Amounts and degradability of dissolved organic carbon from foliar litter at different decomposition stages

Litter is one of the main sources of dissolved organic carbon (DOC) in forest soils and litter decomposition is an important control of carbon storage and DOC dynamics. The aim of our study was to evaluate (i) effects of tree species on DOC production and (ii) relationships between litter decomposition and the amount and quality of DOC. Five different types of leaves and needles were exposed in litterbags at two neighboring forest sites. Within 12 months we sampled the litterbags five times and leached aliquots of field moist litter in the laboratory. In the collected litter percolates we measured DOC concentrations and recorded UV and fluorescence spectra in order to estimate the aromaticity and complexity of the organic molecules. Furthermore, we investigated the biodegradability of DOC from fresh and decomposed litter during 6 weeks incubations. Fresh sycamore maple litter released the largest amounts of DOC reaching about 6.2% of litter C after applying precipitation of 94 mm. We leached 3.9, 1.6, 1.0 and 3.3% carbon from fresh mountain ash, beech, spruce and pine litter, respectively. In the initial phase of litter decomposition significantly decreasing DOC amounts were released with increasing litter mass loss. However, after mass loss exceeds 20% DOC production from needle litter tended to increase. UV and fluorescence spectra of percolates from pine and spruce litter indicated an increasing degree of aromaticity and complexity with increasing mass loss as often described for decomposing litter. However, for deciduous litter the relationship was less obvious. We assume that during litter decomposition the source of produced DOC in coniferous litter tended toward a larger contribution from lignin-derived compounds. Biodegradability of DOC from fresh litter was very high, ranging from 30 to 95% mineralized C. DOC from degraded litter was on average 34% less mineralizable than DOC from fresh litter. Taking into account the large DOC production from decomposed needles we can assume there is an important role for DOC in the accumulation of organic matter in soils during litter decomposition particularly in coniferous forests.     

Fractionation of dissolved organic matter on the XAD resin

Analytical fractionation conditions on the resin XAD-7H of organic substances from gravitational soil moisture influence the proportion and the yield of the extracted fractions. Increasing the column capacity factor k’ is accompanied by a decreased yield of hydrophobic fraction and the change in its composition. The sorption column capacity factor of k’ = 30 is sufficient for the extraction of all hydrophobic components from a soil solution with a concentration of C_DOM of ～25 mg/L.     

Liming effect on dissolved organic matter leaching

The main agrochemical treatment applied in agriculture to lower harmful soil acidity is liming. Long term studies showed that application of calcium carbonate fertilizer brought about higher leaching of dissolved organic matter (DOM) in pot, lysdmetric and field experiments. Outflows obtained from limed soils contained 44.8% more DOM in comparison with nonlimed soils. During four years of lysimetric experiments it was found that the amounts of DOM from limed soils were increased by 52.7%. In the field experiments the amounts leached from one hectare of limed and nonlimed soil ranged 25.6 kg and 19.2 kg per year, respectively. DOM leached from limed soils was characterized by higher (45.4%) contents of carboxylic groups and humic substances (19.7%). Dissolved organic substances particularly humus rich in functional groups, due their complexing properties bound plant nutrients leached from soils and modify geochemical mobility of metals and anions. Therefore, DOM can play a significant role in the migration of chemical substances in agriculture landscape.     

Acid-base analysis of dissolved organic matter in surface waters

A mono-protic multi-site model is developed to obtain a pK(acid) — concentration distribution. Dense and equal interval pH data are required for an accurate characterization. A computer driven titrimetric system is used to obtain the data. The technique is applied to dissolved organic carbon (> 15 mg C L^-1) samples from the Kejimkujik region, Nova Scotia. A calculation shows that the acidic (pH=4.6) dystrophic waters can result from mixing 15 mg C L^-1 of the organic acids with an initial inorganic system of about 75 peq L^-1 alkalinity.     

Seasonal changes in the content of dissolved organic matter in arable soils

Purpose

The aim of this paper has been to determine the seasonal changes in the content of dissolved organic matter (DOM) in the soils under agricultural use based on assaying changes in dissolved organic carbon (DOC) and dissolved nitrogen (DNt) as well as determining the factors which can define the DOM in soils.

Materials and methods

The research has involved the soils under agricultural use sampled in the Kujawsko-Pomorskie province (Poland). Phaeozems and Luvisols were sampled from the depth of 0–30, 30–60, and 60–100 cm, November 2011 through September 2013, in November, March, May, July, and September. The soil samples were assayed for the grain size composition, pH, dry weight content, content of total organic carbon, and total nitrogen. Dissolved organic matter was extracted with 0.004 mol dm³ CaCl₂; in the DOM extracts, the content of dissolved organic carbon (DOC) and dissolved nitrogen (DNt) were assayed. The research results were statistically verified.

Results and discussion

It has been demonstrated that in the first year of research, the content of dissolved organic carbon in the soils was changing throughout the year. The highest differences in the content of that carbon fraction occurred across the soil sampled in autumn and the soil sampled in spring. In the second year of research, an inverse dependence was noted. DOC was migrating to deeper layers of the soil profile; yet, the migration got more intensive in summer. The content of dissolved nitrogen was not changing significantly throughout the year. Higher DNt content in the surface layer, in general, resulted in a higher content of dissolved nitrogen in deeper profile layer, which could have been due to leaching of the nutrient deep down the soil profile.

Conclusions

The content of dissolved organic carbon was significantly related to the content of total organic carbon and total nitrogen. Significant changes in the content of dissolved forms of nitrogen were reported in the profile of Phaeozems due to mineral fertilization and irrigation. The soils where irrigation and higher nitrogen rates had been applied demonstrated a higher content and share of soluble forms of nitrogen, as compared with the soils non-irrigated and the soils where lower nitrogen rates had been supplied.

     

Spectrophotometric determination of polyacrylamide in waters containing dissolved organic matter

Using polyacrylamide (PAM) to reduce soil erosion in irrigated land has increased rapidly in recent years. A simple and reliable method to measure the PAM concentration in waters containing dissolved organic matter (DOM) is of great importance in assessing the fate and efficiency of PAM application. In this research, an analytical method to determine the PAM concentration of waters with correction for DOM interference was developed and tested. The method is based on a combination of determining the total concentration of amide groups by the N-bromination method (NBM) and determining the DOM content spectrophotometrically. The total concentration of amide groups of both PAM and DOM was determined by NBM at 570 nm. The DOM moiety, which is proportional to DOM concentration, was determined by spectrophotometry using a UV 254-nm wavelength. The actual PAM concentration of a water sample (soil extract containing PAM in this study) was obtained from NBM readings subtracted by the interferential DOM contribution using a correction curve. Analysis of PAM in two soil-water samples showed that the recoveries ranged from 94 to 100.3% for the 2 mg/L PAM sample and from 98.4 to 101.4% for the 10 mg/L PAM sample with various DOM concentrations. The coefficients of variation were <6% in all cases.     

Stabilization of dissolved organic matter by sorption to the mineral soil

The main process by which dissolved organic matter (DOM) is retained in forest soils is likely to be sorption in the mineral horizons that adds to stabilized organic matter (OM) pools. The objectives of this study were to determine the extent of degradation of sorbed OM and to investigate changes in its composition during degradation. DOM of different origins was sorbed to a subsoil and incubated for 1 year. We quantified mineralized C by frequent CO₂ measurements in the headspace of the incubation vessels and calculated mean residence times by a double exponential model. Mineralization of C of the corresponding DOM in solution was used as a control to estimate the extent of DOM stabilization by sorption. Changes in the composition of sorbed OM during the incubation were studied by spectroscopic (UV, fluorescence) and isotope (¹³C, ¹⁴C) measurements after hot-water extraction of OM.The fraction of sorbed organic C mineralized during the incubation was only one-third to one-sixth of that mineralized in solution. The mean residence time of the most stable OM sample was estimated to increase from 28 years in solution to 91 years after sorption. For highly degradable DOM samples, the portion of stable C calculated by a double exponential model nearly doubled upon sorption. With less degradable DOM the stability increased by only 20% after sorption. Therefore, the increase in stability due to sorption is large for labile DOM high in carbohydrates and relatively small for stable DOM high in aromatic and complex molecules. Nevertheless, in terms of stability the rank order of OM types after sorption was the same as in solution. Furthermore, the extent of sorption of recalcitrant compounds was much larger than sorption of labile compounds. Thus, sorptive stabilization of this stable DOM sample was four times larger than for the labile ones. We conclude that stabilization of OM by sorption depends on the intrinsic stability of organic compounds sorbed. We propose that the main stabilization processes are selective sorption of intrinsically stable compounds and strong chemical bonds to the mineral soil and/or a physical inaccessibility of OM to microorganisms. The UV, fluorescence and ¹³C measurements indicated that aromatic and complex compounds, probably derived from lignin, were preferentially stabilized by sorption of DOM. The ¹³C and ¹⁴C data showed that degradation of the indigenous OM in the mineral soil decreased after sorption of DOM. We estimated DOM sorption stabilizes about 24 Mg C ha⁻¹ highlighting the importance of sorption for accumulation and preservation of OM in soil.     

Retention and release of dissolved organic matter in Podzol B horizons

The main objectives were to study the effects of pH on the retention and release of organic matter in acid soil, and to determine the main differences in results obtained from batch experiments and experiments in columns. We took soil material from the B horizons of a Podzol at Skånes Värsjö (southern Sweden). In batch experiments, soil was equilibrated with solutions varying in pH and concentration of dissolved organic C. In Bh samples, the release of dissolved C gradually increased with increase in pH. In the Bs1 material there was a minimum at pH 4.1, and in the Bs2 soil the minimum occurred at pH 4.6. The ability to retain added dissolved C increased in the order Bh < Bs1 < Bs2. The column experiment was run for 160 days under unsaturated flow conditions. Columns were packed with Bh, Bh + Bs1 or Bh + Bs1 + Bs2 samples to calculate mass balances for each horizon. Solutions either without any dissolved organic C or ones containing 49 mg C dm^?3 with pH of 4.0 or 3.6 were used to leach columns. The pH of input solutions only little affected the concentration of dissolved C in the effluent. Relative proportions of hydrophobic substances decreased with increasing column length and decreasing pH. For input solutions containing dissolved C, near steady state was achieved for both the Bs1 and Bs2 horizons with approximately 25% dissolved organic matter retention. Thus, no maximum sorption capacity for dissolved C could be defined for these horizons. This behaviour could not have been predicted by batch data, showing that column experiments provide useful additional information on interactions between organic compounds and solid soil material.     

Changes in properties of soil-derived dissolved organic matter induced by biodegradation

Properties of dissolved organic matter (DOM) determine its biodegradation. In turn, biodegradation changes the properties of the remaining DOM, which may be decisive for the formation of stable organic carbon in soil. To gain information on both mechanisms and controlling factors of DOM biodegradation and the properties of biodegraded DOM, we investigated changes in the composition of 13 different DOM samples extracted from maize straw, forest floors, peats, and agricultural soils during a 90-day incubation using UV absorbance, fluorescence emission spectroscopy, FTIR-spectroscopy, ¹H-NMR spectroscopy, pyrolysis-field ionization mass spectroscopy (Py-FIMS), and ¹³C natural abundance before and after incubation. Changes in the DOM properties were related to the extent of biodegradation determined by the release of CO₂. Increasing UV absorption and humification indices deduced from fluorescence emission spectra, and increasing portions of aromatic H indicated relative enrichment of aromatic compounds during biodegradation. This enrichment significantly correlated with the amount of DOC mineralized suggesting that aromatic compounds were relatively stable and slowly mineralized. ¹³C depletion during the incubation of highly degradable DOM solutions indicated an enrichment of lignin-derived aromatic compounds. Py-FI mass spectra indicated increasing contents of phenols and lignin monomers at the expense of lignin dimers and alkylaromatics during incubation. This partial degradation of higher-molecular, lignin-derived DOM compounds was accompanied by relative increases in the proportions of lower-molecular degradation products and microbial metabolites. Carbohydrates, especially when abundant at high initial contents, seem to be the preferred substrate for microorganisms. However, four independent methods suggested also some microbial production of carbohydrates and peptides during DOM degradation. After incubation, the composition of highly degradable DOM samples became similar to relatively stable DOM samples with respect to aromaticity, carbohydrate content, and thermal stability. We conclude that DOM biodegradation seems to result in organic matter properties being a precondition for the formation of stable carbon. These structural changes induced by DOM biodegradation should also result in stronger DOM sorption to the soil matrix additionally affecting DOM stabilization.     

Biochemical properties and biodegradation of dissolved organic matter from soils

Various biologically mediated processes are involved in the turnover of dissolved organic matter (DOM) in soil; however, relatively little is known about the dynamics of either the microbial community or the individual classes of organic molecules during the decomposition of DOM. We examined the net loss of DOC, the mineralisation of C to CO₂ and the degradation of DOC from six different soils by soil microorganisms. We also quantified the changes in the concentrations of protein, carbohydrate and amino acid C during microbial biodegradation. Over a 70-day incubation period at 20°C, the mineralisation of DOC to CO₂ was described by a double exponential model with a labile pool (half-life, 3–8 days) and a stable pool (half-life, 0.4–6 years). However, in nearly all cases, the mass loss of DOC exceeded the C released as CO₂ with significant deviations from the double exponential model. Comparison of mass DOC loss, CO₂ production and microbial cell counts, determined by epifluorescence microscopy, showed that a proportion of the lost DOC mass could be accounted for by microbial assimilation. Carbohydrate and protein C concentrations fluctuated throughout the incubation with a net change of between 3 to 13 and −30 to 22.4% initial DOC, respectively. No amino acid C was detected during the incubation period (level of detection, 0.01 mg C l⁻¹).     

Properties of dissolved organic matter related to soil organic matter quality and nitrogen additions in Norway spruce forest floors

The quality of dissolved organic matter (DOM) is highly variable and little information is available on the relation of DOM quality to the structure and composition of its parent soil organic matter (SOM). The effect of increasing N inputs to forest soils on the structure and composition of both SOM and DOM also remains largely unclear. Here we studied the release of DOM, its specific UV absorption and two humification indices (HIX) derived from fluorescence spectra from Oa material of 15 North- and Central-European Norway spruce (Picea abies (L.) Karst.) stands. The Oa material was incubated aerobically at 15 °C and water holding capacity over a period of 10 months and extracted monthly with an artificial throughfall solution. Soil respiration was determined weekly. The influence of mineral N inputs on composition of DOM and on respiration rates was investigated on periodically NH₄NO₃-treated Oa samples of eight selected sites. Release of dissolved organic carbon (DOC) from untreated Oa material samples ranged from 0.0 to 58.6 μg C day⁻¹ g C⁻¹ and increased with increasing C-to-N ratio. One HIX and UV absorption of DOM were negatively correlated to the degree of oxidation of lignin-derived compounds and positively to the C-to-N ratio and – HIX only – to the aromatic C content of SOM. Mineral N addition had no distinct effect on respiration rates. In six of eight samples the N-treatment caused an increase in specific UV absorption or one HIX of DOM. However, these effects were not statistically significant. Addition of mineral N did not affect the rates of DOM release. Our results show that properties of SOM largely determine the amount and quality of DOM in forest floors. Changes of DOM quality due to mineral N additions are likely, but we cannot confirm significant changes of DOM release.     

Effect of Collembola on mineralization of litter and soil organic matter

Although soil Collembola are known to contribute to soil carbon (C) cycling, their contribution to the mineralization of C sources that differ in bioavailability, such as soil organic C (SOC) and leaf litter, is unknown. Stable C isotopes are often used to quantify the effects of both soil C and litter C on C mineralization. Here, ¹³C-labeled litter was used to investigate the effects of Collembola (Folsomia candida) on the mineralization of both SOC and litter C in laboratory microcosms. The three microcosm treatments were soil alone (S); soil treated with δ¹³C-labeled litter (SL); and soil treated with δ¹³C-labeled litter and Collembola (SLC). The presence of Collembola did not significantly affect soil microbial biomass or litter mass loss and only had a small effect on CO₂ release during the first week of the experiment, when most of the CO₂ was derived from litter rather than from SOC. Later, during the experiment (days 21 and 63), when litter-derived labile C had been depleted and when numbers of Collembola had greatly increased, Collembola substantially increased the emission of SOC-derived CO₂. These results suggest that the effect of Collembola on soil organic C mineralization is negatively related to C availability.     

Contribution of dissolved organic matter to carbon storage in forest mineral soils

Dissolved organic matter (DOM) is often considered the most labile portion of organic matter in soil and to be negligible with respect to the accumulation of soil C. In this short review, we present recent evidence that this view is invalid. The stability of DOM from forest floor horizons, peats, and topsoils against microbial degradation increases with advanced decomposition of the parent organic matter (OM). Aromatic compounds, deriving from lignin, likely are the most stable components of DOM while plant‐derived carbohydrates seem easily degradable. Carbohydrates and N‐rich compounds of microbial origin produced during the degradation of DOM can be relatively stable. Such components contribute much to DOM in the mineral subsoil. Sorption of DOM to soil minerals and (co‐)precipitation with Al (and probably also with Fe), especially of the inherently stable aromatic moieties, result in distinct stabilization. In laboratory incubation experiments, the mean residence time of DOM from the Oa horizon of a Haplic Podzol increased from <30 y in solution to >90 y after sorption to a subsoil. We combined DOM fluxes and mineralization rate constants for DOM sorbed to minerals and a subsoil horizon, and (co‐)precipitated with Al to estimate the potential contribution of DOM to total C in the mineral soil of a Haplic Podzol in Germany. The contribution of roots to DOM was not considered because of lack of data. The DOM‐derived soil C ranges from 20 to 55 Mg ha^–1 in the mineral soil, which represents 19%–50% of the total soil C. The variation of the estimate reflects the variation in mineralization rate constants obtained for sorbed and (co‐)precipitated DOM. Nevertheless, the estimates indicate that DOM contributes significantly to the accumulation of stable OM in soil. A more precise estimation of DOM‐derived C in soils requires mineralization rate constants for DOM sorbed to all relevant minerals or (co‐)precipitated with Fe. Additionally, we need information on the contribution of sorption to distinct minerals as well as of (co‐)precipitation with Al and Fe to DOM retention.     

Water soluble elements in decomposing japanese cedar needle litter

Many studies on the decrease in the contents of elements of decomposing litter have been carried out usually by applying the litter bag method in forest ecosystems. Release of elements from the litter, however, can be divided into three routes, that is, via liquid, solid, and gaseous phases, including leaching, fragmentation, and decomposer respiration, respectively. This study focused on water soluble elements in the decomposing needle litter of Japanese cedar (Cryptomeria japonica, D. Don) as a route of release via the liquid phase. Because rainfall leaches water soluble elements from decomposing litter on forest floor, the leaching process is important for the transport of elements from litter layers to soil horizons, which can affect the composition of soil solutions (Takahashi 1995). Moreover, nutrients in the leachate can be used for microbial and plant communities to control the dynamics of these communities. I investigated 1) the relationships between water soluble elements and the decomposition stages of the litter, and 2) variation in the contents of water soluble elements.     

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

Abstract

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

Recovering decomposing plant residues from the particulate soil organic matter fraction: size versus density separation

 A detailed size separation of particulate organic matter (POM) from soils amended with straw from Hordeum vulgare or Vicia sativa revealed that the loss of C during the first 56 days of incubation mainly occurred from particles >2,000 μm, without a concomitant reduction in the size of these large particles. Preliminary studies of POM from non-amended soil had shown that the stable heavy (>1.4 g cm^–3) POM fraction was mainly (>80%) composed of particles <400 μm, whereas the light fraction was dominated by larger particles (>80%). Therefore we decided to compare the POM <1.4 g cm³ with POM >400 μm. There was a very close relationship between POM>400 μm and POM <1.4 g cm^–3 with regard to amounts of C and N, as well as the appearance of these fractions under the microscope. Similarly there was a close relationship between changes in the C content of the POM fractions and the CO₂ respired, and this was also the case when comparing changes in POM-N with net N mineralization. This indicated that the biological activity during decomposition was actually localized in the POM. Due to the lighter workload and lower expenditure for reagents in connection with size separation of POM, we recommend the size separation procedure in connection with studies of residue decomposition in arable systems. Received: 23 May 2000     

Fungal biomass and nitrogen in decomposing scots pine needle litter

The development of fungal biomass and increase of amounts of N was studied in decomposing pine needle litter for about 3 yr. After a relatively rapid increase of the amount of mycelium the fungal biomass became rather constant after about 2 yr. The absolute amount of N in the needles increased between the 4th and the 16th months and this increase was correlated to the increase of fungal biomass in the needles.     

Direct effects of litter decomposition on soil dissolved organic carbon and nitrogen in a tropical rainforest

To clarify how litter decomposition processes affect soil dissolved organic carbon (DOC) and soil dissolved nitrogen (DN) dynamics, we conducted a field experiment on leaf litter and collected DOC and DN from the underlying soil in a tropical rainforest in Xishuangbanna, southwest China. Principal components analysis (PCA) showed the first PCA axis (corresponding to degraded litter quantity and quality) explained 61.3% and 71.2% of variation in DOC and DN concentrations, respectively. Stepwise linear regression analysis indicated that litter carbon mass controlled DOC and hemicellulose mass controlled DN concentrations. Litter decomposition was the predominant factor controlling surface-soil DOC and DN dynamics in this tropical rainforest.     

Near‐infrared spectroscopy can predict the composition of organic matter in soil and litter

The usefulness and limitations of near‐infrared reflectance spectroscopy (NIRS) for the assessment of several soil characteristics are still not sufficiently explored. The objective of this study was to evaluate the ability of visible and near‐infrared reflectance (VIS‐NIR) spectroscopy to predict the composition of organic matter in soils and litter. Reflectance spectra of the VIS‐NIR region (400–2500 nm) were recorded for 56 soil and litter samples from agricultural and forest sites. Spectra were used to predict general and biological characteristics of the samples as well as the C composition which was measured by ¹³C‐CPMAS‐NMR spectroscopy. A modified partial least‐square method and cross‐validation were used to develop equations for the different constituents over the whole spectrum (1st to 3rd derivation). Near‐infrared spectroscopy predicted well the C : N ratios, the percentages of O‐alkyl C and alkyl C, the ratio of alkyl C to O‐alkyl C, and the sum of phenolic oxidation products: the ratios of standard deviation of the laboratory results to standard error of cross‐validation (RSC) were greater than 2, the regression coefficients (a) of a linear regression (measured against predicted values) ranged from 0.9 to 1.1, and the correlation coefficients (r) were greater than 0.9. Satisfactorily (0.8 ≤ a ≤ 1.2, r ≥ 0.8, and 1.4 ≤ RSC ≤ 2.0) assessed were the contents of C, N, and production of DOC, the percentages of carbonyl C and aromatic C and the ratio of alkyl C to aromatic C. However, the N‐mineralization rate and the microbial biomass were predicted unsatisfactorily (RSC < 1.4). The good and satisfactory predictions reported above indicate a marked usefulness of NIRS in the assessment of biological and chemical characteristics of soils and litter.     

Mobilization of cesium in organic rich soils: Correlation with production of dissolved organic carbon

A study of the downward movement of ¹³⁷Cs in an undisturbed forest soil is presented. Seasonal variations and depth profiles of ¹³⁷Cs activities were measured in seepage water, which is the transport medium for the downward movement of anthropogenic substances in soils. Furthermore the correlation of ¹³⁷Cs mobilization and production of dissolved organic carbon (DOC) was investigated. Seasonal variations of both ¹³⁷Cs and DOC fluxes in the seepage water in a depth of 5 cm depth were observed, where the maximum fluxes in the summer months were about one order of magnitude higher than the minimum fluxes in the winter months. ¹³⁷Cs fluxes are found to be correlated with DOC fluxes with a correlation coefficient of r = 0.63, and both are highly correlated with soil temperature. This indicates that cesium is bound to soil organic material. The production of DOC is controlled by microbial decomposition of soil organic matter and we assume that this holds true for the ¹³⁷Cs release as well. The actual transport velocity (0.2 ± 0.14 mm/a) of ¹³⁷Cs (calculated by the weighed mean of ¹³⁷Cs concentration in the seepage water and the total ¹³⁷Cs content of the soil) is about one order of magnitude less than the mean transport velocity (1.2 ± 0.3 mm/a) over the past 25 years (calculated from the ¹³⁷Cs depth profile). It is possible that the transport velocity of ¹³⁷Cs in undisturbed soils decreases with time as it binds to aged organic material which is less easily decomposable than fresh organic material.