Long-term accumulation and material balance of organic matter in the soil of an effluent infiltration basin

The fate of organic matter (OM) in large-scale infiltration basins used for wastewater treatment by the soil aquifer treatment (SAT) system was investigated. Measured changes in the organic matter concentrations in the soil profiles of the infiltration basins and detailed long-term records of OM concentrations in the recharged effluent and in the observation wells and recovery wells water, were used to calculate OM material balances in the SHAFDAN wastewater treatment plant, serving the City of Tel-Aviv, Israel, since 1977. The average annual total organic matter (TOM) load delivered by the effluents to the soil was ~ 5 kg m^− 2 y^− 1. Soil OM concentrations increased from 0.11% in the pristine soil to ~ 0.8% and ~ 0.6%, in the 0-0.15 m and 0.15-0.30 m soil layers, respectively, after ~ 20 y of effluent recharge, but did not change significantly in the 1.80-2.10 m deep layer. The OM accumulation rates in the top two soil layers were fast initially, then declined slowly and the OM concentrations approached a steady state following 10-15 y of effluent recharge. This suggests that stabilization of the ‘active biofilm’ layer in the infiltration basins' soils is a relatively slow process. Material-balance calculations showed, that accumulated OM in the top 0-2.1 m soil layer amounted to only ~ 4% of the TOM added by the effluents during ~ 20 y of recharge. Along the flow pathway of the effluent through the vertical 50-100 m thick soil-sediment column, DOC concentrations decreased by 70-90% (from ~ 18.9 mg L^− 1 to ~ 3.7 mg L^− 1). Continued flow in the aquifer from the observation wells to the recovery wells further decreased DOC concentrations by about 50% (from ~ 3.7 to ~ 1.5 mg L^− 1).     

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

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

The effects of feeding byOniscus asellus (Isopoda) on nutrient cycling in an incubated hardwood forest soil

Summary Oniscus asellus produced changes in the nutrients leached from Oie and Oa horizons of a hardwood forest soil. Soil with isopods lost more K⁺ (54%) from the Oie horizon and more Ca²⁺ (25%), Mg²⁺ (40%), and water-extractable S (23%) from the Oa horizon than soil without isopods. In contrast, soils with isopods lost less Ca²⁺ (39076) from the Oie horizon and less dissolved C-bonded S (33%) from the Oa horizon than soil without isopods. In addition, the Oia and Oa horizons exhibited different nutrient dynamics. When isopods were present, the Oa horizon leachates accumulated more Na⁺ K⁺, Ca²⁺, Mg²⁺, NO₃ ^– , water-soluble SO₄ ^2–, and dissolved C-bonded S, and the Oie horizon retained more of these nutrients. The type of leaching solution also had a major effect on nutrients. Leaching with a simulated soil solution resulted in smaller nutrient losses for K⁺ and Mg²⁺ in both horizons and for Na⁺, Ca²⁺, and NO₃ ^– in the Oa horizon than leaching with distilled water.     

Fractionation of dissolved organic carbon in soil water: Effects of extraction and storage methods

Abstract

We measured the concentration and composition (sensu Leenheer, 1981) of dissolved organic carbon (DOC) in lysimeter solutions from the forest floor of a spruce stand in Maine and in laboratory extracts of organic (Oa horizon) and mineral soils collected from various forests in Maine, New Hampshire, and Vermont. All soils were acid Spodosols developed from glacial till. The effects of different storage, extraction and filtration methods were compared. Extracts from Oa horizons stored fresh at 3°C contained a larger fraction of hydrophobic neutrals than lysimeter forest floor solutions (31 and 4% of DOC in stored and lysimeter solutions, respectively), whereas extracts from Oa horizons which had been extracted, incubated at 10–15°C, and extracted again had DOC compositions similar to that in lysimeter solutions. Mechanical vacuum and batch extractions of Oa horizons yielded DOC similar in concentration and composition if the extracts were filtered through glass fiber filters. Nylon membrane filters, however, removed more hydrophobic acids from batch extracts. Dissolved organic carbon extracted from frozen, air‐dry, and oven‐dry Oa and Bh horizons was relatively rich in hydrophilic bases and neutrals and was similar to that released after chloroform fumigation, indicating that common soil‐storage methods disrupt microbial biomass.     

The importance of dissolved organic carbon fluxes for the carbon balance of a temperate Scots pine forest

Efforts to increase our understanding of the terrestrial carbon balance have resulted in a dense global network of eddy covariance towers, which are able to measure the net ecosystem exchange of CO₂, H₂O and energy between ecosystems and the atmosphere. However, the typical set-up on an eddy covariance tower does not monitor lateral CO₂- and carbon fluxes such as dissolved organic carbon (DOC). By ignoring DOC fluxes eddy covariance-based CO₂ balances overestimate the carbon sink of ecosystems as part of the DOC drains into the inland waters and get respired outside the footprint of the eddy covariance tower. In this study we quantify 7 years (2000-2006) of DOC fluxes from a temperate Scots pine forest in Belgium and analyse its inter-annual variability. On average, 10 gC m⁻² year⁻¹ is leached from the pine forest as DOC. If the DOC fluxes are considered relative to the gross ecosystem carbon fluxes we see that DOC fluxes are small: 0.8 ± 0.2% relative to gross primary productivity, 1.0 ± 0.3% relative to ecosystem respiration, and (2.4 ± 0.4%) relative to soil respiration. However, when compared to net fluxes such as net ecosystem productivity and net biome productivity the DOC flux is no longer negligible (11 ± 7% and 17%, respectively), especially because the DOC losses constitute a systematic bias and not a random error. The inter-annual variability of the DOC fluxes followed that of annual water drainage. Hence, drainage drives DOC leaching at both short and long time scales. Finally, it is noted that part of the carbon that is leached from the ecosystem as DOC is respired or sequestered elsewhere, so the physical boundaries of accounting should always be reported together with the carbon budget.     

Long-term N addition effects on the C mineralization and DOC production in mor humus under spruce

This study was based on laboratory incubations of mor humus from two N fertilized stands of Norway spruce in Sweden (Skogaby and Stråsan), which had received repeated N additions (100 kg N ha⁻¹ yr⁻¹ as (NH₄)₂SO₄ at Skogaby and 35, 73 and 108 kg ha⁻¹ yr⁻¹ as NH₄NO₃ at Stråsan) during 8 and 24-29 years, respectively. The aim was to investigate long-term N effects on the mineralization of C and production of DOC. Mor humus (Oe and Oa) was incubated in columns at 20 °C for 49 days. Columns were leached once a week with artificial throughfall solution, which was analyzed for DOC, total N, NH₄⁺-N and NO₃⁻-N. Prior to each leaching event, CO₂ evolution from the columns was determined. C-to-N ratios in the N-treated Oe layers at Stråsan (21-24) and Skogaby (24) were significantly lower than those of the controls (Stråsan, 32; Skogaby, 28). The cumulative amount of CO₂-C showed a significant treatment effect in the Oe layer at Skogaby, i.e. 18 and 29 mg C g⁻¹ C in the N treatment and control, respectively. At Stråsan, the cumulative CO₂-C was significantly lower in the N3 treatment compared to the control in both layers (33 compared to 74 mg C g⁻¹ C in the Oe layer and 16 compared to 35 mg C g⁻¹ C in the Oa layer). Neither the DOC nor the DON production showed any significant treatment effects at the two sites. However, DOC was lower in the fertilized Oe layer at Skogaby throughout the incubation. The leaching of DON was highest in the Oe layers at both sites, and DON increased with time at Skogaby while there was a decreasing trend at Stråsan. The DOC-to-DON ratio tended to be lower in the fertilized Oe layers at both sites. The NH₄⁺ leaching at Skogaby decreased in the N-treated Oe and Oa layers. At Stråsan, NH₄⁺ from the Oe layer increased in N2 and control. The NO₃⁻ leaching was low and constant in both Skogaby layers. At Stråsan, NO₃⁻ increased in the Oe layer of N1. Cumulative CO₂ was positively correlated to C-to-N ratio (r2=0.71,p<0.01) and to cumulative DOC (r2=0.63,p<0.05) in the Oe layer at Stråsan. Our conclusion was that long-term N additions caused decreased C-to-N ratios and decreased CO₂ evolution rates. The correlation between CO₂ and C-to-N ratio in the Oe layers at Stråsan may be due to a changed quality of the fertilized forest floor material and presence of more N efficient microorganisms.     

Biochemical characterization of urban soil profiles from Stuttgart, Germany

The knowledge of biochemical properties of urban soils can help to understand nutrient cycling in urban areas and provide a database for urban soil management. Soil samples were taken from 10 soil profiles in the city of Stuttgart, Germany, differing in land use—from an essentially undisturbed garden area to highly disturbed high-density and railway areas. A variety of soil biotic (microbial biomass, enzyme activities) and abiotic properties (total organic C, elemental C, total N) were measured up to 1.9 m depth. Soil organic matter was frequently enriched in the subsoil. Microbial biomass in the top horizons ranged from 0.17 to 1.64 g C kg⁻¹, and from 0.01 to 0.30 g N kg⁻¹, respectively. The deepest soil horizon at 170-190 cm, however, contained 0.12 g C kg⁻¹ and 0.05 kg N kg⁻¹ in the microbial biomass. In general, arylsulphatase and urease activity decreased with depth but in three profiles potentially mineralizable N in the deepest horizons was higher than in soil layers directly overlying. In deeply modified urban soils, subsoil beside topsoil properties have to be included in the evaluation of soil quality. This knowledge is essential because consumption of natural soils for housing and traffic has to be reduced by promoting inner city densification.     

Nitrogen deposition and dissolved organic carbon production in northern temperate forests

Deposition of anthropogenic nitrogen (N) alters the decomposition of organic matter in forest ecosystems by changing the expression of key microbial enzymes. We investigated the effects of experimental N deposition on dissolved organic matter (DOM) in soils of three forest ecosystems representative of the upper Great Lakes region: the sugar maple/basswood (SMBW), sugar maple/red oak (SMRO) and white oak/black oak (WOBO) ecosystems. Mineral soil samples were collected on five dates from ambient and N-amended plots (80 kg N ha⁻¹ yr⁻¹) in three replicate stands of each forest type. DOM was extracted (2:1, water:soil) from each soil sample and analyzed for dissolved organic carbon (DOC). DOC concentration was significantly greater in the N-amended soils (on average: 24% higher for SMBW, 9% for SMRO, and 40% for BOWO). In June and October 2002, bioassays were performed to assess N treatment effects on the composition of DOM and its interacting bacterial community. Within each site, DOM extracts from the ambient and N-amended plots were reciprocally inoculated with bacteria from each plot. After a 48 h incubation at 20 °C, community activity in each microcosm was profiled by measuring 10 extracellular enzyme activities (EEA). MANOVA showed that ecosystem type, sampling date, DOM source (ambient or N-amended plot) and inoculum source (ambient or N-amended plot) all had significant effects on bioassay EEA. Post hoc tests (Tukey's HSD) found significant reductions in oxidative enzyme activity as a result of the N treatment. In general, the bioassay results corroborated a previous report describing losses in soil oxidative enzyme activity in response to N saturation. However, it is not clear whether increased DOC concentration is the direct result of reduced oxidative activity.     

Characterization of hydrophobic acid fractions in water-soluble organic matter in Dystric Cambisol and in a stream in a small forested watershed: Seasonal and vertical variations in chemical properties

Abstract

Chemical properties of hydrophobic acid (HoA) fractions in water-soluble organic matter in soil and water are concerned with its interactions with mineral soil surfaces and organic pollutants. In 2004 we examined the seasonal and vertical changes in chemical properties of the HoA fraction in a Cambisol profile and compared these properties with those in the HoA fraction of an adjacent stream (aquatic humic substances) in a temperate forested watershed using high performance size exclusion chromatography (HPSEC) and ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy. The HoA fractions from Oi, Oe/Oa, A and B horizon soils in summer had lower O-alkyl C proportions than those recorded in samples in other seasons. The proportions of aromatic C in HoA fractions from A and B horizons were highest in summer. These seasonal variations were less significant than variations with soil depth. O-alkyl C proportions in HoA fractions decreased with increasing soil depth from the Oi to the A horizon. The HoA fractions from the B horizon showed a higher alkyl C proportion than samples from other horizons in winter and spring. These changes with soil depth from the Oi to A horizons might result from selective utilization of carbohydrate carbon by microorganisms, whereas those in the B horizon may result from sorption to mineral surfaces. The HoA fractions in the stream were similar in relative molecular weight, distribution of each type of proton and carbon species in HoA fractions from the B horizon, whereas stream HoA fractions collected in summer would be derived from organic horizons. This indicated that vertical changes in the chemical properties of HoA fractions in soil and pathways of water to the stream would largely affect the chemical properties of HoA fractions in the stream.     

Microbial immobilization and mineralization of dissolved organic nitrogen from forest floors

Dissolved organic nitrogen (DON) plays a key role in the N cycle of many ecosystems, as DON availability and biodegradation are important for plant growth, microbial metabolism and N transport in soils. However, biodegradation of DON (defined as the sum of mineralization and microbial immobilization) is only poorly understood. In laboratory incubations, biodegradation of DON and dissolved organic carbon (DOC) from Oi and Oa horizons of spruce, beech and cypress forests ranged from 6 to 72%. Biodegradation of DON and DOC was similar in most samples, and mineralization of DON was more important than microbial immobilization. Nitrate additions (0-10 mg N L⁻¹) never influenced either DON immobilization by microorganisms or mineralization. We conclude that soil microorganisms do not necessarily prefer mineral N over DON for meeting their N demand, and that biodegradation of DON seems to be driven by the microbial demand for C rather than N. Quantifying the dynamics of DON in soils should include consideration of both C and N demands by microbes.     

Loss of low molecular weight dissolved organic carbon (DOC) and nitrogen (DON) in H2O and 0.5 M K2SO4 soil extracts

Soil extracts are routinely used to quantify dissolved organic nutrient concentrations in soil. Here we studied the loss and transformation of low molecular weight (LMW) components of DOC (¹⁴C-glucose, 1 and 100 μM) and DON (¹⁴C-amino acid mixture, 1 and 100 μM) during extraction of soil (0-6 h) with either distilled water or 0.5 M K₂SO₄. The extractions were performed at 20 °C, at 4 °C, or in the presence of an inhibitor of microbial activity (HgCl₂ and Na-azide). We showed that both glucose and amino acids became progressively lost from solution with increasing shaking time. The greatest loss was observed in H₂O extracts at 1 μM for both substances (>90% loss after 15 min). Lower temperature (4 °C) and presence of K₂SO₄ both resulted in reduced loss rates. The presence of microbial inhibitors effectively eliminated the loss of glucose and amino acids. We conclude that microbial transformation of LMW-DOC and DON during H₂O or K₂SO₄ extraction of soil may affect the estimation of their concentrations in soil. This finding has significant implications for methods that rely on chemical extractions to estimate LMW-C components of DOC and DON.     

Carbon accumulation in Umbrisols under Quercus pyrenaica forests: Effects of bedrock and annual precipitation

The objective of this study was to determine the effect of bedrock, mean annual precipitation and slope orientation on soil organic carbon (SOC) accumulation of Quercus pyrenaica Willd forests. Twelve different oak stands, along a rainfall gradient over two bedrock types (granites and schists), were selected for this study. Properties of the diagnostic soil horizons were determined. Bedrock type was the principal factor affecting soil development and fertility. Accumulated SOC varied between 33 and 185 Mg C ha^− 1, the amounts of C accumulated over schists being significantly higher than over granites because of higher soil acidity, ‘silt + clay’ content and metal complexation. The SOC content in the entire profile was over 125 times higher than the C accumulated in the stand forest litter at the schist sites and 50 times higher at the granite sites. Soil acidity and metal complexation hampered microbial decomposition of soil organic matter, producing SOC accumulation and forming thick umbric Ah/AB-horizons, being thicker at sites with high values of mean annual precipitation than at sites with low ones. Mineral N release was low in these soils.     

Modelling low molecular weight organic acid dynamics in forest soils

Low molecular weight organic acids such as citrate and oxalate have been hypothesized to play a key role in rhizosphere ecology and pedogenesis. A mathematical site-specific model, DYNLOW, was constructed to describe the temporal and spatial dynamics of these organic acids in coniferous forest soils using the modelling software STELLA^®. Experimentally derived values for biodegradation, adsorption, and daily values of soil temperature, moisture and hydrological flow were used to parameterize the model. The model describes the dynamics and downward movement of oxalate and citrate through the horizons (O, AE, E, Bhs, Bs) of three podzolic soil profiles in Sweden. After calibration, the model predicted average soil solution organic acid concentrations ranging from <1 to 90 μM, which was in agreement with experimental measurements (<1 to 116 μM). The model results indicated that microbial degradation of organic acids was in quantitative terms the biggest process regulating soil solution concentrations. Primary production rates of organic acid in the soil were predicted to be high (<1 to 1250 nmol g⁻¹ soil d⁻¹) in comparison to the amount present at steady state in the soil solution pool (<0.1 to 240 nmol g⁻¹ soil). The downward transfer of organic acids between soil horizons due to mass flow was predicted to be a small flux (<0.1 to 3% of the total loss) compared to that lost by microbial biodegradation. The model predicted that the amount of basal soil respiration that could be attributable to the microbial turnover of organic acids was on average 19±22% of the basal CO₂ production across all sites and horizons for citrate and 7±7% for oxalate. The model results are discussed in the context of pedogenesis, forest soil respiration and organic matter production.     

Temperature and soil moisture effects on dissolved organic matter release from a moorland Podzol O horizon under field and controlled laboratory conditions

Organic upland soils store large amounts of humified organic matter. The mechanisms controlling the leaching of this C pool are not completely understood. To examine the effects of temperature and microbial cycling on C leaching, we incubated five unvegetated soil cores from a Podzol O horizon (from NE Scotland), over a simulated natural temperature cycle for 1 year, whilst maintaining a constant soil moisture content. Soil cores were leached with artificial rain (177 mm each, monthly) and the leachates analysed for dissolved organic carbon (DOC) and their specific C‐normalized UV absorbance determined (SUVA, 285 nm). Monthly values of respiration of the incubated soils were determined as CO₂ efflux. To examine the effects of vegetation C inputs and soil moisture, in addition to temperature, we sampled O horizon pore waters in situ and collected five additional field soil cores every month. The field cores were leached under controlled laboratory conditions. Hysteresis in the monthly amount of DOC leached from field cores resulted in greater DOC on the rising, than falling temperature phases. This hysteresis suggested that photosynthetic C stimulated greater DOC losses in early summer, whereas limitations in the availability of soil moisture in late summer suppressed microbial decomposition and DOC loss. Greater DOC concentrations of in‐situ pore waters than for any core leachates were attributed to the effects of soil drying and physico‐chemical processes in the field. Variation in the respiration rates for the incubated soils was related to temperature, and respiration provided a greater pathway of C loss (44 g C m⁻² year⁻¹) than DOC (7.2 g C m⁻² year⁻¹). Changes in SUVA over spring and summer observed in all experimental systems were related to the period of increased temperature. During this time, DOC became less aromatic, which suggests that lower molecular weight labile compounds were not completely mineralized. The ultimate DOC source appears to be the incomplete microbial decomposition of recalcitrant humified C. In warmer periods, any labile C that is not respired is leached, but in autumn either labile C production ceases, or it is sequestered in soil biomass.     

Carbohydrate and amino acid composition of dissolved organic matter leached from soil

Low molecular weight organic substances (LMWOS) in soil and soil solution include mainly amino acids, carboxylic acids, and carbohydrates. Due to their high bioavailability they play a crucial role in the cycles of C and nutrients in soils. The variety of soil processes that involve LMWOS requires identifying their composition to elucidate reactions and transformations. In most studies, LMWOS are extracted under artificial conditions, e.g. batch experiments, which may overestimate the actual concentrations. This study measures the composition of carbohydrates and amino acids in solution of a Haplic Luvisol leached in a column experiment. A combined system for simultaneous leaching and blowout of CO₂ was used to estimate LMWOS decomposition. ¹⁴C-labeled glucose was added as a highly sensitive tracer to control the efficiency of the LMWOS extraction by leaching and to estimate LMWOS decomposition during leaching. High performance liquid chromatography (HPLC), optimized for soil extracts, was used to analyze LMWOS composition. For HPLC optimization, different preparations of leached solutions (filtration vs. centrifugation, and drying vs. no-drying) were compared. For sugar determination, drying had no influence on the solution concentrations. In contrast, amino acid concentrations significantly decreased by drying LMWOS eluted substances. Combining the HPLC identification of eluted substances with ¹⁴C tracer application revealed that about 5% of the glucose could be leached unchanged within 786 min (13.1 h), whereas about 84% remained in the soil, 9% were decomposed to CO₂, and 2% were transformed to other LMWOS and recovered in the soil solution. The total amino acid concentration (TAC) in soil solution was about 8.2 μmol l⁻¹, dominated by alanine (14.4% of TAC), glycine (13.4%), glutamic acid (9.9%), serine (9.4%), and leucine (9.3%). The total carbohydrate concentration was about 2.4 μM, dominated by glucose (29.9%), glucuronic acid (26.8%), and galacturonic acid (17.3%). Ratios of hexoses to pentoses, amino sugars glucosamine to galactosamine, and neutral sugars to uronic acids were determined. All three parameters pointed to the dominant influence of plants as the source of LMWOS in the leached soil solution. Within the small contribution of microorganisms, bacteria dominated over fungi. These used biomarker ratios as well as LMWOS concentrations differed widely from the ones obtained with conventional batch extraction. More research is necessary to evaluate the application of these biomarkers to soil solutions.     

Soil organic matter formation along a chronosequence in the Morteratsch proglacial area (Upper Engadine,Switzerland)

Global warming leads to the melting of ice caps and glaciers and, consequently, the exposure of new areas of land to the atmosphere and weathering. These areas usually have a high reactivity to both biotic and abiotic changes. Proglacial areas in the Alps usually have a deglaciation time span of around 150 years (time since the end of the “Little Ice Age” in the 1850's). We investigated a chronosequence of very young soils in the proglacial area Morteratsch (Swiss Alps) to derive time-trends of soil organic matter accumulation and evolution. Total organic C and N contents, C and N contents of the various organic matter (OM) density fractions and of the labile (oxidised by H₂O₂) and stable (H₂O₂-resistant) fractions were measured. Further characterisation of OM and the various fractions was performed using Diffuse Reflection Infrared Fourier Transform (DRIFT). Soil organic matter has been accumulated over 150 years at very high rates, values lay between 7 and 36 g C/m²/year. This led to a soil organic matter abundance of about 1–5.5 kg C/m² after 140 years. Even at the start of soil formation, a very stable fraction of soil organic matter was detectable. Stable organic matter (resistant to the H₂O₂ treatment) comprised about 6% of the total soil organic carbon and 10% of the total nitrogen. At the start of soil formation, a very high proportion of soil organic matter was present in the density fractions < 1.6 g/cm³. After about 140 years, 15% of soil organic carbon and 35–40% of the nitrogen was already present in the highest density fraction (> 2 g/cm³). With time, the quality of soil organic matter changed: a decrease of hydrophobicity, an increase in aromatic compounds in the bulk soil and a decrease in phenolic functional groups in the heaviest density fraction were detectable with increasing age. In general, stable organic matter as well as the density fraction > 2 g/cm³ had a low C/N ratio and were enriched in proteinaceous materials. The adsorption of proteinaceous materials points to a strong organo-mineral association. This process has existed since the very beginning of soil formation.     

Repeated freeze–thaw events affect leaching losses of nitrogen and dissolved organic matter in a forest soil

Freezing and thawing may substantially influence the rates of C and N cycling in soils, and soil frost was proposed to induce NO losses with seepage from forest ecosystems. Here, we test the hypothesis that freezing and thawing triggers N and dissolved organic matter (DOM) release from a forest soil after thawing and that low freezing temperatures enhance the effect. Undisturbed soil columns were taken from a soil at a Norway spruce site either comprising only O horizons or O horizons + mineral soil horizons. The columns were subjected to three cycles of freezing and thawing at temperatures of –3°C, –8°C, and –13°C. The control columns were kept at constant +5°C. Following the frost events, the columns were irrigated for 20 d at a rate of 4 mm d^–1. Percolates were analyzed for total N, mineral N, and dissolved organic carbon (DOC). The total amount of mineral N extracted from the O horizons in the control amounted to 8.6 g N m^–2 during the experimental period of 170 d. Frost reduced the amount of mineral N leached from the soil columns with –8°C and –13°C being most effective. In these treatments, only 3.1 and 4.0 g N m^–2 were extracted from the O horizons. Net nitrification was more negatively affected than net ammonification. Severe soil frost increased the release of DOC from the O horizons, but the effect was only observed in the first freeze–thaw cycle. We found no evidence for lysis of microorganisms after soil frost. Our experiment did not confirm the hypothesis that soil frost increases N mineralization after thawing. The total amount of additionally released DOC was rather low in relation to the expected annual fluxes.     

Clear-cutting of a Norway spruce stand: implications for controls on the dynamics of dissolved organic matter in the forest floor

Clear‐cutting of forest provides a unique opportunity to study the response of dynamic controls on dissolved organic matter. We examined differences in concentrations, fluxes and properties of dissolved organic matter from a control and a clear‐cut stand to reveal controlling factors on its dynamics. We measured dissolved organic C and N concentrations and fluxes in the Oi, Oe and Oa horizons of a Norway spruce stand and an adjacent clear‐cutting over 3 years. Aromaticity and complexity of organic molecules were determined by UV and fluorescence spectroscopy, and we measured δ¹³C ratios over 1 year. Annual fluxes of dissolved organic C and N remained unchanged in the thin Oi horizon (～ 260 kg C ha^?1, ～ 8.5 kg N ha^?1), despite the large reduction in fresh organic matter inputs after clear‐cutting. We conclude that production of dissolved organic matter is not limited by lack of resource. Gross fluxes of dissolved organic C and N increased by about 60% in the Oe and 40% in the Oa horizon upon clear‐cutting. Increasing organic C and N concentrations and increasing water fluxes resulted in 380 kg C ha^?1 year^?1 and 10.5 kg N ha^?1 year^?1 entering the mineral soil of the clear‐cut plots. We found numerous indications that the greater microbial activity induced by an increased temperature of 1.5°C in the forest floor is the major factor controlling the enhanced production of dissolved organic matter. Increasing aromaticity and complexity of organic molecules and depletion of ¹³C pointed to an accelerated processing of more strongly decomposed parts of the forest floor resulting in increased release of lignin‐derived molecules after clear‐cutting. The largest net fluxes of dissolved organic C and N were in the Oi horizon, yet dissolved organic matter sampled in the Oa horizon did not originate mainly from the Oi horizon. Largest gross fluxes in the Oa horizon (control 282 kg C ha^?1) and increased aromaticity and complexity of the molecules with increasing depth suggested that dissolved organic matter was derived mainly from decomposition, transformation and leaching of more decomposed material of the forest floor. Our results imply that clear‐cutting releases additional dissolved organic matter which is sequestered in the mineral soil where it has greater resistance to microbial decay.     

Leaching of soil organic carbon and nitrogen in sandy soils after cultivating grass-clover swards

Several studies have focused on the formation and losses of dissolved organic matter in forest systems, whereas a limited number have dealt with this aspect in agricultural soils. The purpose of this study was to estimate the leaching of dissolved organic carbon (DOC) and nitrogen (DON), with focus on the period after cultivating grass-clover swards. Grass-clovers were ploughed in the spring prior to sowing cereals followed by either catch crops or bare soil. The concentrations of DOC and DON decreased with soil depth and ranged at 90-cm soil depth between 7 and 21 mg C L⁻¹ and between 1 and 3 mg N L⁻¹, respectively, in a sandy loam soil, and between 16 and 63 mg C L⁻¹ and between 1 and 10 mg N L⁻¹, respectively, in a coarse sandy soil. The resulting DOC/DON ratios were in the range between 2 and 42, with higher values in the coarse sandy soil than in the sandy loam soil. The total percolation was 218 mm in the sandy loam soil and 596–645 mm in the coarse sandy soil, which resulted in an annual leaching of 22–40 kg DOC ha⁻¹ year⁻¹ and 3–4 kg DON ha⁻¹ year⁻¹ in the sandy loam soil, and 174–310 kg DOC ha⁻¹ year⁻¹ and 10–31 kg DON ha⁻¹ year⁻¹ in the coarse sandy soil. It was shown that higher amounts of DOC were lost by leaching under the catch crops than from bare soil, that losses of DON were higher from bare soil than from soils with catch crops and that DON contributed significantly to the total N loss. Thus, DON needs to be taken into account in N-balance calculations.     

Experimental evaluation of methods to quantify dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) in soil

A significant proportion of the total nutrient in soil solution can be bound to organic molecules and these often constitute a major loss from soil to freshwater. Our purpose was to determine whether chemical extractants used for measuring inorganic N could also be used to quantify dissolved organic nitrogen (DON) and carbon (DOC) in soil. In a range of soils, DOC and DON were extracted with either distilled water or 2 M KCl and the amount recovered compared with that present in soil solution recovered by centrifugal-drainage. The recovery of DON and DOC from soil was highly dependent upon the method of extraction. Factors such as soil sampling strategy (number of samples over space and time), sample preparation (sieving and drying), soil storage, extraction temperature, shaking time, and soil-to-extractant volume ratio all significantly affected the amount of DOC and DON extracted from soil. To allow direct comparison between independent studies we therefore propose the introduction of a standardized extraction procedure: Replicate samples of unsieved, field-moist soil extracted as soon as possible after collection with distilled water, 0.5 M K₂SO₄ or 2 M KCl at a 1:5 w/v ratio for 1 h at 20 °C.