Dissolved Organic Matter Retention Research Articles

Complementary effects of sorption and biochemical processing of dissolved organic matter for emerging structure formation controlled by soil texture

AbstractBackgroundPercolating dissolved organic matter (DOM) from the topsoil is considered the main source of subsoil organic carbon (OC) in temperate soils, but knowledge about its influence on OC storage and structure‐forming processes is limited.AimsWe conducted a 30‐day incubation experiment with artificial soils to study the effects of percolating DOM and soil texture on OC turnover and initial structure formation.MethodsArtificial soils with contrasting texture, but identical mineral composition, were used to mimic subsoil conditions, where mineral surfaces free of OM come into contact with percolating DOM. After the incubation, we assessed the solution exchange, OM covers on minerals, microbial community and OC turnover, and aggregate formation and stability.ResultsA higher sand content caused a lower porosity, accompanied by a lower moisture content. In contrast, the OC retention (21% of the OC input), microbial activity, and community size were unaffected by soil texture. The OM covered 10% of the mineral surfaces within an otherwise OC‐free mineral matrix. The formation of large, water‐stable aggregates occurred in all soils, but was pronounced in the clay‐rich soils (58% mass contribution), which also supported a higher mechanical stability of the aggregates.ConclusionsThe initial retention and microbial mineralization of DOM are decoupled from pore sizes and soil solution exchange but are driven by the mineral composition and OC input. The biochemical processing of the percolating DOM can induce large aggregates. Here, the presence of fine mineral particles enhances the formation and mechanical stability of the aggregates, irrespective of their surface charge or sorptive properties.

Retention of dissolved organic matter during podzolisation: Testing processes in laboratory experiments and at the submicron scale

Immobilisation of organic matter in illuvial subsoil horizons is a characteristic result of Podzol formation and development. We tested dissolved organic matter (DOM) immobilisation by mineral mixtures as models of initial illuvial horizons and conducted laboratory experiments by irrigating mineral mixtures with a DOM solution and analysing the eluate solutions. The mineral mixtures consisted of quartz with accessory H+-saturated illite, or Ca2+-saturated illite, or quartz with ferrihydrite. Furthermore, we studied the transport of colloidal proto-imogolite in the ferrihydrite-containing mixture and analysed the spatial distribution of elements by nanoscale secondary ion mass spectrometry (NanoSIMS). Retention of DOM in the presence of Ca2+-saturated illite did not exceed that in the presence of H+-saturated illite, as the promoting effect of the divalent cation was counterbalanced by the initially higher pH. The mineral mixture containing ferrihydrite retained most DOM. In all experiments, aromatic and oxidised DOM moieties were preferentially adsorbed. Colloidal proto-imogolite co-precipitated with DOM and reduced the pore space of the mineral mixture, which increased the residence time and DOM adsorption on ferrihydrite. NanoSIMS analyses showed proto-imogolite deposited on ferrihydrite, with ferrihydrite as the more important DOM adsorbent in ferrihydrite-proto-imogolite assemblages. Furthermore, NanoSIMS revealed less N in adsorbed DOM closer to the mineral surface. Thus, we differentiated between processes of DOM retention in simulated early podzolisation, detecting preferentially adsorbed species and compositional differences within adsorbed DOM layers.

Secondary Mineral Formation and Carbon Dynamics during FeS Oxidation in the Presence of Dissolved Organic Matter.

Iron (Fe) minerals constitute a major control on organic carbon (OC) storage in soils and sediments. While previous research has mainly targeted Fe (oxyhydr)oxides, the impact of Fe sulfides and their subsequent oxidation on OC dynamics remains unresolved in redox-fluctuating environments. Here, we investigated the impact of dissolved organic matter (DOM) on FeS oxidation and how FeS and its oxidation may alter the retention and nature of DOM. After the anoxic reaction of DOM with FeS, FeS preferentially removed high-molecular-weight and nitrogen-rich compounds and promoted the formation of aqueous sulfurized organic molecules, according to Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) analysis. When exposed to O2, FeS oxidized to nanocrystalline lepidocrocite and additional aqueous sulfurized organic compounds were generated. The presence of DOM decreased the particle size of the resulting nano-lepidocrocite based on Mössbauer spectroscopy. Following FeS oxidation, most solid-phase OC remained associated with the newly formed lepidocrocite via a monodentate chelating mechanism (based on FTIR analysis), and FeS oxidation caused only a slight increase in the solubilization of solid-phase OC. Collectively, this work highlights the under-appreciated role of Fe sulfides and their oxidation in driving OC transformation and preservation.

Selective sorption and desorption of DOM in podzol horizons — FTIR and Py-GC/MS of leachates from a column experiment

The sorption of dissolved organic matter (DOM) depends on its interaction with the soil matrix. In hydromorphic podzols, DOM reacts mainly with aluminium (Al), which is responsible for the formation of the Bh-horizon in the subsoil. In this work, we investigated whether the retention of DOM in the soil during the podzolization process is selective in relation to the molecular composition of DOM. A column experiment was conducted to study the selective retention of sorption and desorption processes under controlled conditions. Materials used in the column experiment were representative for Brazilian coastal podzols under tropical rainforest. Materials were collected from this tropical coastal podzol ecosystem, and included soil from E- and Bh-horizons, and DOM from a stream (Stream), peat water (Peat), litter (Litter) and charred litter (Char). To evaluate selective retention of DOM, both the initial DOM and its leachates were analyzed by Fourier transform infrared spectra absorption (FTIR) and pyrolysis gas-chromatography/mass spectrometry (Py-GC/MS). The results showed preferential retention of DOM associated with biopolymers for soil columns with E-horizon material (E), E with Al nitrate (E-n), E with kaolinite (E-k) and E with gibbsite (E-h), except for Char. The composition of leachates after percolation through B horizon columns was mainly determined by desorption, and had a relatively large contribution from phenolic and carboxylic groups associated with Al and low molecular weight aromatic and N-containing pyrolysis products, while products from macromolecular materials such as cellulose were selectively retained in the columns for all DOM types. DOM from the Stream (taken during the rainy season) resembled that of desorbed OM from the B columns, reinforcing substantial desorption in the field as well. Our results suggest that sorption and desorption of OM in the hydromorphic Bh-horizon is continuous and that the selectivity of sorption is dependent on DOM source.

Mobilisation of Al, Fe, and DOM from topsoil during simulated early Podzol development and subsequent DOM adsorption on model minerals

Podzols are characterised by mobilisation of metals, particularly Al and Fe, and dissolved organic matter (DOM) in topsoil horizons, and by immobilisation in subsoil horizons. We mimicked element mobilisation during early podzolisation by irrigating the AE horizon of a Dystric Arenosol with acetic acid at different flow velocities and applying flow interruptions to study rate-limited release in experiments with soil cylinders. We used eluates in batch experiments with goethite and Al-saturated montmorillonite to investigate DOM reactivity towards minerals. Both the flow velocity and flow interruptions affected element release, pointing to chemical non-equilibrium of release and to particles, containing Fe and OM mobilised at larger flow velocity, characteristic of heavy rain or snowmelt. Based on chemical extractions, the source of mobilised Al and Fe, the vast majority of which was complexed by DOM, was no oxide phase, but rather organic. Rate limitation also affected the composition of DOM released. Carboxyl and phenolic species were the most important species adsorbed by both minerals. However, DOM composition affected the extent of DOM adsorption on goethite more distinctly than that on montmorillonite. Our findings evidence that the intensity of soil percolation affects quantitative and qualitative element release during early podzolisation and adsorptive DOM retention in subsoil horizons.

Release of dissolved organic matter (DOM) in an autotrophic and productive stream in Pampean region

The aim of our study was to analyze dissolved organic carbon processing at a reach of a productive Pampean stream and to determine DOM retention efficiencies under a wide range of hydrological conditions. This study was carried out in Las Flores stream, a third-order stream located in the Lujan River basin in the northeast of the Buenos Aires province, Argentina. We selected a 2.2 km reach of Las Flores stream. On each sampling occasion, we collected water samples at the end of the reach, at two upstream tributaries, and at the potential hydrological contributors (end members) to stream flow. We determined DOC concentration and DOM optical properties in the stream and end members by combining absorbance–fluorescence spectroscopy techniques. The reach acts as a reactor for DOC concentration at baseflow condition, when in situ production processes prevailed. However, DOC is passively transported downstream at high flow condition. We found that humic and more degraded compounds predominate in DOM at the end of the reach. According to this, we proposed that both protein-like and humic-like compounds are produced at the study reach through in situ production processes, such as biological production from periphyton and macrophytes decomposition.

Plant source and soil interact to determine characteristics of dissolved organic matter leached into waterways from riparian leaf litter

Wetting of leaf litter accumulated in riparian zones during rainfall events provides pulses of dissolved organic matter (DOM) to rivers. Restoring riparian vegetation aims to reduce sediment and nutrient transport into rivers, however DOM from leaf litter can stimulate phytoplankton growth and interfere with water treatment processes. Improved understanding of the loads and chemical composition of DOM leached from leaf litter of different plant species, and how subsequent leaching through soils affects DOM retention or transformation, is needed to predict the outcomes of riparian revegetation. To investigate this, we simulated rapid leaching of rainfall through the leaf litter of two riparian tree species with and without subsequent leaching through soil, comparing dissolved organic carbon (DOC) and nitrogen (DON) loads, and DOM chemical composition (via spectroscopic and novel NMR-fingerprinting techniques). Plant source affected the load and composition of DOM leaching, with Eucalyptus tereticornis leaching more DOC than Casuarina cunninghamiana. Additionally, E. tereticornis DOM had a higher sugar, myo-inositol, benzoic acid, flavonoid and oxygenated aromatic content. More than 90% of leaf litter DOM was retained in the soil under simulated repeated heavy rainfall. The DOM chemistry of these species determined the total loads and changes in DOM composition leaching through soil. Less E. tereticornis DOM was retained by the soil than C. cunninghamiana DOM, with sugars, myo-inositol and amino acids being poorly retained compared to fatty acids and aromatic compounds. It also appears that DOM from E. tereticornis litter primed the soil, resulting in more DON being leached compared with bare soil. In comparison, C.cunninghamiana litter resulted in greater retention of DON, oxygenated aromatic compounds and the amino acid tryptophan. This study provides new information on how a range of DOM sources and transformations affect the DOM ultimately leached into waterways, key to developing improved models of DOM transformations in catchments.

Supramolecular Association Impacts Biomolecule Adsorption onto Goethite.

Formation of biomolecule-rich supramolecular complexes in dissolved organic matter (DOM) and subsequent adsorption onto minerals is important for the development of mineral-stabilized organic matter, yet the impact of supramolecular association on interfacial behavior is seldom studied. A series of supramolecular complexes of model biomolecules (deoxyribonucleic acid (DNA) and bovine serum albumin (BSA)) are synthesized, characterized, and adsorbed onto goethite. Complexes represent 0.1 mg/mL DNA mixed with BSA concentrations from 0.05 to 0.5 mg/mL in 5 mM KCl at pH = 5.0. Circular dichroism demonstrates strong binding between DNA and BSA, with DNA saturation when (BSA) ≈ 0.4 mg/mL. Dynamic light scattering and electrophoretic mobility measurements suggest DNA-BSA binding reduces DNA-DNA electrostatic repulsion. Spectroscopic studies of DNA/BSA complex adsorption show complexation hinders coordination of DNA phosphodiester groups with goethite. Increasing BSA (≤0.4 mg/mL) in DNA/BSA complexes enhances DNA adsorption, due to reduced repulsion between adsorbed DNA helices. When (BSA) > 0.4 mg/mL, however, DNA adsorption is decreased. We hypothesize this results from blocking of surface sites by fast adsorption of BSA loosely associated with DNA/BSA complexes. We posit an additional mechanism describing multilayered architecture formation of organo-mineral associations in soil, suggesting solution interactions may represent an overlooked factor when considering mineral retention of DOM.

Molecular size-dependent abundance and composition of dissolved organic matter in river, lake and sea waters

Dissolved organic matter (DOM) is ubiquitous in natural waters. The ecological role and environmental fate of DOM are highly related to the chemical composition and size distribution. To evaluate size-dependent DOM quantity and quality, water samples were collected from river, lake, and coastal marine environments and size fractionated through a series of micro- and ultra-filtrations with different membranes having different pore-sizes/cutoffs, including 0.7, 0.4, and 0.2 μm and 100, 10, 3, and 1 kDa. Abundance of dissolved organic carbon, total carbohydrates, chromophoric and fluorescent components in the filtrates decreased consistently with decreasing filter/membrane cutoffs, but with a rapid decline when the filter cutoff reached 3 kDa, showing an evident size-dependent DOM abundance and composition. About 70% of carbohydrates and 90% of humic- and protein-like components were measured in the <3 kDa fraction in freshwater samples, but these percentages were higher in the seawater sample. Spectroscopic properties of DOM, such as specific ultraviolet absorbance, spectral slope, and biological and humification indices also varied significantly with membrane cutoffs. In addition, different ultrafiltration membranes with the same manufacture-rated cutoff also gave rise to different DOM retention efficiencies and thus different colloidal abundances and size spectra. Thus, the size-dependent DOM properties were related to both sample types and membranes used. Our results here provide not only baseline data for filter pore-size selection when exploring DOM ecological and environmental roles, but also new insights into better understanding the physical definition of DOM and its size continuum in quantity and quality in aquatic environments.

Small scale variability of vertical water and dissolved organic matter fluxes in sandy Cambisol subsoils as revealed by segmented suction plates

Dissolved organic matter (DOM) is considered as a major carbon source in subsoils. As soil water fluxes are highly variable at small scale, and transport versus sorptive retention of DOM is related to water flux and associated contact time with minerals, knowledge of the small scale spatial variability of the dissolved organic carbon (DOC) concentrations and fluxes into the subsoil is decisive for a solid estimation of organic carbon (OC) translocation into the subsoil. Here, we made advantage of novel segmented suction plates (4 × 4 segments, each 36 cm2) to analyze the small scale spatial and temporal variability of DOC transport at 10, 50 and 150 cm depth of three subsoil observatories (approximately 50 m apart) in a sandy Dystric Cambisol under beech in the Grinderwald, 40 km northwest from Hannover, Germany. Water fluxes, DOC concentrations and fluxes as well as the specific UV absorbance (SUVA) at 280 nm were determined in weekly samples from August 2014 to November 2015 for each individual segment. The DOC fluxes decreased with depth (19.6 g C m−2 year−1, 10 cm; 1.2 g C m−2 year−1, 150 cm) and were strongly related to the water fluxes. The SUVA at 280 nm also decreased with depth (0.03 L mg C−1 cm−1, 10 cm; 0.01 L mg C−1 cm−1, 150 cm), indicating a selective retention of aromatic moieties, that was eased with increasing water flux at least in the subsoil. The proportion of temporal fluctuations and small scale variability on the total variance of each parameter where determined by the calculation of intra class correlations. The seasonal heterogeneity and the small scale spatial heterogeneity were identified to be of major importance. The importance of the small scale spatial heterogeneity strongly increased with depth, pointing towards the stability of flow paths and suggesting that at a given substrate hydrological processes rather than physicochemical processes are decisive for the sorptive retention of DOM and the variability of OC accumulation in the subsoil. Our results clearly show the demand of small scale sampling for the identification of processes regarding carbon cycling in the subsoil.

Hydrological conditions control in situ DOM retention and release along a Mediterranean river

Uncertainties exist regarding the magnitude of in situ dissolved organic matter (DOM) processing in lotic systems. In addition, little is known about the effects of extreme hydrological events on in-stream DOM retention or release during downriver transport.This study quantified the net in-stream retention/release efficiencies (η) of dissolved organic carbon (DOC) and its humic and protein-like fluorescent fractions along a Mediterranean river during drought, baseflow and flood conditions. High performance size exclusion chromatography was used to describe the apparent size distributions of the humic and protein-like DOM moieties. A snapshot mass balance allowed estimating the η values of DOC and humic and protein-like fractions. Significant DOM net retention (η < 0) was detected during the drought condition and the protein-like fraction was more retained than the humic-like fraction and bulk DOC. In addition, small substances were more efficiently retained than larger substances. DOC retention decreased under baseflow conditions, but it remained significant. The humic and protein-like net efficiencies exhibited high variability, but the net retention were not significant. From a longitudinal perspective, the entire fluvial corridor contributed net retention of DOC and humic and protein-like moieties net retention during drought condition. In contrast, net retention/release efficiencies exhibited spatial variability during baseflow condition. The flood preferentially mobilized large size DOM molecules and the fluvial corridor behaved as a homogeneous passive DOM (η = 0) conduit.This research highlights the relevance of hydrological extreme events on the magnitude of DOM retention/release mass balance and emphasizes the need to perform measurements during these conditions to quantify the impact of fluvial corridors on DOM fate and transport.

Tracking litter-derived dissolved organic matter along a soil chronosequence using 14C imaging: Biodegradation, physico-chemical retention or preferential flow?

The cycling of dissolved organic matter (DOM) in soils is controversial. While DOM is believed to be a C source for soil microorganisms, DOM sorption to the mineral phase is regarded as a key stabilization mechanism of soil organic matter (SOM). In this study, we added 14C-labelled DOM derived from Leucanthemopsis alpina to undisturbed soil columns of a chronosequence ranging from initial unweathered soils of a glacier forefield to alpine soils with thick organic layers. We traced the 14C label in mineralized and leached DOM and quantified the spatial distribution of DO14C retained in soils using a new autoradiographic technique. Leaching of DO14C through the 10 cm-long soil columns amounted up to 28% of the added DO14C in the initial soils, but to less than 5% in the developed soils. Biodegradation hardly contributed to the removal of litter-DO14C as only 2–9% were mineralized, with the highest rates in mature soils. In line with the mass balance of 14C fluxes, measured 14C activities in soils indicated that the major part of litter DO14C was retained in soils (>80% on average). Autoradiographic images showed an effective retention of almost all DO14C in the upper 3 cm of the soil columns. In the deeper soil, the 14C label was concentrated along soil pores and textural discontinuities with similarly high 14C activities than in the uppermost soil. These findings indicate DOM transport via preferential flow, although this was quantitatively less important than DOM retention in soils. The leaching of DO14C correlated negatively with oxalate-extractable Al, Fe, and Mn. In conjunction with the rapidity of DO14C immobilization, this strongly suggests that sorptive retention DOM was the dominating pathway of litter-derived DOM in topsoils, thereby contributing to SOM stabilization.

Potential adsorption of dissolved organic matter in poorly podzolised, high-latitude soils

Dissolved organic matter (DOM) plays an essential role in many biogeochemical processes in soil and water, and the retention of DOM in soils influences the amount and composition of DOM exported from terrestrial to aquatic ecosystems. Adsorption in soil mineral horizons is considered a main mechanism by which DOM is retained and stabilised in soils, and podzol soils have received attention due to their capacity to retain DOM in the mineral B horizon. With the projected increase in temperature and precipitation at higher latitudes, the transport of organic matter is expected to increase, but few studies have investigated the adsorption of DOM in poorly podzolised soils at higher latitudes. In this study, we used batch equilibrium experiments to investigate the potential for DOM adsorption in the B horizon of poorly podzolised soils from a confined area in Neiden, northern Norway. We used the Initial Mass (IM) isotherm to describe the amount of dissolved organic carbon (DOC) removed from or released to solution by the soil as a linear function of the amount of DOC added to the soil–water suspension.Our results suggest that the affinity for DOC of B horizons in poorly podzolised high-latitude soils is largely determined by the content of amorphous Fe and Al oxides, where adsorption is limited by the saturation of amorphous Fe and Al with organic matter. The B horizons show a relatively large variation in DOC sorption characteristics, with an overall low potential to adsorb added DOC at field relevant DOC concentrations. Six out of twelve B horizons show a net DOC desorption over the whole range of added DOC (0–480mgCkg−1 soil). These findings indicate a limited potential for retention and stabilisation of DOM in poorly podzolised soils at higher latitudes in a warmer and wetter climate.

Structural stability of coprecipitated natural organic matter and ferric iron under reducing conditions

The objective was to assess the interaction of Fe coprecipitated with dissolved organic matter (DOM) and its effect on Fe (hydr)oxide crystallinity and DOM retention under abiotic reducing conditions. A Fe-based coagulant was reacted with DOM from an agricultural drain and the resulting precipitate (floc) was exposed to S(-II) and Fe(II). Solution concentrations of Fe(II/III) and DOM were monitored, floc crystallinity was determined using X-ray diffraction, and the composition and distribution of functional groups were assessed using scanning transmission X-ray microscopy (STXM) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Results indicate coprecipitation of Fe(III) with DOM forms a non-crystalline floc that withstands crystallization regardless of change in pH, Fe:DOM ratio and type of reductant added. There was no evidence that exposure to reducing conditions led to release of DOM from the floc, indicating that coprecipitation with complex natural DOM in aquatic environments may stabilize Fe (hydr)oxides against crystallization upon reaction with reduced species and lead to long term sequestration of the DOM. STXM analysis identified spatially distinct regions with remarkable functional group purity, contrary to the model of DOM as a relatively uniform complex polymer lacking identifiable organic compounds. Polysaccharide-like OM was strongly and directly correlated with the presence of Fe but showed different Fe binding strength depending on the presence of carboxylic acid functional groups, whereas amide and aromatic functional groups were inversely correlated with Fe content.

A Review of Dissolved Organic Matter Transport Processes Affecting Soil and Environmental Quality

Dissolved organic matter (DOM) affects several processes in soil and water including nutrient cycling, soil and water pollution and CO2 flux between the soil and atmosphere. The aim of this review is to collate and synthesize the literature on the transport processes of DOM in soil. The DOM normally comprises of only a small fraction of soil organic matter (SOM) and originates mainly from the decomposition and solubilization of SOM, which is accumulated on soil surface or soil profile from plant residues and additions of organic amendments such as animal and poultry manures and other biosolids. The DOM is one of the most reactive and mobile SOM fractions and has a major influence on biogeochemical processes in both terrestrial and aquatic environments. Terrestrially borne DOM is subjected to microbial decomposition, photodegradation and adsorption on soil mineral surfaces. It is sorbed on mineral surfaces and high adsorption capacities of clay minerals and oxides for DOM sorption are demonstrated in laboratory studies. However, these high sorption values are not reproduced in limited field studies. Similarly, a few data available on the transport of DOM through macropores also demonstrate the limited control of sorption on DOM retention in soil profile. Thus, there is a need to further investigate the physical and chemical protection mechanisms, as well as the biodegradability of DOM shown in laboratory studies. There is an increasing need to clearly understand the formation, fate and transport of DOM at field scales. The environmental factors such as precipitation and temperature, land use change, land management, and biological factors have profound and discrete influences on DOM dynamics in soil profile. Future research efforts must focus on the assessment of the influences of these factors by conducting field studies in different climatic zones, soils, and land use and management systems.

Concentrations and fluxes of dissolved organic carbon and nitrogen in a Picea abies chronosequence on former arable land in Sweden

Shifting land use from agriculture to forestry induces major changes in the carbon (C) and nitrogen (N) cycles, including fluxes of dissolved organic carbon (DOC) and nitrogen (DON). This study investigated the long-term effects of afforestation on ecosystem DOC and DON dynamics using a chronosequence approach comprising four arable fields and nine differently aged (10–92 years) Norway spruce stands growing on similar former arable soils in the same area. Along the chronosequence, concentrations and fluxes of DOC and DON were determined in bulk precipitation, throughfall, O horizon leachate and mineral soil solution during a 2–3-year period. Soil water fluxes were calculated using a soil hydrological model (SWAP). Results showed that DOC concentrations and fluxes with throughfall were strongly positively correlated with tree height (r2=0.95; P<0.05 for both conc. and flux) and stand age, while DON showed no such trends, suggesting different origins of DOC and DON in throughfall. The highest concentrations and fluxes of DOC and DON occurred in soil leachate from the O horizon. Here, DOC flux was 250–310kgCha−1yr−1 and DON flux 8–9kgNha−1yr−1 in stands afforested between 65 and 92 years ago. Concentrations and fluxes of DOC and DON in the mineral subsoil were consistently low. Flux calculations suggest that there was a net loss of >90% (230–280kgha−1yr−1) of DOC leached from the O horizon within 0–60cm of the mineral soil. There was no significant effect of land use or forest age on DOC concentrations in solution from the lower part of the A horizon. The effect of time since afforestation was masked by soil properties that influence DOM retention in the mineral soil. Our data indicate that DOC concentrations in the A horizon of the sites studied were primarily related to the oxalate-extractable Al and Fe amounts in the same horizon. Afforestation of arable land induced a gradual qualitative change in soil organic matter (SOM) and dissolved organic matter (DOM), with significantly increasing C:N ratios in soil and soil solution over time. The development of an O horizon and the subsequent leaching of DOC and DON to the underlying mineral soil are important drivers of a changing soil C and N turnover following afforestation.

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

AbstractDissolved 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 &lt;30 y in solution to &gt;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.

Chemistry and Dynamics of Dissolved Organic Matter in a Temperate Coniferous Forest on Andic Soils: Effects of Litter Quality

Dissolved organic matter (DOM) plays an important role in transporting carbon and nitrogen from forest floor to mineral soils in temperate forest ecosystems. Thus, the retention of DOM via sorption or microbial assimilation is one of the critical steps for soil organic matter formation in mineral soils. The chemical properties of DOM are assumed to control these processes, yet we lack fundamental information that links litter quality, DOM chemistry, and DOM retention. Here, we studied whether differences in litter quality affect solution chemistry and whether changes in litter inputs affect DOM quality and removal in the field. The effects of litter quality on solution chemistry were evaluated using chemical fractionation methods for laboratory extracts and for soil water collected from a temperate coniferous forest where litter inputs had been altered. In a laboratory extraction, litter type (needle, wood, root) and the degree of decomposition strongly influenced solution chemistry. Root litter produced more than 10 times more water-extractable dissolved organic N (DON) than any other litter type, suggesting that root litter may be most responsible for DON production in this forest ecosystem. The chemical composition of the O-horizon leachate was similar under all field treatments (doubled needle, doubled wood, and normal litter inputs). O-horizon leachate most resembled laboratory extracts of well-decomposed litter (that is, a high proportion of hydrophobic acids), in spite of the significant amount of litter C added to the forest floor and a tendency toward higher mean DOM under doubled-Litter treatments. A lag in DOM production from added litter or microbial modification might have obscured chemical differences in DOM under the different treatments. Net DOM removal in this forest soil was strong; DOM concentration in the water deep in the mineral soil was always low regardless of concentrations in water that entered the mineral soil and of litter input manipulation. High net removal of DOM from O-horizon leachate, in spite of extremely low initial hydrophilic neutral content (labile DOM), coupled with the lack of influence by season or soil depth, suggests that DOM retention in the soil was mostly by abiotic sorption.

CONCENTRATIONS AND PROPERTIES OF DISSOLVED ORGANIC MATTER IN FOREST SOILS AS AFFECTED BY THE REDOX REGIME

Changes in concentrations and properties of dissolved organic matter (DOM) caused by oxygen deficiency are poorly understood. We estimated the influence of redox conditions on DOM dynamics in the field, sampling soil solutions at different depths of three soils (Humic and Histic Gleysol, Chromic Cambisol) along a soil catena in the cool-humid Black Forest (Germany) over a period of 2 years. We measured dissolved organic carbon (DOC) and determined the specific absorbance at 280 nm and two humification indices derived from fluorescence spectra to describe the aromaticity and complexity of DOM. Redox potential (Eh) was monitored continuously in situ. In the forest floor, DOC concentrations ranged independent of soil organic matter content and redox regime between 40 and 60 mg C L−1. DOC concentrations in all soils decreased with depth, accompanied by a decrease in DOM aromaticity and complexity. In the mineral subsoil, DOC concentrations, aromaticity, and DOM complexity were smallest in the aerobic soil (Chromic Cambisol; Eh > 500 mV) and largest in the most anaerobic soil (Histic Gleysol; Eh < 100 mV). Large DOM retention in the aerobic soil could be related to high contents of Fe oxides, highlighting their importance for DOM adsorption. Despite significantly reduced DOM retention under anaerobic conditions, it remains relatively large because the main DOM adsorbents changed from Fe oxides under oxic conditions to clay minerals, which were about 100 times more abundant under anaerobic conditions than Fe oxides. We found indications that biodegradation of DOM contributes more to DOM retention under anaerobic conditions, and we conclude that large DOM fluxes from anaerobic forest soils are the result of limited DOM adsorption in the subsoil rather than large DOM release from the topsoil.

Dissolved organic matter in soil: challenging the paradigm of sorptive preservation

During the last decade, research in sedimentary systems led to the paradigm of sorptive stabilization of organic matter (OM). Studies on soils also show that sorptive interactions between dissolved organic matter (DOM) and mineral phases contribute to the preservation of soil OM. In the first part of the paper, we summarize evidence for sorptive stabilization of OM in forest soils including (a) pronounced retention of DOM in most subsoils, (b) strong chemisorptive binding exhibiting strong hysteresis, and (c) similarity in the composition of DOM and OM in illuvial soil horizons and clay-sized separates. However, the capacity of soils for sorption of DOM is not infinite. In the second part of the paper, we present a case study where we relate the yearly retention of dissolved organic carbon (DOC) in the mineral soil to the available sorption capacity of seven forest soils. We estimate that the saturation of the sorption complex would occur within 4–30 years. Assuming these soils are in steady-state equilibrium with respect to carbon cycling, this suggests a mean residence time of the sorbed organic carbon (OC) of about the same time, therefore providing little evidence for a long-term stabilization of sorbed OM. One explanation for this discrepancy may be because in forest soils most surfaces are not characterized by juvenile minerals but are covered with OM and colonized by microorganisms. This is the case mainly in topsoil horizons but occurs also along preferential flow paths and on aggregate surfaces. Biofilms develop particularly at sites receiving high input of nutrients and organic substrates, i.e., DOM, such as preferential flow paths. The OM input enhances the heterotrophic activity in the biofilm, converting the DOM into either organic compounds by microbial resynthesis or inorganic mineralization products. Recent studies suggest that Fe hydrous oxides embedded within the biofilms may serve as a sorbent and shuttle for dissolved organic compounds from the surrounding aqueous media. We assume that sorption of DOM to the biofilm does not lead to a stabilization of OM but is a prerequisite for its rapid turnover. Only when DOM is transported by mass flow or diffusion to fresh, juvenile mineral surfaces, may sorption effectively stabilize OM. This stabilization would involve complexation of functional groups, changed conformation, and incalation in small pores.