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Natural clay minerals can play an important role in crude remediation of wastewater polluted with the heavy metals (HMs) Cu, Zn and Ni. The presence and timing of addition of natural dissolved organic matter (DOM) have a significant effect on the HM removal by clay mineral sorbents. However, the influence of the presence of DOM on the remediation of the used clay mineral sorbents once saturated with HMs is largely unknown. To resolve this, clay mineral-rich soil column of varying composition, loaded (i) with Cu, Zn and Ni only, (ii) first with DOM followed by Cu, Zn and Ni, or (iii) with DOM, Cu, Zn and Ni simultaneously, was used in a set of desorption experiments. The soil columns were leached with 0.001 mol L−1CaCl2dissolved in water as control eluent and 0.001 mol L−1CaCl2dissolved in DOM as treatment eluent. During the preceding loading phase of the sorbent, the timing of DOM addition (sequential or concurrent with HMs) was found to have a significant influence on the subsequent removal of the HMs. In particular when the column was loaded with DOM and HMs simultaneously, largely irreversible co-precipitation took place. Our results indicate that the regeneration potential of clay mineral sorbents in wastewater treatment will be significantly reduced when the treated water is rich in DOM. In contrast, in manured agricultural fields (where HMs enter together with DOM), HM mobility will be lower than expected from interaction dynamics of HMs and clay minerals.

The fate of soil‐derived organic C (SOC) transported during erosion is a large uncertainty in assessing the impact of soil erosion on aquatic environments and in balancing C budgets. In our study, we determined C mineralization from solid soil organic C and dissolved organic C (DOC) translocated from a loess soil into surface water. We used runoff generated during rainfall simulation experiments. Both total runoff C and DOC were incubated to measure CO2evolution during 28‐d experiments. Cumulative CO2emissions from runoff accounted for 3.9 to 4.8% of initial runoff C. It was estimated that 3.3 to 3.7% of initial solid SOC was mineralized contributing to 69 to 80% of total C mineralization from runoff. Mineralization of DOC was larger (7.3–30.2% of initial DOC) and showed a much larger variability than mineralization from solid SOC. However, DOC mineralization contributed to 20 to 31% of total C mineralization from runoff only because of the much smaller amounts of DOC than solid SOC. We could confirm a preferential removal of labile C from soils by water erosion. Nevertheless, the majority of this C will contribute to an aquatic C sink with less than 5% being potentially mineralizable. Our results indicated that the base level of C mineralization from translocated C was derived from the solid phase whereas the variability depends largely on DOC.

The precipitation of dissolved organic matter DOM by aluminum Al results in a stable soil organic matter OM fraction. Extracellular enzymes can also be removed from soil solution by sorption or precipitation, but whether this affects their activity and their importance for carbon C mineralization is largely unknown. We studied the activity of eight extracellular enzymes, precipitated by Al together with DOM, in relation to C mineralization of the precipitated OM. Dissolved OM was obtained from the Oi and Oa horizon of two forest soils and precipitated at different Al : C ratios and pH values to achieve a large variation in composition and C mineralization of precipitated OM. All eight enzymes were present in a functional state in precipitated OM. On average 53 of DOM was precipitated, containing on average 17–41 of the enzyme activity EA involved in C degradation chitinase, cellobiohydrolase, glucosidase, glucuronidase, lacasse, and xylosidase previously present in soil solution. In contrast, on average only 4–7 of leucineaminopeptidase and acidphosphatase activity was found in precipitated OM. The EA found in precipitates significantly increased the percentage of C mineralized of precipitated OM, with a stronger influence of Cdegrading enzymes than enzymes involved in N and P cycling. However, after 8 weeks of incubation the correlations between EA and C mineralization disappeared, despite substantial EA being still present and only 0.5–7.7 of C mineralized. Thus, degradation of precipitated OM seems to be governed by EA during the first degradation phase, but the longterm stability of precipitated OM is probably related to its chemical properties.

Traditionally, the selective preservation of certain recalcitrant organic compounds and the formation of recalcitrant humic substances have been regarded as an important mechanism for soil organic matter (SOM) stabilization. Based on a critical overview of available methods and on results from a cooperative research program, this paper evaluates how relevant recalcitrance is for the long-term stabilization of SOM or its fractions. Methodologically, recalcitrance is difficult to assess, since the persistence of certain SOM fractions or specific compounds may also be caused by other stabilization mechanisms, such as physical protection or chemical interactions with mineral surfaces. If only free particulate SOM obtained from density fractionation is considered, it rarely reaches ages exceeding 50 y. Older light particles have often been identified as charred plant residues or as fossil C. The degradability of the readily bioavailable dissolved or water-extractable OM fraction is often negatively correlated with its content in aromatic compounds, which therefore has been associated with recalcitrance. But in subsoils, dissolved organic matter aromaticity and biodegradability both are very low, indicating that other factors or compounds limit its degradation. Among the investigated specific compounds, lignin, lipids, and their derivatives have mean turnover times faster or similar as that of bulk SOM. Only a small fraction of the lignin inputs seems to persist in soils and is mainly found in the fine textural size fraction (<20 µm), indicating physico-chemical stabilization. Compound-specific analysis of 13C : 12C ratios of SOM pyrolysis products in soils with C3-C4 crop changes revealed no compounds with mean residence times of > 40–50 y, unless fossil C was present in substantial amounts, as at a site exposed to lignite inputs in the past. Here, turnover of pyrolysis products seemed to be much longer, even for those attributed to carbohydrates or proteins. Apparently, fossil C from lignite coal is also utilized by soil organisms, which is further evidenced by low 14C concentrations in microbial phospholipid fatty acids from this site. Also, black C from charred plant materials was susceptible to microbial degradation in a short-term (60 d) and a long-term (2 y) incubation experiment. This degradation was enhanced, when glucose was supplied as an easily available microbial substrate. Similarly, SOM mineralization in many soils generally increased after addition of carbohydrates, amino acids, or simple organic acids, thus indicating that stability may also be caused by substrate limitations. It is concluded that the presented results do not provide much evidence that the selective preservation of recalcitrant primary biogenic compounds is a major SOM-stabilization mechanism. Old SOM fractions with slow turnover rates were generally only found in association with soil minerals. The only not mineral-associated SOM components that may be persistent in soils appear to be black and fossil C.

We summarize progress with respect to (1) different approaches to isolate, extract, and quantify organo-mineral compounds from soils, (2) types of mineral surfaces and associated interactions, (3) the distribution and function of soil biota at organo-mineral surfaces, (4) the distribution and content of organo-mineral associations, and (5) the factors controlling the turnover of organic matter (OM) in organo-mineral associations from temperate soils. Physical fractionation achieves a rough separation between plant residues and mineral-associated OM, which makes density or particle-size fractionation a useful pretreatment for further differentiation of functional fractions. A part of the OM in organo-mineral associations resists different chemical treatments, but the data obtained cannot readily be compared among each other, and more research is necessary on the processes underlying resistance to treatments for certain OM components. Studies using physical-fractionation procedures followed by soil-microbiological analyses revealed that organo-mineral associations spatially isolate C sources from soil biota, making quantity and quality of OM in microhabitats an important factor controlling community composition. The distribution and activity of soil microorganisms at organo-mineral surfaces can additionally be modified by faunal activities. Composition of OM in organo-mineral associations is highly variable, with loamy soils having generally a higher contribution of polysaccharides, whereas mineral-associated OM in sandy soils is often more aliphatic. Though highly reactive towards Fe oxide surfaces, lignin and phenolic components are usually depleted in organo-mineral associations. Charred OM associated with the mineral surface contributes to a higher aromaticity in heavy fractions. The relative proportion of OC bound in organo-mineral fractions increases with soil depth. Likewise does the strength of the bonding. Organic molecules sorbed to the mineral surfaces or precipitated by Al are effectively stabilized, indicated by reduced susceptibility towards oxidative attack, higher thermal stability, and lower bioavailability. At higher surface loading, organic C is much better bioavailable, also indicated by little 14C age. In the subsurface horizons of the soils investigated in this study, Fe oxides seem to be the most important sorbents, whereas phyllosilicate surfaces may be comparatively more important in topsoils. Specific surface area of soil minerals is not always a good predictor for C-stabilization potentials because surface coverage is discontinuous. Recalcitrance and accessibility/aggregation seem to determine the turnover dynamics in fast and intermediate cycling OM pools, but for long-term OC preservation the interactions with mineral surfaces, and especially with Fe oxide surfaces, are a major control in all soils investigated here.

Based on recent findings in the literature, we developed a process-oriented conceptual model that integrates all three process groups of organic matter (OM) stabilization in soils namely (1) selective preservation of recalcitrant compounds, (2) spatial inaccessibility to decomposer organisms, and (3) interactions of OM with minerals and metal ions. The model concept relates the diverse stabilization mechanisms to active, intermediate, and passive pools. The formation of the passive pool is regarded as hierarchical structured co-action of various processes that are active under specific pedogenetic conditions.To evaluate the model, we used data of pool sizes and turnover times of soil OM fractions from horizons of two acid forest and two agricultural soils. Selective preservation of recalcitrant compounds is relevant in the active pool and particularly in soil horizons with high C contents. Biogenic aggregation preserves OM in the intermediate pool and is limited to topsoil horizons. Spatial inaccessibility due to the occlusion of OM in clay microstructures and due to the formation of hydrophobic surfaces stabilizes OM in the passive pool. If present, charcoal contributes to the passive pool mainly in topsoil horizons. The importance of organo-mineral interactions for OM stabilization in the passive pool is well-known and increases with soil depth. Hydrophobicity is particularly relevant in acid soils and in soils with considerable inputs of charcoal. We conclude that the stabilization potentials of soils are site- and horizon-specific. Furthermore, management affects key stabilization mechanisms. Tillage increases the importance of organo-mineral interactions for OM stabilization, and in Ap horizons with high microbial activity and C turnover, organo-mineral interactions can contribute to OM stabilization in the intermediate pool. The application of our model showed that we need a better understanding of processes causing spatial inaccessibility of OM to decomposers in the passive pool.

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.

The forest floor was shown to be an effective sink of atmospherically deposited methylmercury (MeHg) but less for total mercury (Hgtotal). We studied factors controlling the difference in dynamics of MeHg and Hgtotal 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 Hgtotal 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 Hgtotal 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 Hgtotal 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.

In many forest ecosystems chronically large atmospheric deposition of N has caused considerable losses of inorganic N by seepage. Freezing and thawing of soil may alter the N turnover in soils and thereby the interannual variation of N seepage fluxes, which in turn makes it difficult to evaluate the N status of forest ecosystems. Here, we analyzed long-term monitoring data of concentrations and fluxes of dissolved inorganic N (DIN) in throughfall and seepage from a Norway spruce stand at the Fichtelgebirge (SE Germany) between 1993 and 2004. Despite constant or even slightly increasing N inputs in throughfall, N losses with seepage at 90 cm declined from 15–32 kg N ha–1 y–1 in the first years of the study period (1993–1999) to 3–10 kg N ha–1 y–1 in 2000 to 2004. The large N losses in the first years coincided with extreme soil frost in the winter of 1995/96, ranging from –3.3°C to –1.0°C at 35 cm soil depth. Over the entire observation period, maximum fluxes of nitrate and ammonium were observed in the mineral soil following thawing of the soil. The elevated ammonium and nitrate fluxes resulted apparently from increased net ammonification and nitrification rates in the mineral soil, whereas mineral-N fluxes in the O horizon were less affected by frost. Our data suggest that (1) extreme soil frost may cause substantial annual variations of nitrate losses with seepage and that (2) the assessment of the N status of forest ecosystems requires long periods of monitoring. Time series of biogeochemical data collected over the last 20–30 y include years with extreme cold winters and warm summers as well as unusual precipitation patterns. Analysis of such long-term monitoring data should address climate extremes as a cause of variation in N outputs via leaching. The mean loss of 14.7 kg N with seepage water during 12 y of observation suggests that the forest ecosystem was saturated with N.

The amount and composition of rhizo-deposition as an important component of dissolved organic carbon (DOC) in forest soils is largely unknown. The objective of our study was to examine one component of DOC production in forest soils, namely rhizo-deposition, under conditions of low, moderate, and high inorganic-nitrogen (N) input. Two-year-old Picea abies saplings were incubated in hydroponic solution for a period of 2 weeks, comprising an adaptation and an experimental period of each 1 week. Treatments included 0 (zero N), 1, 10 (moderate N), and 100 (high N) mg N L–1 nitrogen addition split equally between nitrate and ammonium. Quantity and quality of dissolved-organic-carbon (DOC) production were investigated. Cumulative DOC production ranged between 3.7 and 8.8 mg C (g root dry weight)–1 (7 d)–1 and was significantly reduced in zero- and high-N treatments. In turn, the specific UV absorbance as a measure of the aromaticity increased significantly in zero- and high-N treatments, indicating a reduced contribution of microbial products to rhizo-deposited C. Mineralization of rhizo-deposited DOC was not significantly different between N treatments at day 1 of the experimental period, but by day 7, 69% of rhizo-deposition in zero-N hydroponic solution was mineralizable compared to 37%–49% for the moderate- and high-N treatments. Increasing N input tended to result in decreased degradability. We conclude from this short-term experiment that rhizo-deposited DOC from Norway spruce displays a fast response to changes in inorganic N.

Dissolved organic carbon (DOC) constitutes an important carbon input flux to forested mineral soils. Seepage from mineral subsoils contains only small amounts of DOC because of mineralization, sorption or the formation of particulate organic matter (POM). However, the relation between these processes is largely unknown. Therefore, the objective of this study was to quantify the mineralization of DOC from different depths of forest soils, and to determine degradation rate constants for rapidly and slowly degradable DOC pools. Mineralization of DOC and formation of POM in mineral soil solution from two forested sites in northern Bavaria (Germany) were quantified in a 97 days laboratory incubation experiment. Furthermore, spectroscopic properties such as specific UV absorption and a humification index derived from fluorescence emission spectrometry were measured before and after incubation. DOC in all samples turned out to belong mainly to the stable DOC pool (> 95 %) with half-lives ranging from years to decades. Spectroscopic properties were not suitable to predict the mineralization of DOC from mineral soils. However, together with data on DOC from the forest floor and long-term data on DOC concentrations in the field they helped to identify the processes involved in C sequestration in mineral subsoils. Mineralization, formation of POM, and probably sorption seem all to be responsible for maintaining low concentrations of DOC in the upper mineral soil. DOC below the upper mineral soil is highly resistant to mineralization, and thus the further decrease of DOC concentrations in the subsoil as observed under field conditions cannot be attributed to mineralization. Our results suggest that sorption and to some minor extent the formation of POM may be responsible for C sequestration in the subsoil.

Fulvic acids (FAs) from topsoil and ground water solutions were investigated to discover effects of land use and peat degradation on their molecular chemical composition and thermal properties. The FAs were extracted from three Gleysols under arable land, intensive and extensive grassland, and from three Histosols under alder forest, extensive grassland, and a natural succession in a long-term (> 200 years) cultivated fen area. Functional groups and molecular subunits of the FAs were investigated by ¹³C Nuclear Magnetic Resonance (¹³C NMR) spectroscopy. Thermal properties and structural molecular subunits were investigated by off-line pyrolysis, and Pyrolysis-Field Ionization Mass Spectrometry (Py-FIMS). The ¹³C NMR spectra showed that the FAs from topsoil solutions had smaller proportions of alkyl C (mean: —8 %) and more aromatic C (mean: + 6 %) than FAs from ground water. This clear differentiation of dissolved FAs in the soil profiles is consistent with Py-FIMS data which have shown enrichments of lipids in ground water FAs. Furthermore, Py-FIMS revealed that the FAs from topsoils were richer in phenols + lignin monomers, carbohydrates, as well as mostly aromatic N-containing compounds. These molecular subunits of FAs, relatively enriched in topsoil, were also the main indicators of land use and peat degradation. For topsoil solutions, the proportions of phenols + lignin monomers and carbohydrates increased stepwise with peat degradation in Gleysols and Histosols. Correspondingly, the thermal properties indicated the incorporation of these compounds into FAs by chemical bonds of larger thermal stability. Statistical evaluation by principal component analysis of Py-FIMS clearly supported the differentiation of FAs according to the origin from topsoils and ground water, different soil types, and land use and peat degradation. Hence, it is concluded that water soluble FAs can be utilized as objective ecological indicators for soil effects on adjacent ground and surface waters.

The high variability of dissolved organic matter (DOM) in natural systems (concentration, composition) means rapid methods are required for its characterization so that a high number of samples can be analyzed. The objective of the present study was to quantify the effects of spectrometer type and dissolved organic carbon (DOC) concentration on the humification indices of water-soluble fulvic acids (FAs) derived from synchronous fluorescence spectra, and thus enable the broader application of this method for DOM characterization. We used three standard FAs from the International Humic Substances Society, 24 water-soluble FAs isolated from topsoil, groundwater and surface water in a fen area, and two different spectrometers. The wavelengths at which bands occurred were similar for all the FAs. Therefore, the differences between the spectra of the FAs studied could be described by humification indices (band ratios). The humification indices calculated correlated very well between spectrometers despite small differences in the wavelengths of bands and shoulders. The absolute values of these indices deduced from two spectrometers can only be directly compared if the spectra are corrected using a standard substance. Increasing DOC concentration resulted in a linear increase in humification indices with a sample specific slope. Therefore, we recommend using an uniformly low DOC concentration of about 10 mg C l^—1 for recording the spectra of samples with typically low DOC concentrations (aquatic samples, soil solutions). This value is a compromise between relatively low absorption to minimize inner filter effects and a sufficient signal-to-noise ratio.