Physico-chemical and biological controls on dissolved organic matter in peat aggregate columns

We investigated the importance of physico‐chemical mechanisms responsible for the release of dissolved organic matter (DOM) from a peaty soil. Columns containing peat aggregates (embedded within a sand matrix) provided an experimental system in which both convective and diffusive processes contributed to DOM leaching. The use of aggregated peat avoided the problems associated with traditional batch equilibration experiments in which soil structure is destroyed. Biotic and abiotic processes operating in the columns were manipulated by working with two unsterilized columns (at 5°C and 22°C) and one gamma irradiation‐sterilized column (5°C). Continuous solute flows (< 80 hours) and periods of flow interruption (five interruptions of 6 hours to 384 hours) were applied to the columns (using a 1‐mm NaCl electrolyte) to investigate mechanisms of diffusion‐controlled release of DOM. For all columns, dissolved organic carbon and nitrogen (DOC and DON) effluent concentrations increased after resumption of flow and the maximum concentrations increased with increased flow‐interruption duration. Measurements of effluent UV absorbance (λ= 285 nm) showed that the DOM leached immediately after the flow interruptions contained fewer aromatic moieties of lower molecular weight than the DOM leached after periods of steady flow. The sterilized column had larger DOC and DON effluent concentration spikes than those from the unsterilized column at 5°C (38 mg C dm⁻³ and 6.5 mg N dm⁻³ versus 13 mg C dm⁻³ and 6.5 mg N dm⁻³ after the 384 hours flow interruption). This result suggested that the concentrations of DOM resulting from physico‐chemical release mechanisms (sterilized column) were attenuated by biological activity (unsterilized columns). Our results indicate that the peat’s microporous structure provides reservoirs of DOM that interact with solute in transport pores via abiotic, rate‐controlled mass transport. Hence, diffusion can influence the quantity and composition of DOM leached from peat in the field depending on intensity and duration of rainfall.     

Variation in the sensitivity of DOC release between different organic soils following H2SO4 and sea‐salt additions

Long‐term monitoring data from eastern North America and Europe indicate a link between increased dissolved organic carbon (DOC) concentrations in surface waters over the last two decades and decreased atmospheric pollutant and marine sulphur (S) deposition. The hypothesis is that decreased acidity and ionic strength associated with declining S deposition has increased the solubility of DOC. However, the sign and magnitude of DOC trends have varied between sites, and in some cases at sites where S deposition has declined, no significant increase in DOC has been observed, creating uncertainty about the causal mechanisms driving the observed trends. In this paper, we demonstrate chemical regulation of DOC release from organic soils in batch experiments caused by changes in acidity and conductivity (measured as a proxy for ionic strength) associated with controlled SO₄^2? additions. DOC release from the top 10 cm of the O‐horizon of organo‐mineral soils and peats decreased by 21–60% in response to additions of 0–437 µeq SO₄^2? l^?1 sulphuric acid (H₂SO₄) and neutral sea‐salt solutions (containing Na⁺, Mg²⁺, Cl^?, SO₄^2?) over a 20‐hour extraction period. A significant decrease in the proportion of the acid‐sensitive coloured aromatic humic acids (measured by specific ultra‐violet absorbance (SUVA) at 254 nm) was also found with increasing acidity (P < 0.05) in most, but not all, soils, confirming that DOC quality, as well as quantity, changed with SO₄^2? additions. DOC release appeared to be more sensitive to increased acidity than to increased conductivity. By comparing the change in DOC release with bulk soil properties, we found that DOC release from the O‐horizon of organo‐mineral soils and semi‐confined peats, which contained greater exchangeable aluminium (Al) and had lower base saturation (BS), were more sensitive to SO₄^2? additions than DOC release from blanket peats with low concentrations of exchangeable Al and greater BS. Therefore, variation in soil type and acid/base status between sites may partly explain the difference in the magnitude of DOC changes seen at different sites where declines in S deposition have been similar.     

Effects of throughfall and litterfall manipulation on concentrations of methylmercury and mercury in forest‐floor percolates

The forest floor was shown to be an effective sink of atmospherically deposited methylmercury (MeHg) but less for total mercury (Hg_total). We studied factors controlling the difference in dynamics of MeHg and Hg_total in the forest floor by doubling the throughfall input and manipulating aboveground litter inputs (litter removal and doubling litter addition) in the snow‐free period in a Norway spruce forest in NE Bavaria, Germany, for 14 weeks. The MeHg concentrations in the forest‐floor percolates were not affected by any of the manipulation and ranged between 0.03 (Oa horizon) and 0.11 (Oi horizon) ng Hg L^–1. The Hg_total concentrations were largest in the Oa horizon (24 ng Hg L^–1) and increased under double litterfall (statistically significant in the Oi horizon). Similarly, concentrations of dissolved organic C (DOC) increased after doubling of litterfall. The concentrations of Hg_total and DOC correlated significantly in forest‐floor percolates from all plots. However, we did not find any effect of DOC on MeHg concentrations. The difference in the coupling of Hg_total and MeHg to DOC might be one reason for the differences in the mobility of Hg species in forest floors with a lower mobility of MeHg not controlled by DOC.     

Physicochemical factors controlling the release of dissolved organic carbon from columns of forest subsoils

Retention of dissolved organic carbon in soil depends on the chemical and physical environment. We studied the release of organic carbon from three carbonate-free forest subsoil materials (Bs1, Bs2, Bg) in unsaturated column experiments as influenced by (i) variations of the flow regime and (ii) varied chemical properties of the irrigation solution. We investigated the effect of flow initiation, constant irrigation, interruptions to flow, and variation in the effective pore water velocity on the release of organic C. The influence of ionic strength and cation valence in the irrigation solution was studied by stepped pulses of NaCl and CaCl₂. The release of C from all materials was characterized by an initial large output and a decline to constant concentrations under long-term irrigation. Interrupting the flow increased its release when flow was resumed. The release from the Bs1 material was not related to the duration of the interruption. The Bs2 material, in contrast, released organic carbon in a way that was successfully described by a kinetic first-order model. Increased pore water velocity decreased the concentrations of C in the effluent from it. The pH of the irrigation solution had negligible effects on the mobilization of C. Increased ionic strength reduced the release, whereas rinsing with distilled water increased the concentrations of C in the effluent. The response of dissolved C to pulses of weak solutions, however, was sensitive to the type of cation in the previous step with strong solutions. The results suggest that the release of organic matter in the soils depends on its colloidal properties.     

Effects of acid sulphate on DOC release in mineral soils: the influence of SO42− retention and Al release

Dissolved organic carbon (DOC) in acid‐sensitive upland waters is dominated by allochthonous inputs from organic‐rich soils, yet inter‐site variability in soil DOC release to changes in acidity has received scant attention in spite of the reported differences between locations in surface water DOC trends over the last few decades. In a previous paper, we demonstrated that pH‐related retention of DOC in O horizon soils was influenced by acid‐base status, particularly the exchangeable Al content. In the present paper, we investigate the effect of sulphate additions (0–437 µeq l^?1) on DOC release in the mineral B horizon soils from the same locations. Dissolved organic carbon release decreased with declining pH in all soils, although the shape of the pH‐DOC relationships differed between locations, reflecting the multiple factors controlling DOC mobility. The release of DOC decreased by 32–91% in the treatment with the largest acid input (437 µeq l^?1), with the greatest decreases occurring in soils with very small % base saturation (BS, < 3%) and/or large capacity for sulphate (SO₄^2?) retention (up to 35% of added SO₄^2?). The greatest DOC release occurred in the soil with the largest initial base status (12% BS). These results support our earlier conclusions that differences in acid‐base status between soils alter the sensitivity of DOC release to similar sulphur deposition declines. However, superimposed on this is the capacity of mineral soils to sorb DOC and SO₄^2?, and more work is needed to determine the fate of sorbed DOC under conditions of increasing pH and decreasing SO₄^2?.     

Effect of long-term irrigation with untreated sewage effluents on soil properties and heavy metal adsorption of leptosols and vertisols in Central Mexico

The effect of long-term irrigation with untreated sewage effluents from Mexico City on soil properties and heavy metal adsorption behaviour of soils at Irrigation District 03 in the Mezquital Valley, Central Mexico, was studied. General soil parameters of 25 samples from Ap-horizons of Mollic Leptosols and Eutric Vertisols taken at sites which have been under irrigation for different periods of time were compared with samples from fields under rainfed agriculture. The adsorption of Pb, Cd, Cu and Zn was analysed in 8 selected samples by batch experiments. The long-term irrigation of soils with untreated sewage effluent has increased the contents of total (TOC) and dissolved (DOC) organic carbon in Ap-horizons of Leptosols and Vertisols, and diminished the contents of manganese oxides in Vertisols. This influences the heavy metal adsorption behaviour of both soils, since DOC enhances metal solubility at low loading rates and TOC improves metal adsorption capacities at high loading rates. The possibility of an increase in the chloride content in soils due to wastewater irrigation and its relation to higher Cd mobility are discussed, as is the importance of humus balance control through oriented management practices in order to minimize heavy metal mobility in soils, which have been under irrigation with wastewater during long periods.     

Modeling ferricyanide adsorption on goethite: Basics and applications

Ferricyanide, [Fe^III(CN)₆]^3–, is an anthropogenic and potentially toxic contaminant in soil. Its adsorption on goethite has been previously studied, but not evaluated with a surface complexation model (SCM) considering the effects of pH and ionic strength. Therefore, we carried out batch experiments with ferricyanide and goethite suspensions with different ferricyanide concentrations (0.075 mM and 0.15 mM), ionic strengths (0.01 and 0.1 M), and pH (ranging from 4 to 7.4). Adsorption data were then interpreted with the 1‐pK Stern and the charge distribution model assuming monodentate inner‐sphere ferricyanide surface complexes on goethite (lg K = 10.6), which are known from infrared spectroscopy. Furthermore, we applied the SCM to ferricyanide adsorption in previous studies on ferricyanide adsorption in the presence of sulfate and on the solubility of Fe‐cyanide complexes in a suspension of a loess loam. The SCM correctly reflected ferricyanide adsorption in the batch experiments as well as the effects of pH and ionic strength. The SCM also described ferricyanide adsorption in the presence of sulfate, because the ferricyanide adsorption measured and that modeled were significantly correlated (R² = 0.80). Furthermore, we applied the SCM to a study on the solubility of Fe‐cyanide complexes in soil under varying redox conditions so that ferricyanide adsorption on goethite and precipitation of Fe‐cyanide complexes were considered. The actual ferricyanide concentrations were rather reflected when applying the SCM compared to those modeled in an approach in which exclusively precipitation was taken into account. We conclude that ferricyanide adsorption on goethite should be included into geochemical modeling approaches on the mobility of Fe‐cyanide complexes in subsoils.     

General purpose Freundlich isotherms for cadmium,copper and zinc in soils

Assessing the accumulation and transport of trace metals in soils and the associated toxicological risks on a national scale requires generally applicable sorption equations. Therefore Freundlich equations were derived for Cd, Zn and Cu using multiple linear regression on batch sorption data from the literature with a wide variety of soil and experimental characteristics, and metal concentrations ranging over five orders of magnitude. Equations were derived based on both total dissolved metal concentrations and free metal activities in solution. Free metal activities were calculated from total metal concentrations taking into account ionic activity, and inorganic (all metals) and organic complexation (Cu only). Cadmium and Zn were present in solution predominantly as free ions, while Cu was present as organic complexes. Since actual dissolved organic carbon (DOC) concentrations were not available they were estimated using an empirical field relation between DOC and organic matter content. The logarithmic transformation of the Freundlich constant for Cd was regressed on the logarithmic transformations of cation exchange capacity (CEC) (H⁺) and dissolved Ca, and for Zn with CEC and (H⁺). For Cu the log–log regression model of the Freundlich constant included the solid:solution ratio of the batch to account for dilution of DOC in the batch as compared with the field. The explained variance for the fitted Freundlich equations was 79% for Cd, 65% for Cu and 83% for Zn, using log-transformed adsorbed concentrations and soil solution activities. The Freundlich adsorption models underestimated metal contents determined from 1 m HNO₃ digestion on field samples, up to a factor of 6 (Cd and Cu) or 10 (Zn).     

Low‐molecular‐weight organic acids enhance desorption of polycyclic aromatic hydrocarbons from soil

The impact of low‐molecular‐weight organic acids (LMWOAs) on desorption of phenanthrene and pyrene as representative polycyclic aromatic hydrocarbons (PAHs) from a contaminated soil was investigated by using a laboratory batch experiment. Three LMWOAs were used in this study and were citric, oxalic and malic acids. The LMWOAs in aqueous solution promoted desorption of PAHs from soil significantly and demonstrated an increasing trend as the concentration of LMWOAs increased. When compared with desorption of phenanthrene and pyrene from soil to water, the addition of LMWOAs enhanced desorption of test PAHs by up to 285 and 299%, respectively. Among the three LMWOAs studied, citric acid demonstrated the greatest efficiency in promoting PAH desorption from soil. Solutions of LMWOA continuously promoted PAH removal from soil during the multiple cycles of desorption. Overall, the experimental results suggest that LMWOAs in aqueous solution could disrupt soil organic matter (SOM)–metal cation–mineral linkages in soils, resulting in the release of SOM from soil and simultaneous increase of dissolved organic carbon (DOC) in solution. The loss of SOM from soil and increase of DOC in solution are responsible for the enhanced PAH desorption from soil. The positive correlation between DOC in solution and desorbed PAHs from soil suggests that the loss of SOM from soil plays an important role in LMWOA‐enhanced desorption of PAHs from soil.     

Cyanides in a soil of a former coking plant site

Soils of former coking plant sites are frequently contaminated with cyanide, which mainly occurs as the pigment Berlin blue, Fe^III4 [Fe^IICN)₆]₃, and soluble iron cyanide complexes, Fe(CN)₆^3-/4?. Berlin blue is only slightly soluble under acidic conditions. The cyanide mobility in a strongly acid soil (pH about 3) of such a site was studied by assessing the distribution of cyanides in the soil and sediment and by conducting batch experiments at different pH levels. The soil is based on a disposal layer (0–32 cm) overlying sandy loess (32–145 cm) overlying glaciofluvial sand (145–250 cm) overlying marlstone (250–500 cm). Highest cyanide concentrations were found in the disposal layer (62–2865 mg CN^? kg^?1), medium concentrations in the sandy loess (16–29 mg CN^? kg^?1), concentrations of about 100 mg CN^? kg^?1 in the glaciofluvial sand and lowest concentrations in the marlstone (0.22–0.49 mg CN^?1 kg^?1). The surfaces of macropores, which occur in the sandy loess and finish in the glaciofluvial sand, are dark-blue dyed and have much higher cyanide concentrations than the surrounding bulk soil. Thus, the accumulation of cyanides in the sand layer may partly be the result of macropore flow. Batch experiments show a strong pH dependence of the solubility of cyanide in the soil as well as of Berlin blue which was found by Mössbauer spectroscopy to be the dominating or sole iron cyanide. The time necessary to transport the cyanides solely as hexacyanoferrate into the undisturbed horizons is estimated to 1000 yr. However, because Berlin blue is known to form colloids, we discuss the possibility of cyanide transport as colloid not requiring dissolution and reprecipitation. We postulate that colloidal Berlin blue transported by macropore flow is responsible for the high mobility of cyanides in this acid soil.     

Simple modelling of the sorption of iron‐cyanide complexes on ferrihydrite

The sorption of the iron‐cyanide complexes ferricyanide, [Fe(CN)₆]^3—, and ferrocyanide, [Fe(CN)₆]^4—, on ferrihydrite was investigated in batch experiments including the effects of pH (pH 3.5 to 8) and ionic strength (0.001 to 0.1 M). The pH‐dependent sorption data were evaluated with a model approach by Barrow (1999): c = a exp(bS)S/(S_max‐S), where c is the solution concentration; S is the sorbed amount; S_max is maximum sorption; b is a parameter; and a is a parameter at constant pH. Ferricyanide sorption was negatively affected by increasing ionic strength, ferrocyanide sorption not at all. More ferricyanide than ferrocyanide was sorbed in the acidic range. In the neutral range the opposite was true. Fitting the pH‐dependent sorption to the model resulted in a strong correlation for both iron‐cyanide complexes with a common sorption maximum of 1.6 μmol m^—2. Only little negative charge was conveyed to the ferrihydrite surface by sorption of iron‐cyanide complexes. The sorption of iron‐cyanide complexes on ferrihydrite is weaker than that on goethite, as a comparison of the model calculations shows. This may be caused by the lower relative amount of high‐affinity sites present on the ferrihydrite surface.     

Leaching of carbon and nitrogen from a sandy soil substrate: A comparison between suction plates and ion exchange resins

To determine boundary effects on leaching, we investigated (1) how filter materials affect the concentrations of dissolved organic carbon (DOC) and nitrate (NO₃‐N) in soil percolates and (2) whether ion exchange resins and suction plates are equally suited to capture NO₃‐N. DOC leaching was higher with PE suction plates and plate material did not affect NO₃‐N leachate concentrations. Cumulative NO₃‐N leaching was similar for glass suction plates and ion exchange resins.     

The UV‐absorbance of dissolved organic matter predicts the fivefold variation in its affinity for mobilizing Cu in an agricultural soil horizon

It is well established that dissolved organic matter (DOM) mobilizes copper (Cu) in soils but it is unknown to what extent variable DOM quality affects this. During a 5 month period, 250 leachates of an uncontaminated agricultural soil were sampled at 45 cm depth using passive capillary wick samplers. The dissolved Cu and organic carbon (DOC) concentrations varied sevenfold and were weakly correlated (r = 0.56). The [Cu] : [DOC] ratio varied fivefold and exhibited a significant positive correlation (r = 0.77) with the specific UV‐absorbance of DOM at 254 nm (SUVA), indicating that the more aromatic DOM had higher Cu affinity. The dissolved Cu concentrations were predicted by an assemblage model in WHAM6 using the composition of the solid phase above the wick samplers and that of the solution, including DOC. The predicted [Cu] : [DOC] ratio was almost constant when assuming default DOM properties with 65% of all DOM active as fulvic acid (%AFA). The %AFA was subsequently varied proportionally to the SUVA of DOM and using the SUVA of pure FA (SUVA_FA) as a fitting parameter. In that case, the variation in the predicted [Cu] : [DOC] ratio was much larger and the predicted Cu concentrations were within a factor of 1.4 of the measured values for 90% of the samples. The fitted SUVA_FA was 38 l g^?1 cm^?1, in excellent agreement with that of Suwannee River FA (SUVA_FA = 37 l g^?1 cm^?1). It is concluded that the DOM quality, e.g. the aromaticity, should be taken into account when estimating Cu mobility in soils.     

Fate of anthracene in contaminated soil: transport and biochemical transformation under unsaturated flow conditions

To obtain reliable estimates for the loss of polycyclic aromatic hydrocarbons (PAHs) from contaminated soils, one has to distinguish between (i) losses due to release and solute transport and (ii) losses resulting from degradation. We studied the interplay of these processes in a column experiment representing a typical soil contamination scenario: in the upper part of the column was a contaminated layer, spiked with 9‐¹³C‐labelled anthracene, and beneath it uncontaminated pristine soil. The experimental course comprised a steady‐state flow phase (constant irrigation for 4 months) followed by several periods during which flow was halted. The effects of varied residence time on anthracene biodegradation and on anthracene mass transfer were investigated. We monitored labelled anthracene and its transformation products, dissolved organic carbon, electric conductivity (EC), pH, and inorganic carbonate content in the column effluent, and the CO₂ evolved. Under steady‐state flow, pH, dissolved organic C, and EC approached steady states after 350 pore volumes. Concentrations of anthracene in the effluent, however, increased continuously and levelled off after 800 pore volumes. This marked retardation reflects the great affinity of anthracene to soil organic matter. The response to interruptions in the flow revealed that mass is transferred without equilibrium between solid and liquid phase for both anthracene and dissolved organic C. Thus, residence time is one factor controlling the concentration of anthracene in the effluent and therefore the export of contaminant to the aquifer. In the course of the experiment several labelled anthracene degradation products appeared in the effluent. At least three of them were identified as transformation products showing a dramatic increase in mobility relative to the parent compound. A third of the overall anthracene loss from the column was due to solute transport, and biodegradation was responsible for the remaining two thirds. The incomplete degradation of anthracene leads to the formation of highly mobile transformation products and thus promotes the export of carbon, derived from the contaminant.     

Different effect of drying on the fluxes of dissolved organic carbon and nitrogen from a Norway spruce forest floor

The forest floor represents the major source of dissolved organic carbon (DOC) and nitrogen (DON) in forest soils. The release mechanisms of DOC and DON from forest floors and their environmental controls as well as the dynamics of concentrations and fluxes are still poorly understood. We investigated the effect of drying and rewetting on the release of DOC and DON from a Norway spruce forest floor. Undisturbed soil columns of 17 cm diameter and 15—20 cm height were taken with 7 replicates from the forest floor of a mature Norway spruce (Picea abies [L.] Karst.) site and established at 10°C in the laboratory. Columns were exposed to different periods of drying (3, 5, 10, 20 days). Each drying period was followed by a rewetting for 5 days at an irrigation rate of 10 mm d^—1 with a natural throughfall solution. The percolates from the forest floor were collected daily and analyzed for DOC, total N, NH₄, NO₃, pH, electrical conductivity and major ions. Drying for 10 and 20 days decreased the water content of the Oi horizon from 280% dry weight to about 30%. The water content of the Oe and the Oa horizon only changed from about 300% to 200%. The fluxes of DOC from the forest floor were moderately effected by drying and rewetting with an increase after 3 and 5 days of drying, but a decrease after 10 and 20 days. On the contrary, the drying for 10 and 20 days resulted in a drastic increase of the DON fluxes and a subsequent decrease of the DOC/DON ratios in the forest floor percolates from about 50 to 3.3. These results suggest that the mechanisms for DOC release in forest floors differ from those for DON and that drying and rewetting cause temporal variations in the DOC/DON ratios in forest floor percolates.     

Relationship between rate of carbon dioxide evolution,microbial biomass carbon,and amount of dissolved organic carbon as affected by temperature and water content of a forest and an arable soil

Abstract

Using an Ochrept soil of a forest at climax stage or of an arable site at Kita‐Ibaraki, a city in central Japan, the rates of carbon dioxide (CO₂)‐carbon (C) evolution, the amounts of microbial biomass carbon (MBC) and the amounts of dissolved organic carbon (DOC) were measured in a laboratory with special reference to the incubation temperature and the soil water content. The rates of CO₂‐C evolution increased exponentially with increase in the incubation temperature in the range of 4–40°C. The temperature coefficients (Q₁₀) were 2.0 for the forest and 1.9 for the arable soil. The amounts of MBC were almost constant of 980 μg g^‐1 soil in the incubation temperature up to 25°C for the forest, and 340 μg g^‐1 soil in the incubation temperature up to 31 °C for the arable soil. The amounts of DOC in soil solutions were almost constant at 3.1 μg g^‐1 soil in the incubation temperature up to 25°C for the forest, and 3.8 μg g^‐1 soil in the incubation temperature up to 31°C for the arable soil. The rates of CO₂‐C evolution and the amounts of DOC increased with increase in soil water content (% of soil dry weight) up to 91% for the forest or up to 26% for the arable soil. However, the rates of CO₂‐C evolution and the amounts of DOC were almost constant within soil water content in the range of 91–160% or 26–53%, respectively. The amounts of MBC of the forest or arable soil were almost constant over a wide range of soil water content in the range of 41–220% or 8–73%, respectively. The rates of CO₂‐C evolution of both the forest and the arable soils were highly correlated with the amounts of DOC, but not with the amounts of MBC, under laboratory conditions in the case that the amounts of DOC were changed by various treatments. The regression equation,     

Batch‐Determined Elements Release from Wood Ash Mixed with an Acidic Forest Soil Sample

Abstract

A serial batch leaching experiment was carried out to evaluate the release of elements from wood ash mixed with a strongly acidic forest soil sample. Wood ash application resulted in increased leachate pH, dissolved organic carbon (DOC), and electrical conductivity (EC). Increasing application of wood ash increased cumulative release of inorganic carbon (IC), chloride (Cl), nitrate (NO₃), sulfate (SO₄), potassium (K), sodium (Na), calcium (Ca), magnesium (Mg), manganese (Mn), phosphorus (P), and copper (Cu). Release of NO₃, P, iron (Fe), aluminum (Al), Cu, and lead (Pb) continued. Large amounts of DOC, K, Ca, and SO₄ were mobilized. Inorganic C, Fe, and Mg were released in moderate quantities. Manganese, Na, Al, Cl, and NO₃ were released in limited amounts. Amounts of leached P, Pb, and Cu were lower. The mixed order equation adequately described the release of elements in the soil‐ash mixture. Accumulation of elevated amounts of trace elements does not appear to be a problem when higher wood ash rates are avoided. Wood ash should be applied in split application to avoid short‐term concentrated alkaline and salty conditions that could interfere with plant growth.     

Biochar addition enhanced growth of Dactylis glomerata L. and immobilized Zn and Cd but mobilized Cu and Pb on a former sewage field soil

The mobility and bioavailability of heavy metals in soils contaminated by irrigation with wastewater increase with increasing mineralization of accumulated organic substance and decreasing pH. In laboratory experiments addition of biochar reduced heavy metal availability to plants and enhanced plant growth. However, literature from field trials is scarce. Therefore, we conducted a 2‐year field experiment with orchard grass (Dactylis glomerata L.) to study the effects of miscanthus‐derived biochar applied to sewage field soil on biomass production and concentrations of zinc (Zn), copper (Cu), cadmium (Cd), lead (Pb) and various nutrients in plants and in the soil solution. Biochar was mixed into the contaminated topsoil (30 cm) with addition rates of 0, 1, 2.5 and 5% on a dry‐mass basis (g 100 g^?1). The soil solution was collected with suction plates installed at a depth of 30 cm. Addition of biochar increased biomass production and reduced Zn and Cd concentrations in the soil solution. Zinc concentrations were also reduced in plants. This effect seems to be attributable to an increase in pH caused by biochar addition. In contrast, Pb and Cu concentrations in the soil solution generally increased and were related to the concentrations of dissolved organic carbon (DOC). Copper concentrations also increased in the plants; however, only at the beginning of the measurement period. Our data indicate that increased concentrations of DOC, Cu and Pb in the soil solution might be a transient effect. Therefore, further research is needed to determine the long‐term effect of biochar amendment on element immobilization and leaching into groundwater.     

Polyzyklische Aromatische Kohlenwasserstoffe (PAKs) in den Böden der Rieselfelder der Stadt Münster (Westfalen)

Polycyclic Aromatic Hydrocarbons (PAHs) in soils of the Münster (Westfalia) irrigation fields From 1901 to 1975 municipal sewage waters with a daily amount of finally about 40.000 m³ were seeped in the irrigation fields of the city of Münster (Westfalia). Soil samples of former seeped sites and non-seeped neighbour sites have been examined with regard to 6 polycyclic aromatic hydrocarbons (PAHs) of the German “drinking water regulation”. The irrigation caused an enrichment of PAH in the topsoils of the former irrigation basins up to an average concentration of 0,57 mg PAH/kg soil, while the topsoils outside of the irrigation fields show an average concentration of only 0,14 mg PAH/kg soil. Significant correlations between different soil properties, e.g. the organic matter content, and the concentration of PAHs are not evident. After termination the irrigation of sewage water, single irrigation bassins were filled with substratum, which is contaminated with scoriaceous materials. The average PAH concentration of such an area amounts to 7 mg PAH/kg topsoil. The PAH distribution in a soil profile shows a decreasing concentration by depth but a clear enrichment in oxidized gleyic horizons (Go) of different age. Probably the PAHs were leached under recent conditions by chemical bond to dissolved organic carbon (DOC) and finally adsorbed by Fe oxides. Since the end of irrigation Fluoranthene, the most mobile PAH compound, was leached down to 110 cm depth. A contamination of the superficial groundwater at such sites cannot be excluded.     

Plant roots are more important than temperature in modulating carbon release in a limed acidic soil

Lime is a common amendment to overcome soil acidity in agricultural production systems. However, plant root effects on lime and soil carbon (C) dynamics in acidic soils under varied temperature remain largely unknown. We monitored root effects of soybean on the fate of lime applied to an acidic soil at 20 and 30°C in growth chambers. Soil respired CO₂ was continuously trapped in columns without and with plants until the final stage of vegetative growth. Lime‐derived CO₂ was separated from total respired CO₂ based on δ¹³C measurements in CO₂. Leaching was induced at early and late vegetative growth stages, and the leachates were analysed for dissolved organic (DOC) and inorganic C (DIC) concentrations. Soil respiration significantly increased with lime addition at both temperatures (p < 0.001). The presence of soybean doubled the recovery of lime‐derived CO₂‐C at 20°C at the early growth stage; however, by the end of the experiment, the contribution of lime‐derived CO₂‐C to soil respiration was negligible in all treatments, indicating that the contribution of lime to soil respiration was shortlived. In contrast, DIC and DOC concentrations in leachates remained elevated with liming and were greater in the presence of soybean. We observed no main temperature effects and no interactive effects of temperature and soybean presence on lime‐derived CO₂‐C, DIC and DOC. These results highlight the role of plant‐modulated processes in CO₂ release and C leaching from lime in acidic soils, whereas an increase in temperature may be less important. Temperature and plant roots alter the rate of key processes controlling C dynamics in a limed acidic soil. Lime‐derived CO₂‐C, DIC and DOC increased more in the presence of plants than with increased temperature. Root effects are more important than temperature for inorganic and organic carbon dynamics in limed acidic soils.