Sorption of Dissolved Organic Carbon by Ceramic P 80 Suction Cups

Comparison is made between the chemical composition of acid soil solutions percolated through new, acid-washed ceramic P 80 suction cups, and old, over 3 years field-equilibrated suction cups with respect to quantitative and qualitative changes of dissolved organic carbon (DOC). While new suction cups sorb DOC in significant amounts with hydrophobic constituents preferred, field-equilibrated suction cups alter DOC neither in concentration nor in composition. But at changes of DOC concentrations a percolation volume of 300 ml is necessary for reaching equilibrium. It is, therefore, concluded that field-equilibrated ceramic P 80 suction cups can be used for collecting DOC from mineral B and C horizons of acid forest soils, where DOC concentrations remain constant. In contrast, the suction cups investigated are unsuitable for collecting A horizon solutions, which show greater variations in DOC concentration.     

Sorption of trace metals by suction cups of aluminium oxide,ceramic and plastics

The sorption properties of ceramic, aluminium oxide and plastic suction cups in respect to trace metals (Be, Cd, Co, Cu, Mn, Ni, Pb, Zn) were compared in laboratory and field experiments. The sorption effect is determined by the level of the cation exchange capacity of the cup material, the pH-value of the soil solution, the content of dissolved organic carbon, the sampling rate and the sampled volume. Sorption was generally negligible only in case of cobalt, manganese and nickel. At low pH-values no retention of trace metals occurred with the exception of lead in the aluminium oxide and the ceramic cups. At pH-values of about 8 cadmium and zinc were strongly sorbed only by aluminium oxide and ceramic cups whereas beryllium, copper and lead were markedly sorbed at this pH-range by all cup types. These results are only valid for the boundary conditions used. Whenever a suction cup's suitability is in doubt it should be tested after a conditioning procedure using realistic boundary conditions.     

Schwermetall-Sorptionsverhalten einer Saugkerze aus porösem Borosilikatglas

Heavy metal sorption of a new developed porous borosilicate glass suction cup In relation to conventional ceramic P 80-suction cups, a porous borosilicate glass suction cup shows distinctly lower sorption and desorption regarding the heavy metals Pb and Cu.     

Challenges in modelling dissolved organic matter dynamics in agricultural soil using DAISY

Because dissolved organic matter (DOM) plays an important role is terrestrial C-, N- and P-balances and transport of these three components to aquatic environments, there is a need to include it in models. This paper presents the concept of the newly developed DOM modules implemented in the DAISY model with focus on the quantification of DOM sorption/desorption and microbial-driven DOM turnover. The kinetics of DOM sorption/desorption is described by the deviation of the actual DOM concentration in solution from the equilibrium concentration, C_eq. The C_eq is soil specific and estimated from pedotransfer functions taking into account the soil content of organic matter, Al and Fe oxides. The turnover of several organic matter pools including one DOM pool are described by first-order kinetics.The DOM module was tested at field scale for three soil treatments applied after cultivating grass–clover swards. Suction cups were installed at depths 30, 60 and 90 cm and soil solution was sampled for quantification of dissolved organic C (DOC) and dissolved organic N (DON). In the topsoil, the observed fluctuations in DOC were successfully simulated when the sorption/desorption rate coefficient k was low. In the subsoil, the observed concentrations of DOC were steadier and the best simulations were obtained using a high k. The model shows that DOC and DON concentrations are levelled out in the subsoils due to soil buffering. The steady concentration levels were based on the C_eq for each horizon and the kinetic concept for sorption/desorption of DOC appeared a viable approach. If C_eq was successfully estimated by the pedotransfer function it was possible to simulate the DOC concentration in the subsoil. In spite of difficulties in describing the DOC dynamics of the topsoil, the DOM module simulates the subsoil concentration level of DOC well, and also—but with more uncertainty—the DON concentration level.     

An evaluation of methods for measurement of pesticides in sorption experiments

Abstract

The concentration of four pesticides (2,4‐D, atrazine, phorate, and terbufos) in soil solution during sorption experiments was measured using UV spectrophotometry, Gas Liquid Chromatography (GLC), High Performance Liquid Chromatography (HPLC), and radiotracer technique. The presence of water soluble organic matter in soil solution interfered with the measurement of pesticide using the UV spectrophotometry. The use of GLC, HPLC, and radiotracer technique involving ¹⁴C gave a good estimate of the concentration of pesticide in soil solution. The pesticide remaining in soil can be quantitatively analyzed by extracting with a scintillation solution containing an organic solvent such as toluene or dioxane. Among the various centrifuge tubes glass tubes with Teflon caps sorbed negligible amount of pesticides and these tubes can be used for the sorption measurements.     

Dissolved organic matter leaching in some contrasting New Zealand pasture soils

Leaching of dissolved organic matter (DOM) from pastoral soils is increasingly seen as an important but poorly understood process. This paper examined the relationship between soil chemical properties, microbial activity and the losses of dissolved organic carbon (DOC) and nitrogen (DON) through leaching from six pasture soils. These soils differed in carbon (C) (4.6–14.9%) and nitrogen (N) (0.4–1.4%) contents and in the amount of organic C and N that had accumulated or been lost in the preceding 20+ years (i.e. −5131 to +1624 kg C ha⁻¹ year⁻¹ and −263 to +220 kg N ha⁻¹ year⁻¹, respectively). The paper also examined whether between‐soil‐type differences in DOC and DON leaching was a major explanatory factor in the observed range of soil organic matter (SOM) changes in these soils. Between 280 and 1690 kg C ha⁻¹ year⁻¹ and 28–117 kg N ha⁻¹ year⁻¹ leached as DOC and DON, respectively, from the six soils in a lysimeter study, with losses being greater from two poorly drained gley soils. Losses of C and N of this magnitude, while at the upper end relative to published data, could not fully explain the losses at Rawerawe, Bruntwood and Lepperton sites reported by Schipper et al. (2007) . The study highlights the leaching of DOM as a significant pathway of loss of C and N in pasture soils that is often ignored or given little attention in predictive models and nutrient budgeting. Leaching losses of DOC and DON alone, or in combination with slightly increased respiration losses of SOM given a 0.2°C increase in the mean annual soil temperature, do not fully explain long‐term changes in the SOM observed at these sites. When soils examined in the present study were separated on the basis of drainage class, the losses of DOC by leaching were correlated with both total and hot‐water extractable C (HWC), the latter being a measure of the labile SOM fraction. Basal microbial CO₂ respiration rates, which varied between 1 and 3.5 µg CO₂‐C g⁻¹ soil hour⁻¹ in surface soils (0–75‐mm depth), was also linked to HWC and the quantities of C lost as DOC. Adoption of the HWC method as an approach that could be used as a proxy for the direct measurement of the soil organic C lost by leaching as DOC or respired needs to be examined further with a greater number of soils. In comparison, a poor relationship was found between the hot‐water extractable N (HWN) and loss of DON by leaching, despite HWN previously being shown to be a measure of the mineralizable pool of N in soils, possibly reflecting the greater competition for N than C in these soils.     

Effect of Ionic Strength and Index Cation on the Sorption of Phenanthrene

Sorption characteristics of phenanthrene were studied in batch equilibrium experiments with 32 Australian soils that varied widely in physicochemical properties. Sorption of phenanthrene varied widely among the soils and was generally nonlinear, with the nonlinearity index (n) of the Freundlich isotherm varying from 0.62 to 1.01. Simple regression analyses revealed that total organic carbon (TOC) accounts for about 68 % of the variation in the partition coefficient (K′ _f ) for sorption among the soils at an equilibrium concentration (C _e ) of 0.05 mg/L. The organic carbon normalized distribution coefficient (K _OC ), varied considerably between soils with >70 % of the variance of logK _OC being accounted for by logTOC, clay and log dissolved organic carbon (DOC). These results show that the phenanthrene C _e is influenced by both TOC as well as the DOC in soil suspensions. The effects of ionic strength (IS) and index cation were investigated using four contrasting soils. Results show that with an increase in IS from 0.03 to 0.15 M sorption of phenanthrene generally increased in CaCl₂ background solutions, whereas the effect was less significant and variable in NaCl background solutions. Sorption of phenanthrene was slightly higher at low IS (0.03 M) with Na⁺ as index cation compared with that of Ca²⁺, whereas an opposite trend was observed at higher IS (0.15 M). For two soils high in TOC, the flocculation of endogenous DOC in the presence of Ca²⁺ reduced the influence of background electrolyte and resulted in a more linear sorption isotherm as well as higher sorption capacity. This trend was more significant with Ca²⁺ relative to Na⁺.     

Water flow patterns and pesticide fluxes in an upland soil in northern Thailand

Rapid percolation of water through soil facilitates both the recharge and the contamination of groundwater reservoirs. We have studied the variation of water flux and pesticide leaching through a soil in northern Thailand. At a depth of 55 cm, two pits were equipped with tensiometer-controlled glass suction lysimeters that were connected to a novel on-line solid-phase extraction device. Nine insecticides varying in water solubility from 10⁻² to 10⁺⁶ mg l⁻¹ were applied on the soil surface, and leaching was monitored for 8 weeks. Measured water fluxes were compared with simulated values. Total recovery ranged from traces (malathion, triazophos) to 1.3% (dimethoate) of the applied amount, showing a decreasing retardation with increasing polarity of the substances. All pesticides were detectable in the soil solution during the first rain after application. Due to fingering, 83% of the leachate was transported through 38% of the area at leaching rates of < 2 mm per day. A new adaptation of the Simpson Index revealed that the diversity of the flow pattern increased exponentially with decreasing rates of seepage water flux (R² = 0.80). No such correlation was found when leaching was faster, indicating that the flow pattern switched from a fingering- to a matric-dominated flux. No long-term leaching of insecticides was observed. The two profiles studied behaved similarly in terms of both water and pesticide transport. Therefore we suggest that the flow pattern is a stable property of the soil that can be accurately described by our combination of novel experimental setup and statistical analysis of the flow field.     

Sickerwassergewinnung mittels Saugkerzen – eine Literaturstudie

Sampling seepage water with suction cups – a literature study The article presents a literature study concerning the sampling of seepage water with suction cups. It gives a general view about construction and materials of different types of suction cups as well as proposals for the installation and arrangement of sampling equipments. Furthermore the article deals with questions related to the release or the sorption of substances by the cup material. Other problems like the influence of the applied suction on the concentration of the solutants in the seepage water sample and the potential field around the suction cup are discussed.     

Zusammensetzung und Speziierung der Bodenlösung eines mit Schwermetallen belasteten kalkreichen Bodens

Composition and Speciation of Soil Solution collected in a Heavy Metal polluted calcareous Soil Close to a brass foundry, which had emitted heavy metal containing dusts for over 80 years, soil water was collected in the topsoil (18 cm) and in the subsoil (40 cm) of a severely polluted Calcic Fluvisol by means of polyethylen suction cups over a period of 2 years. The total metal content of the topsoil (extracted with 2M HNO₃ at 100 °C for 2 hours) was 38 nmol g^?1, 24 μmol g^?1, and 25 μmol g^?1 for Cd, Cu, and Zn, respectively. The mean heavy metal concentrations of the soil solution were 0.5 mol L^?1, 300 nmol L^?1, and 200 nmol L^?1 in the topsoil and 0.6 nmol L^?1, 90 nmol L^?1, and 30 nmol L^?1 in the subsoil for Cd, Cu, and Zn, respectively. Solubility calculations showed that the soil solutions were undersaturated with respect to heavy metal carbonates as well as to hydroxides. It seems that the heavy metal concentration is determined by sorption processes rather than by precipitation. The composition of the soil solution has been shown to be governed by the presence, of calcite, by the soil temperature and by the partial pressure of CO₂ in the soil air. The pCO₂ in the soil air (in both depths) has been estimated at 2 mbar during the winter term and at 20 mbar during the summer term. A corresponding increase of the concentration of macroelements (Ca, Mg, Na) as well as of total dissolved carbonate and of dissolved organic matter (DOC) has been measured in the summer half year. No significant seasonal variations of the heavy metal concentrations were detected and no correlations with concentrations of other components could be found.     

Über die Sorptionseigenschaften von Waldböden bezüglich gelöster organischer Substanzen

About the sorption of dissolved organic matter to forest soils This investigation characterizes the major forest soils of the temperate climatic zones (leptosols, vertisols, cambisols, luvisols, podzols, stagnosols, gleysols) as sorbents for dissolved organic matter (DOM). Sorption isotherms were obtained for 135 soil horizons from 36 profiles. When solutions containing no DOC were added, the release of dissolved organic carbon (DOC) was highest for horizons rich in organic C (A and Bh horizons). In subsoil horizons DOC release was much lower. Most of the investigated top soils (A and E horizons) and Bh, Bg, and C horizons showed a weak DOC sorption. This was caused by low contents of sorbents (clay and sesquioxides) and/or high contents of organic C. Organic C seems to reduce the DOC sorption by occupying binding sites. Subsoils rich in clay and sesquioxides like Bs, Bt, and Bw horizons showed a strong retention of DOC. Under the aerobic conditions of the experiments, some of the subsoils of stagnosols and gleysols also showed a strong sorption of DOC. However, in sorption experiments conducted after an anaerobic incubation, the DOC sorption decreased significantly.     

Predicting and modelling availability of fluoride in soil from sorption properties

Usage of alkaline and saline groundwater with elevated concentrations of fluoride (F⁻) for irrigation of pastures requires an assessment of the mobility of F⁻ within the plant-water-soil continuum. Factors influencing F⁻ sorption and desorption in 95 Australian soils were examined. Pronounced differences in F⁻ sorption were observed across the soils, but these differences could not be explained by differences in soil pH, electrical conductivity or organic carbon. Rather, sorption was correlated with the content of Fe/Al hydrous oxides and kaolinite in soil. Sorption of F⁻ onto soils increased the solution pH and the ratio of (F⁻ adsorbed) to (OH⁻ desorbed) was consistently below 1, thereby indicating that adsorption of F⁻ results in the release of water and hydroxyl groups from, or co-adsorption of protons to, the sorbent surface. Maximum sorption occurred at pH values of approximately 5–6, whilst sulphate slightly increased (<5%) F⁻ sorption. Desorption was slightly decreased (~3%–7%) in presence of sulphate anions. Hence, it is unlikely that irrigation of soil with alkaline and saline groundwater, in combination with soil applications of gypsum and sulphur to limit pH fluctuations, would increase mobility of F⁻ in soil. Finally, the irrigation of soil columns with 1200–1500 mm of alkaline and saline groundwater containing 0.18 mM F⁻, as would commonly occur in irrigation systems using coal seam gas associated water in Australia, resulted in a F⁻ concentration of 0.05 mM at 10 cm depth and the measured values were in excellent agreement with modelled F⁻ movement based on sorption parameters. Thus, sorption parameters can be used to identify soils which minimize movement of F⁻ because of their strong F⁻ sorption. Based on these results, safe limits for irrigation of soil can be established which avoid F⁻ toxicity risks to plants, animals and contamination of water resources.     

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.     

Nitrat‐Austräge auf intensiv und extensiv beweidetem Grünland,erfasst mittels Saugkerzen‐ und Nmin‐Beprobung II Variabilität der NO3‐ und NH4‐Werte und Aussagegenauigkeit der Messmethoden

Nitrate leaching from intensively and extensively grazed grassland measured with suction cup samplers and sampling of soil mineral‐N II Variability of NO₃ and NH₄ values and degree of accuracy of the measurement methods Data from a grazing experiment — comparison of mean values, see Anger et al. (2002) — were used to estimate within‐field variability to asses the accuracy of two frequently used methods of estimating NO₃ leaching on pastures: (1) the ceramic suction cup sampling (with 34 cups ha^—1 minimum, calculated climatic water balance, 4 leaching periods) and (2) using the soil mineral‐N method (vertical soil NO₃ and NH₄ content in 0—0.9 m (N_min) measured at the beginning and end of two winters on a minimum of 10 different areas of 50 m² each with a minimum of 7 different sample cores). These methods were used on two permanent pastures with high mean stocking density of cattle of 4.9 LU ha^—1 on 1.3 ha with N‐fertilization of 250 kg N ha^—1 (= intensive [I]) and 2.9 LU without N fertilization on a 2.2 ha pasture (= extensive [E]). The results show that NO₃ leaching on pastures was largely due to few selectively extremely high NO₃ amounts under a few excrement spots — mainly urine spots — which would not be sampled representatively with an acceptable effort in a conventional grazing experiment. In both grazing treatments, very large spatial variation occurred. This was greater between the different suction cups than between the compound mineral N samples of each area. Therefore, a marked skewness and kurtosis demonstrated a non‐normal distribution of samples from suction cups, while mineral N values did not show this effect consistently. Sampling selected mostly spots without noticeable influence of excrement, but a few samples with very high values identified evidently urine spots from summer or autumn grazing. The differences in mean coefficient of variation (CV) between the grazing treatments and estimation methods were mainly based on the stocking rate and the density of excrement spots. CV values were 131 % [I] / 242 % [E] for NO₃ leaching measured with suction cup samplers and of 71 % [I] / 116 % [E] for soil NO₃ values and 24 % [I] / 34 % [E] for soil NH₄ values in 0—0.9 m according N_min‐method. Results of the N_min method are obviously inaccurate even with a sampling intensity much greater than 70 cores ha^—1; and so making an estimation of NO₃ leaching by this method is unsatisfactory for pastures. Compared to this, the results of suction cup sampling are more convincing; but even with a tolerated deviation of ± 20 % from the empirically estimated average and with a 95 %‐confidence interval, the calculated mean minimum number of samples in our experiment should be increased to 146 and 265 suction cups ha^—1 for the intensively and extensively grazed treatments, respectively. This requirement would be prohibitive for many field experiments.     

DOC leaching from a coniferous forest floor: modeling a manipulation experiment

The DyDOC model simulates the C dynamics of forest soils, including the production and transport of dissolved organic matter (DOM), on the basis of soil hydrology, metabolic processes, and sorption reactions. The model recognizes three main pools of soil C: litter, substrate (an intermediate transformation product), and humic substances. The model was used to simulate the behavior of C in the O horizon of soil under a Norway spruce stand at Asa, Sweden, that had been subjected to experimental manipulations (addition and removal) of above‐ground litter inputs and to removal of the Oi and Oe layers. Initially, the model was calibrated using results for the control plots and was able to reproduce the observed total soil C pool and ¹⁴C content, DOC flux and DO¹⁴C content, and the pool of litter C, together with the assumed content of C in humic substances (20% of the total soil C), and the assumed distribution of DOC between hydrophilic and hydrophobic fractions. The constant describing DOC exchange between micro‐ and macropores was estimated from short‐term variations in DOC concentration. When the calibrated model was used to predict the effects of litter and soil manipulations, it underestimated the additional DOC export (up to 33%) caused by litter addition, and underestimated the 22% reduction in DOC export caused by litter withdrawal. Therefore, an additional metabolic process, the direct conversion of litter to DOC, was added to the model. The addition of this process permitted reasonably accurate simulation of the results of the manipulation experiments, without affecting the goodness‐of‐fit in the model calibration. The results suggest that, under normal conditions, DOC exported from the Asa forest floor is a mixture of compounds derived from soil C pools with a range of residence times. Approximately equal amounts come from the litter pool (turnover time 4.6 yr), the substrate pool (26 yr), and the humic‐substances pool (36 yr).     

Retention of dissolved organic matter by illitic soils and clay fractions: Influence of mineral phase properties

The dependency of the retention of dissolved organic carbon (DOC) on mineral phase properties in soils remains uncertain especially at neutral pH. To specifically elucidate the role of mineral surfaces and pedogenic oxides for DOC retention at pH 7, we sorbed DOC to bulk soil (illitic surface soils of a toposequence) and corresponding clay fraction (< 2 μm) samples after the removal of organic matter and after removal of organic matter and pedogenic oxides. The DOC retention was related to the content of dithionite‐extractable iron, specific surface area (SSA, BET‐N₂ method) and cation exchange capacity (pH 7). The reversibility of DOC sorption was determined by a desorption experiment. All samples sorbed 20–40 % of the DOC added. The DOC sorption of the clay fractions explained the total sorption of the bulk soils. None of the mineral phase properties investigated was able to solely explain the DOC retention. A sorption of 9 to 24 μg DOC m^–2 indicated that DOC interacted only with a fraction of the mineral surface, since loadings above 500 μg m^–2 would be expected for a carbon monolayer. Under the experimental conditions used, the surface of the silicate clay minerals seemed to be more important for the DOC sorption than the surface of the iron oxides. The desorption experiment removed 11 to 31 % of the DOC sorbed. Most of the DOC was strongly sorbed.     

The contribution of fresh litter to dissolved organic carbon leached from a coniferous forest floor

The relative contributions of litter and humified organic matter as the source of dissolved organic carbon (DOC) leached from organic layers of forest soils are poorly understood. In the present investigation, ¹³C labelled spruce litter was used to study the role of recent litter in the leaching of DOC from a coniferous forest floor in southern Sweden, while litterbags were used to quantify the total loss of C from the labelled litter. The labelled litter applied on bare lysimeters released considerable amounts of DOC during the first weeks, but the concentration of DOC originating from labelled litter decreased gradually from 176 mg litre⁻¹ during the first sampling period in May to 5 mg litre⁻¹ in the last sampling period in October. Only a moderate flush of DOC from the labelled litter occurred under the Oe and Oa horizons, with concentrations of 20 and 6 mg litre⁻¹ from labelled litter, equal to 19 and 9% of the total DOC flux, respectively, during the first sampling period. Total flux of DOC from labelled litter from May to September was 16 g m⁻², whereas only 2.2 and 0.9 g m⁻² were captured under the Oe and Oa horizons, respectively. The almost complete loss of new DOC implies that DOC leached from the Oe and Oa horizons consists not of recent litter‐derived carbon, but of DOC produced in these two horizons themselves. Water‐extractable organic carbon from labelled litter left in litterbags in the field for 4 months consisted of about one‐third native carbon from external sources at the experimental site and two‐thirds of the labelled litter. In contrast, the ¹³C content of the bulk litter from the litterbags was not changed by the incubation in the field. We suggest that the soluble native carbon in water extracts originated from throughfall DOC that had been assimilated by microorganisms in the litterbags.     

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.     

Release of glucose from dissolved and mineral-bound organic matter by enzymatic hydrolysis

Sorption of dissolved organic matter (DOM) by poorly crystalline minerals during their formation may protect large amounts of carbon in soils from mineralization. We investigated the bioavailability of carbohydrates in DOM and after co-precipitation with short-range ordered aluminosilicates. Carbohydrates originated from soil solutions collected in situ at two depths of a Dystric Cambisol, and from litter extracts. Quantification of substrate-specific degradability was achieved by the addition of β-glucosidase at an optimal concentration and subsequent determination of glucose release. Depending on DOM composition, 0.6–41.4 mg g⁻¹ C⁻¹ of glucose was enzymatically released from dissolved carbohydrates. Co-precipitated carbohydrates were partially accessible, resulting in a glucose release of 0.7–5.2 mg g⁻¹ C⁻¹. Restricted enzymatic depolymerization due to co-precipitation may contribute to accumulation of easily degradable substrates in soils.     

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.