils.     

Chloride retention and release in a boreal forest soil: effects of soil water residence time and nitrogen and chloride loads

The common assumption that chloride (Cl-) is conservative in soils and can be used as a groundwater tracer is currently being questioned, and an increasing number of studies indicate that Cl- can be retained in soils. We performed lysimeter experiments with soil from a coniferous forest in southeast Sweden to determine whether pore water residence time and nitrogen and Cl- loads affected Cl- retention. Over the first 42 days there was a net retention of Cl- with retention rates averaging 3.1 mg CI- m(-2) d(-1) (68% of the added Cl- retained over 42 days). Thereafter, a net release of Cl- at similar rates was observed for the remaining experimental period (85 d). Longer soil water residence time and higher Cl- load gave higher initial retention and subsequent release rates than shorter residence time and lower Cl- load did. Nitrogen load did not affect Cl transformation rates. This study indicates that simultaneous retention and release of Cl- can occur in soils, and that rates may be considerable relative to the load. The retention of Cl- observed was probably due to chlorination of soil organic matter or ion exchange. The cause of the shift between net retention and net release is unclear, but we hypothesize that the presence of O2 or the presence of microbially available organic matter regulates Cl- retention and release rates.     

大理州植烟土壤肥力质量现状与演变趋势

在大理州乡镇、村、户典型的连片烟田作了调查访问,并采集了506个耕层土样进行系统的化验。结果发现,本州植烟土壤肥力质量的主要指标有蜕化的趋势,土壤有机质平均27.7 g/kg,有下降趋势;pH平均值6.47,呈酸化趋势;有效磷P₂O₅平均27.23 mg/kg,老烟区已累积偏高,且有继续上升趋势,对烟叶生产不利;速效钾K₂O平均189.15 mg/kg,总体水平较高,有上升趋势,但也有水平偏低的;交换性镁Mg平均139.5 mg/kg,约一半土壤偏低;速效硼B0.31 mg/kg,普遍缺乏或偏低;有效钼Mo平均0.24 mg/kg,约1/3缺乏;速效硫、钙、锰、铁、铜含量偏高,而含氯量偏低。烤烟施肥应减氮减磷稳钾,普遍施硼,因土施镁。在土壤pH<5.5时,应适施石灰,调节酸度。有机无机相结合,适当改变氮肥形态防治土壤进一步酸化。实施科学施肥确保植烟土壤可持续利用。      

烟田施用有机肥对土壤理化性状和烟叶香气成分含量的影响

本试验研究了烟田施用不同数量草炭对土壤的有机质含量、理化性状、土壤微生物数量及烟叶中香气成分含量的影响。结果表明,烟田施用有机肥草炭8250~16500 kg/hm2,随着施肥量的增加,土壤有机质、腐殖质含量,土壤真菌、细菌和放线菌数量均增加;烟叶香气成分中总醇量增加,总醛量减少,总酮量稍有增加。      

Solid-solution speciation and phytoavailability of copper and zinc in soils

The soil solution speciation and solid-phase fractionation of copper (Cu) and zinc (Zn) in 11 typical uncontaminated soils of South Australia were assessed in relation to heavy metal phytoavailability. The soils were analyzed for pH (4.9-8.4), soil organic matter content (3.5 to 23.8 g of C kg(-1)), total soil solution metal concentrations, Cu8 (49-358 microg kg(-1)) and Zn8 (121-582 microg kg(-1)), and dissolved organic matter (DOM) (69-827 mg of C L(-1)). The solid-liquid partition coefficient (Kd) ranged from between 13.9 and 152.4 L kg(-1) for Cu and 22.6 to 266.3 L kg(-1) for Zn. The phytoavailability of Cu and Zn could be predicted significantly using an empirical model with the solid-phase fractions of Cu and Zn, as obtained from selective sequential extraction scheme, as components. Phytoavailable Cu and Zn were found to significantly correlate with fulvic complex Cu (r= 0.944, P < 0.0001) and exchangeable Zn (r = 0.832, P = 0.002), respectively. The fulvic complex Cu was found to explain 89.2% of the variation in phytoavailable Cu, where as, the exchangeable Zn together with fulvic complex Zn could explain 78.9% of the variation in phytoavailable Zn. The data presented demonstrate the role of solid-phase metal fractions in understanding the heavy metal phytoavailability. The assessment of the role of solid-phase fractions in heavy metal phytoavailability is a neglected area of study and deserves close attention.     

Origin of PCDDs in ball clay assessed with compound-specific chlorine isotope analysis and radiocarbon dating

Polychlorinated dibenzo-p-dioxins (PCDDs) of high concentrations in a ball clay deposit from the Mississippi Embayment were found to be consistent with a natural abiotic and non-pyrogenic origin by investigation with bulk radiocarbon analysis, compound-specific chlorine isotope analysis (CSIA-delta37Cl) of octachlorodibenzo-p-dioxin (OCDD), and black carbon (BC) analysis. The conventional radiocarbon date of total organic carbon from a depth of approximately 10 m in three parallel cores ranged from 14 700 years to >48 000 years, indicating that the strata with elevated levels of PCDDs have remained isolated from recent anthropogenic input in these >40 Ma old clay sediments. The CSIA-delta37Cl of OCDD yielded a delta37Cl of -0.2 per thousandth, which is significantly higher than the postulated range for biotic chlorination by chloroperoxidase enzymes, -11 to -10 per thousandth, and falls within the known range for abiotic organochlorines, -6 to +3 per thousandth. The absence of correlations between concentrations of PCDDs and corresponding pyrogenic black carbon (BC), together with estimations of BC sorptive loadings and the absence of polychlorinated dibenzofurans (PCDFs), suggest that vegetation fires did not form these ball-clay PCDDs. Results from this study indicate that the high levels of the toxic and carcinogenic PCDDs found in kaolinite-bearing clays may result from natural abiotic formation via in situ surface-promoted reactions on the clay mineral, including a so-far unknown organic precursor, rather than being the result of anthropogenic contamination.     

Long-term dynamics of phosphorus forms and retention in manure-amended soils

Phosphorus (P) leaching from soils with elevated P levels due to manure applications is increasingly becoming a concern as a source of eutrophication of streams and lakes. This study investigates the relationship between organic and inorganic P in soil pools and equilibrium leachate along a chronosequence of poultry and dairy manure additions in New York state. Resin-extractable P (molybdate-reactive P, RP) and total soil P reached very high levels of 2330 and 7343 mg of P kg(-1), respectively, after more than 25 years of continuous manure applications. After long-term manuring, the ability of these soils to retain additional P was low (Langmuir maximum sorption potential of 51-59 g of P kg(-1)) and equilibrium leachate concentrations of total dissolved P (TDP) were high (5.5-7.6 mg L(-1); saturated conditions, 0.15-m lysimeters in closed loop). Total dissolved P concentrations in equilibrium leachate increased linearly (r= 0.737) to a total soil P of 4500 mg kg(-1) and increased to a greater extent above 4500-5500 g kg(-1) (change point equivalent to about 1500 mg kg(-1) Mehlich 3-extractable RP). The proportion of dissolved unreactive P (DUP) in equilibrium leachate decreased from 90% of TDP in fields with a short manure application history to 2% of TDP where mainly poultry manure had been applied for >25 years, while unreactive P (UP) in soil decreased from 44% to 6%. Dissolved RP (DRP) was less mobile than DUP in soils with short duration of manure applications (p < 0.05), while differences between DUP and DRP mobility disappeared with longer duration of manure application and greater total soil P. Organic P forms in NaOH/NaF extracts determined by 31P NMR did not change with manure history, but sequential fractionation showed that the relative distribution of RP pools in soils changed. Dilute acid Pi increased from 10% to 62% with longer poultry manure additions, suggesting the formation of calcium phosphates as the soil pH increased from 4.1 to 6.0-7.2. The precipitation of P as calcium phosphates appeared to influence leachable P upon high and long-term applications of manure dominated by poultry litter.     

Sorption and degradation of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine in soil

The sorption/desorption and long-term fate of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) was examined using sterilized and nonsterilized soils. Two soils were used that differ mainly by the amount of total organic carbon (TOC): an agricultural topsoil (VT, 8.4% TOC) and a sandy soil (SSL, 0.33% TOC). The adsorption isotherms performed at room temperature were well-described by a linear model, which led to sorption distribution coefficients of 2.5 and 0.7 L kg(-1) for VT and SSL soils, respectively. The organic content of soil did not significantly affect HMX sorption. Over a period of 20 weeks, HMX degraded (60% disappearance) in static anaerobic nonsterile VT soil preparations. In separate experiments using UL-[14C]-HMX, 19% mineralization (liberated 14CO2) was obtained in 30 weeks. In addition, four nitroso derivatives of HMX were detected. Knowing the sorption/desorption behavior and the long-term fate of HMX in soil will help assess the effectiveness of natural attenuation for HMX removal.     

Evolution of the concentrations of polycyclic aromatic hydrocarbons in burnt woodland soils

Little is known of the fate of polycyclic aromatic hydrocarbons (PAHs) in soils under burnt woodland. It is not clear what the behavior of the overlying wood ash layer will be along months. In this study, the levels of eight representative PAHs in the 1-5 cm layer of a periurban woodland soil that had undergone wildfire were compared with those measured in nearby and distant unburnt periurban woodland soils and in a distant unburnt rural woodland soil, and the levels at the burnt site were monitored during some 10 months. The analytical method optimized for the purpose afforded recoveries of 74-111% (depending on PAH) and repeatabilities (RSDs) better than 9%, with limits of detection ranging from 1 to 7 microg/kg. PAH levels in the 1-5 cm layer of the burnt periurban soil were very similar to those of distant unburnt periurban soil (188 vs 173 microg/kg), about seven times the 26 microg/kg measured in unburnt rural soil, which furthermore contained no detectable quantities of the highest molecular weight PAHs typical of traffic and other urban sources, as the periurban soils did. At the burnt site, PAH levels fell along the months (the total PAH level from 188 to 119 microg/kg), apparently as the result of rainfall and the prevention of further input from the atmosphere by the overlying layer of wood ash, which had a very high PAH adsorption capacity (1169 microg/kg) and did not itself appear to act as a source of PAHs. PAH transport may have been assisted by increased mobilization of PAHs associated with dissolvable organic matter due to an increase in soil pH due to alkaline ash components.     

Impacts of land use change on nitrogen cycling archived in semiarid unsaturated zone nitrate profiles, southern High Plains, Texas

Nitrate (NO3) profiles in semiarid unsaturated zones archive land use change (LUC) impacts on nitrogen (N) cycling with implications for agricultural N management and groundwater quality. This study quantified LUC impacts on NO3 inventories and fluxes by measuring NO3 profiles beneath natural and rainfed (nonirrigated) agricultural ecosystems in the southern High Plains (SHP). Inventories of NO3-N under natural ecosystems in the SHP normalized by profile depth are extremely low (2-10 kg NO3-N/ha/m), in contrast to those in many semiarid regions in the southwestern U.S. Many profiles beneath cropland (9 of 19 profiles) have inventories at depth that range from 28-580 kg NO3--N/ha/m (median 135 kg/ha/m) that correspond to initial cultivation, dated using soil water Cl. These inventories represent 74% (median) of the total inventories in these profiles. This NO3 most likely originated from cultivation causing mineralization and nitrification of soil organic nitrogen (SON) in old soil water (precultivation) and is attributed to enhanced microbial activity caused by increased soil wetness beneath cropland (median matric potential -42 m) relative to that beneath natural ecosystems (median -211 m). The SON source is supported by isotopes of NO3 (delta15N: +5.3 to +11.6; delta18O: +3.6 to +12.1). Limited data in South Australia suggest similar processes beneath cropland. Mobilization of the total inventories in these profiles caused by increased drainage/ recharge related to cultivation in the SHP could increase current NO3-N levels in the underlying Ogallala aquifer by an additional 2-26 mg/L (median 17 mg/L).     

Dissolved free and combined amino acids in surface runoff and drainage waters from drained and undrained grassland under different fertilizer management

Organic matter is a valuable resource on which the sustainability and productivity of soils relies heavily. Thus, it is important to understand the mechanisms for the loss of organic compounds from soil. It is also essential to determine how these losses can be minimized, especially those resulting from anthropogenic activity. Grazed grassland lysimeters (1 hectare) were used to examine the contribution and distribution patterns of dissolved free and combined amino acids to dissolved organic nitrogen and carbon in surface runoff and drainage waters from a grassland soil over three winter drainage periods. The waters were collected from soils beneath drained and undrained permanent ryegrass swards, receiving 0 and 280 kg ha(-1) year(-1) mineral nitrogen (N) input. Total dissolved free amino acid (DFAA) and dissolved combined amino acid (DCAA) concentrations ranged between 1.9 nM and 6.1 microM and between 1.3 and 87 microM, respectively. Although addition of mineral N fertilizer increased both DFAA and DCAA concentrations in waters, there was no detectable effect of soil hydrology or fertilizer addition on distribution patterns.     

Effect of a nonionic surfactant on biodegradation of slowly desorbing PAHs in contaminated soils

The influence of the nonionic surfactant Brij 35 on biodegradation of slowly desorbing polycyclic aromatic hydrocarbons (PAHs) was determined in contaminated soils. We employed a soil originated from a creosote-polluted site, and a manufactured gas plant soil that had been treated by bioremediation. The two soils differed in their total content in five indicator 3-, 4-, and 5-ring PAHs (2923 mg kg(-1) and 183 mg kg(-1) in the creosote-polluted and bioremediated soils, respectively) but had a similar content (140 mg kg(-1) vs 156 mg kg(-1)) of slowly desorbing PAHs. The PAHs present in the bioremediated soil were highly recalcitrant. The surfactant at a concentration above its critical micelle concentration enhanced the biodegradation of slowly desorbing PAHs in suspensions of both soils, but it was especially efficient with bioremediated soil, causing a 62% loss of the total PAH content. An inhibition of biodegradation was observed with the high-molecular-weight PAHs pyrene and benzo[a]pyrene in the untreated soil, possibly due to competition effects with other solubilized PAHs present at relatively high concentrations. We suggest that nonionic surfactants may improve bioremediation performance with soils that have previously undergone extensive bioremediation to enrich for a slowly desorbing profile.     

Solid-solution partitioning of Cd, Cu, Ni, Pb, and Zn in the organic horizons of a forest soil

We report the solid-liquid partitioning of Cd, Cu, Ni, Pb, and Zn in 60 organic horizon samples of forest soils from the Hermine Watershed (St-Hippolyte, PQ, Canada). The mean Kd values are respectively 1132, 966, 802, 3337 and 561. Comparison of those Kd coefficients to published compilation values show that the Kd values are lower in acidic organic soil horizons relative to the overall mean Kd values compiled for mineral soils. But, once normalized to a mean pH of 4.4, the Kd values in organic soil horizons demonstrate the high sorption affinity of organic matter, which is either as good as or up to 30 times higher than mineral soil materials for sorbing trace metals. Regression analysis shows that, within our data set, pH and total metal contents are not consistent predictors of metal partitioning. Indeed, metal sorption by the solid phase must be studied in relation to complexation by dissolved organic ligands, and both processes may sometime counteract one another.     

Solid- and solution-phase organics dictate copper distribution and speciation in multicomponent systems containing ferrihydrite, organic matter, and montmorillonite

Copper retention by ferrihydrite, leaf compost, and montmorillonite was studied over 8 months in systems that emulate a natural soil where different solid phases compete for Cu through a common solution in a compartmentalized batch reactor. Copper speciation in solution (total dissolved, DPASV-labile, and free) and exchangeable and total Cu in individual solid phases were determined. Organic carbon in solution (DOC) and that retained by the mineral phases were also determined. Cu sorption reached steady-state after 4 months and accounted for 80% of the Cu initially added to the system (0.15 mg L(-1)). The remaining 20% stayed in solution as nonlabile (82.8%), labile (17%), and free (0.2%) Cu species. Copper sorption followed the order organic matter > silicate clays > iron oxides. Within each solid phase, exchangeable Cu was < or = 10% of the total Cu sorbed. DOC reached steady state (22 mg L(-1)) after 4 months and seemed to control Cu solubility and sorption behavior by the formation of soluble Cu-DOC complexes and by sorbing onto the mineral phases. DOC sorption onto ferrihydrite prevented Cu retention by this solid phase. Using a multicomponent system and 8 months equilibrations, we were able to capture some of the more important aspects of the complexity of soil environments bytaking into account diffusion processes and competition among solid- and solution-phase soil constituents in the retention of a metal cation.