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

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

Biochemical properties and biodegradation of dissolved organic matter from soils

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

Effects of hydrophobic and hydrophilic organic matter on the water repellency of model sandy soils

Soil water repellency affects the hydrological functions of soil systems. Water repellency is associated with the content and the composition of soil organic matter. In the present study, we examined the effects of hydrophobic and hydrophilic organic matter contents, the hydrophobic/hydrophilic organic matter ratio and the total organic matter content on water repellency using model sandy soils. Stearic acid and guar gum were used as the hydrophobic and hydrophilic organic compounds, respectively. Water repellency was estimated using the sessile drop method. Hydrophobic organic matter content was found to be the dominant factor affecting soil water repellency. Hydrophilic organic matter was found to increase the contact angle to some extent without the presence of hydrophobic organic matter. With the presence of both hydrophobic and hydrophilic organic matter, the effects of the hydrophilic organic matter content on contact angle were found to be dependent on the hydrophobic organic matter content of the soil. This relationship was explained by the differences in the surface free energies of different organic matter and mineral surfaces. The contact angle increased with increasing hydrophobic/hydrophilic organic matter ratio when the hydrophilic organic matter content was constant. When the hydrophobic organic matter content was constant, contact angles were roughly comparable, irrespective of the hydrophobic/hydrophilic organic matter ratio. The contact angles were not comparable at each total organic matter content. Accordingly, the hydrophobic/hydrophilic organic matter ratio and the total organic matter content in soil may not provide satisfactory information about soil water repellency.     

Physical protection and biochemical quality of organic matter in mediterranean calcareous forest soils: a density fractionation approach

Physical protection is one of the most important ways for stabilization of organic carbon (OC) in soils, and in order to properly manage soils as a sink for carbon, it is necessary to know how much OC a given soil could protect. To this end, we studied individual horizons taken from 16 soil profiles under Quercus rotundifolia stands, all over calcareous parent materials. Horizons were subjected to a sequential extraction using solutions of sodium polytungstate (NaPT) of increasing density: (i) NaPT d=1.6, using slight hand agitation, to obtain the free light fraction (FL); (ii) NaPT d=1.6 and ultrasonic dispersion, to obtain the Occluded Fraction I (Ocl I); (iii) NaPT d=1.8, to obtain the Occluded Fraction II (Ocl II); and (iv) NaPT d=2.0, to obtain the Occluded Fraction III (Ocl III). The fraction of density>2.0 are taken as dense fraction (DF). The free organic matter was further divided into FL>50 (retained by a 50 μm mesh: coarse organic fragments) and FL<50 (non-retained: fine organic fragments). The fractions FL>50 and FL<50 were taken together as free organic matter. The rest of the fractions are taken together as protected organic matter. The obtained fractions were analyzed for total OC, total N, and carbohydrate content. The percentage of non-hydrolyzable OC and N in each fraction was taken as an indicator of OC and N recalcitrance, respectively.For both OC and N, the fractions FL>50 and DF are dominant; the rest of the fractions are of much lower quantitative importance. In H horizons and in most A horizons, most of the OC and N are free, whereas in B horizons both OC and N are mostly protected. Overall, the percentages of free OC and N are very high and are currently amongst the highest ever recorded.Organic matter recalcitrance is lowest in the two most protected fractions (Ocl III and especially DF), and highest in the first occluded fractions (Ocl II and especially Ocl I). The free organic matter (FL>50 fraction) has an intermediate quality: it includes recognizable plant fragments, but the indicators tested (recalcitrance, carbohydrate content, cellulose to total carbohydrates ratio) suggest that it is not always the most fresh and non-decomposed fraction.There are clear maxima for both protected OC and N, which can be approached by curve fitting. By exponential fit, the obtained maxima are 84.1 g of OC and 7.7 g of N kg⁻¹ of mineral particles <20 μm. These maxima are much higher than the upper limits obtained by other authors. Differences in the sampling approach are suggested as the reason for such discrepancies.     

Limited effects of land use on soil dissolved organic matter chemistry as assessed by excitation–emission fluorescence spectroscopy and molecular weight fractionation

Dissolved organic matter (DOM) in soil solution represents a complex mixture of organic molecules and plays a central role in carbon and nitrogen cycling in plant–microbial–soil systems. We tested whether excitation–emission matrix (EEM) fluorescence spectroscopy can be used to characterize DOM and support previous findings that the majority of DOM is of high molecular weight (MW). EEM fluorescence spectroscopy was used in conjunction with MW fractionation to characterize DOM in soil solution from a grassland soil land management gradient in North Wales, UK. Data analysis suggested that three distinct fluorescence components could be separated and identified from the EEM data. These components were identified as being of humic‐like or fulvic‐like origin. Contrary to expectations, the majority of the fluorescence signal occurred in the small MW (<1 kDa) fraction, although differences between soils from the differently managed grasslands were more apparent in larger MW fractions. We conclude that following further characterization of the chemical composition of the fluorophores, EEM has potential as a sensitive technique for characterizing the small MW phenolic fraction of DOM in soils.     

Detectability of degradable organic matter in agricultural soils by thermogravimetry

Sustainable agricultural land use requires an assessment of degradable soil organic matter (SOM) because of its key function for soil fertility and plant nutrition. Such an assessment for practical land use should consider transformation processes of SOM and its sources of different origin. In this study, we combined a 120‐day incubation experiment with thermal decay dynamics of agricultural soils altered by added organic amendments. The aim was to determine the abilities and limits of thermal analysis as a rapid approach revealing differences in the degradability of SOM. The carried out experiments based on two independent sampling sets. The first sample set consisted of soil samples taken from non‐fertilized plots of three German long‐term agricultural field experiments (LTAEs), then artificially mixed with straw, farmyard manure, sheep faeces, and charcoal equal to 60 Mg ha^?1 under laboratory conditions. The second sample set based on soil samples of different treatments (e.g., crop type, fertilization, cultivation) in LTAEs at Bad Lauchstädt and Müncheberg, Germany. Before and after the incubation experiment, thermal mass losses (TML) at selected temperatures were determined by thermogravimetry indicating the degradability of organic amendments mixed in soils. The results confirmed different microbial degradability of organic amendments and SOM under laboratory conditions. Thermal decay dynamics revealed incubation‐induced changes in the artificial soil mixtures primarily at TML around 300°C in the case of applied straw and sheep faeces, whereas farmyard manure showed mainly changes in TML around 450°C. Charcoal did not show significant degradation during incubation, which was confirmed by TML. Detailed analyses of the artificial soil mixtures revealed close correlations between CO₂‐C evolution during incubation and changes in TML at 300°C with R² > 0.96. Results of the soils from LTAEs showed similar incubation‐induced changes in thermal decay dynamics for fresh plant residues and farmyard manure. We conclude that the practical assessment of SOM could be facilitated by thermal decay dynamics if modified sample preparation and evaluation algorithms are used beyond traditional peak analysis.     

Potential nitrogen immobilization as influenced by available carbon in Japanese arable and forest soils

Abstract

Iron oxide is the most important electron acceptor in paddy fields. We aimed to suppress the methane emission from paddy fields over the long term by single application of iron materials. A revolving furnace slag (RFS; 245 g Fe kg^-1) and a spent disposable portable body warmer (PBW; 550 g Fe kg_-1) were used as iron materials. Samples of a soil with a low iron level (18.5 g Fe kg^-1), hearafter referred to as “a low-iron soil” and of a soil with a high iron level (28.5 g Fe kg^-1), hearafter referred to as “an iron-rich soil,” were put into 3 L pots. At the beginning of the experiment, RFS was applied to the pots at the rate of 20 and 40 t ha^-1, while PBW was applied at the rate of 10 t ha^-1 only, and in the control both were not applied. Methane and nitrous oxide emissions from the potted soils with rice plants were measured by the closed chamber method in 2001 and 2002. When RFS was applied at the rates of 20 and 40 t ha^-1 to the low-iron soil, the total methane emission during the cultivation period significantly decreased by 25–50% without a loss of grain yield. Applied iron materials clearly acted as electron acceptors, based on the increase in the amount of ferrous iron in soil. However, the suppressive effect was not evident in the iron-rich soil treated with RFS or PBW. On the other hand, nitrous oxide emission increased by 30–95%. As a whole, when the total methane and nitrous oxide emissions in the low-iron soil were converted to total greenhouse gas emissions expressed as CO₂- C equivalents in line with the global warming potential, the total greenhouse gas emissions decreased by about 50% due to the application of RFS.     

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.     

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

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

Qualitative and quantitative differences in particulate organic matter fractions in organic and conventional farming systems

To quantify functionally important differences in soil organic matter (SOM) that result from use of different farming practices, soils from 9 long-term trials comparing manure+legume-based organic, legume-based organic, and conventional farming systems were collected and particulate organic matter (POM) was fractionated to reflect its position within the soil matrix. The free, light POM (FPOM; <1.6 g cm⁻³) not occluded within aggregates and occluded POM (OPOM; <2.0 g cm⁻³) were compared to an undifferentiated POM fraction (coarse fraction, CF; >53 μm) obtained by wet sieving. Fraction C, N, and hydrolyzable N (quantified using the Illinois test (IL-N)) were determined. Organic farming systems had greater quantities of C and N in the OPOM and CF and, greater IL-N contents in all POM fractions considered. The OPOM's C:N ratio (16-19) and was least in the manure+legume-based organic, intermediate in the legume-based organic, and greatest in the conventional systems (P<0.10). Trends in OPOM C:N and IL-N abundance suggested occluded POM was most decomposed, and possibly a greater N reservoir, in the manured soils. The FPOM quality reflected the residues added to each system and its removal improved resolution of quality-based differences in POM associated with long-term management. Subdivision of POM revealed differences in its quality that were not evident using the undifferentiated CF. Quantification of hydrolysable N (IL-N) in POM did not enhance our understanding of management's affect on SOM quality. This multi-site comparison showed organic management simultaneously increased the size of the labile N reservoir and the amount of POM protected within aggregates; and that, occluded POM is more decomposed in manure+legume- than in legume-based organic systems. The characteristics of POM reveal how organic practices improve SOM and suggest the nutrient and substrate decay dynamics of organic systems may differ as a result of the N fertilization strategies they employ.     

海泡石添加对猪粪堆肥腐熟和水溶性有机质的影响

为明确黏土矿物的投加对畜禽粪便堆肥腐熟和稳定化的影响,该研究以猪粪和杨木木屑为原料,探究添加海泡石对堆肥基本理化性质、不同成分有机质含量以及溶解性有机质（Dissolved Organic Matter,DOM）结构的影响。结果表明,添加海泡石后堆体最高温度比对照有所下降且电导率上升9.69%,而C/N则降低2.81%,同时种子发芽指数提高11.96%,显示腐熟状况更好;DOM含量降低7.84%而胡敏酸占比提高9.71%,使得堆体有机质更加稳定。荧光光谱分析表明,添加海泡石堆体DOM的荧光谱图中,长波长的峰强在较短时间内出现了明显增加;三维荧光光谱-平行因子分析显示,添加海泡石增加了堆体中高芳香性组分的占比。相关性分析结果表明,添加海泡石后,高芳香性组分与总有机碳之间相关性更为显著,说明海泡石在碳素分解的同时促进了其聚合,从而出现了胡敏酸与高芳香性荧光组分的增长。添加海泡石既能促进堆体腐熟,又可转化调控碳素进而提高堆体稳定性,有利于堆肥的后续农田施用。     

黄土丘陵区不同植被下土壤可溶性有机物的荧光特征研究

【目的】土壤水溶性有机质的组成和结构是土壤质量的重要判别指标,具有重要的生态意义。研究黄土丘陵地区不同植被下土壤水溶性有机物的数量和荧光结构特征,可以为该地的植被修复及土壤质量评价提供科学依据。【方法】利用传统荧光和三维荧光技术,选取激发发射荧光光谱、 同步荧光光谱得到的腐殖化指标对土壤水溶性有机物的来源和结构进行评估,通过三维荧光技术探究不同植被下土壤水溶性有机物之间组分的差异。【结果】辽东栎(Quercus liaotungensis)和油松林(Pinus tabulaeformia)地具有较高的水溶性有机物含量,含量均为0.16 mg/g; 而荒坡地和农地的水溶性有机物较低,分别为0.04和0.05 mg/g,灌木荆条(Vitex negundo var. heterophylla )和狼牙刺(Sophora viciifolia)林地的含量介于两者之间。发射激发荧光光谱、 同步荧光光谱以及三维荧光光谱表明土壤水溶性有机物大多来源于植物和微生物的混合作用; 油松林地的水溶性有机物结构较简单、 腐殖化程度较低; 而灌木林地的水溶性有机物结构较为复杂、 腐殖化程度较高。从不同植被下土壤水溶性有机物的组成来看,蛋白类的物质差异不明显; 油松和辽东栎林地土壤水溶性有机物的类酪氨酸蛋白质、 类色氨酸蛋白质、 类溶解性微生物代谢产物含量较高,灌木林地较低。最主要的差异是富里酸类和胡敏酸类物质,油松林地的水溶性小分子量的富里酸类物质占主导地位,而灌木林地的水溶性有机物芳香化程度较高,农地和抛荒地类富里酸物质和类胡敏酸类物质含量的比值最低,其水溶性腐殖质的缩聚度高。油松、 辽东栎林地水溶性有机物由于植物残体分解形成的富里酸较易被氧化,同时阴坡林地较高的含水量使得这些产物较难缩合; 而灌木林地和农田及荒地较为干旱,枯枝落叶少,腐殖酸有充分时间进行缩合,导致了水溶性有机物的高芳香化和腐殖化。【结论】不同植被下的土壤水溶性有机物的组成和结构是存在差异的,同时说明荧光技术可用于揭示水溶性有机物的组成和缩合特性研究。     

Effect of grassland harvesting frequency and N-fertilization on stocks and dynamics of soil organic matter in the temperate climate

ABSTRACT

Management of grassland may affect the dynamics of soil organic carbon (SOC). Objectives were to analyze the effect of different harvesting frequencies and nitrogen fertilization regimes on SOC and total N stocks in a field trial on a sandy loam to loamy sand soil of a grassland site near Kiel (Germany). Additionally, effects on microbial biomass C (C_mic) and ergosterol (as proxy for fungi) contents, water-stable aggregate size-classes and density fractions were studied. In the surface soil (0–10 cm), SOC and total N stocks, amounts of large water-stable macroaggregates (> 2000 µm) and contents of C_mic and ergosterol were significantly higher under a five cut regime. C_mic (r_Spearman = 0.61) and ergosterol contents (r_Spearman = 0.67) were correlated with amounts of large water-stable macroaggregates suggesting that fungi and microbial biomass play an important role in binding of small macroaggregates into large macroaggregates. The free light fraction of SOM showed significantly higher C concentrations under three cut compared to five cut at 30–60 cm, presumably related to the C/N ratio and the decomposability of root litter. This study indicates the importance of cutting frequency on SOC and total N stocks, amounts of large macroaggregates and contents of C_mic and ergosterol.     

Active and passive organic matter fractions in Mediterranean forest soils

Soil organic carbon (C) is a complex set of pools, and to understand its dynamics it is necessary to know which of these pools are active at a given moment, and which act as passive, due to either physical protection or biochemical recalcitrance, or both. This matter has been studied mainly in agricultural soils. For forest soils, especially in Mediterranean areas, there is a data gap that needs to be filled. Therefore, we studied three profiles in Catalonia (NE Spain) over marl and under Pinus halepensis stands. Soil horizons were incubated under optimal conditions for 45 days. The respiration rate on day 45 was taken as basal respiration rate (BR_R). The following fractions were quantified: (1) soluble C, (2) microbial C, both corrected (MC_C) and uncorrected (MC_UC) (i.e., applying or not a correction factor to account for the non-extractable microbial carbon), (3) C in size fractions, isolated by ultrasonic dispersion and sieving plus sedimentation, and (4) labile and recalcitrant C, quantified by acid hydrolysis, applied to both the whole soil horizons and the size fractions. The basal respiration rate (BR_R) correlated best with the sum soluble + MC_UC, which altogether seem the best estimator of the active C pool. The correlation between BR_R and MC_C was worse, thus suggesting that not all microbial C should be included in the active pool. The correlation of BR_R with the C associated to coarse fractions (>50 μm) was positive, whereas that\with C associated to fine fractions (<20 μm) was negative. The correlations were lower than those obtained with the soluble + MC_UC, thus suggesting that the coarse organic fractions are probably the main source of active C, but not active C itself. Alone, the pools obtained by acid hydrolysis (labile and recalcitrant) correlated poorly with BR_R, but the combination of size fractionation with acid hydrolysis resulted in some of the best predictors of microbial activity. Hydrolyzable polyphenolic compounds inhibited microbial activity. Unhydrolyzable C associated to fine fractions (<20 μm) seemed the most stable of all the C pools studied. By contrast the unhydrolyzable part was apparently as unstable as the hydrolyzable part in the coarse organic debris. Overall, our results point to a hierarchy of constraints: both the physical protection and the biochemical quality affect microbial activity, but the physical protection goes first. In the profiles studied, C did not appear to be more stable in deep horizons than in surface horizons.     

可溶性有机物对土壤中绿麦隆吸附与解吸的影响

A batch equilibrium techniques was used to examine the effect of dissolved organic matter （DOM） extracted from both non-treated sludge （NTS） and heat-expanded sludge （HES） on the sorption and desorption of chlorotoluron （3-（3-chloro-p-tolyl）-1,1-dimethylurea） in two types of soils, a yellow fluvo-aquic and a red soil from China. Without DOM,sorption of chlorotoluron was significantly greater （P 〈 0.05） in the red soil than in the yellow fluvo-aquic soil. However,with DOM the effect was dependent on the soil type and nature of DOM. Chlorotoluron sorption was lower in the yellow fluvo-aquic soil than in the red soil, suggesting that with the same DOM levels the yellow fluvo-aquic soil had a lower sorption capacity for this herbicide. Application of DOM from both NTS and HES led to a general decrease in sorption to the soils and an increase in desorption from the soils. Desorption of chlorotoluron also significantly increased （P 〈 0.05） with an increase in the DOM concentration. Additionally, for sorption and desorption, at each DOM treatment level the NTS treatments were significantly lower （P 〈 0.05） than the HES treatments. This implied that non-treated sludge had a greater effect on the sorption and desorption of chlorotoluron than heat-expanded sludge.     

Degradation of organotin compounds in organic and mineral forest soils

Broad industrial application of organotin compounds (OTC) leads to their release into the environment. OTC are deposited from the atmosphere into forest ecosystems and may accumulate in soils. Here, we studied the degradation of methyltin and butyltin compounds in a forest floor, a mineral, and a wetland soil with incubation experiments at 20 °C in the dark. OTC degraded slowly in soils with half‐lives estimated from 0.5 to 15 years. The first order degradation rate constants of OTC in soils ranged from 0.05 to 1.54 yr^–1. The degradation rates in soils were generally in the order mono‐ ≥ di‐ > tri‐substituted OTC. Stepwise dealkylation was observed in all cases of di‐substituted OTC, but only in some cases of tri‐substituted OTC. Decomposition rates of OTC in the forest floor were higher than in wetland and mineral soils. Tetramethyltin in the gas phase was not detected, suggesting little tin methylation in the wetland soils. Slow degradation of OTC in soils might lead to long‐term storage of atmospherically deposited OTC in soils.     

Biological degradation of pyrogenic organic matter in temperate forest soils

Pyrogenic organic matter (PyOM), derived from the incomplete combustion of plant biomass and fossil fuels, has been considered one of the most stable pools of soil organic matter (SOM) and a potentially important terrestrial sink for atmospheric CO₂. Recent evidence suggests that PyOM may degrade faster in soil than previously thought, and can affect native SOM turnover rates. We conducted a six-month laboratory incubation study to better understand the processes controlling the degradation of PyOM in soils using dual-enriched (¹³C/¹⁵N) PyOM and its precursor wood (Pinus ponderosa). We examined the effects of soil type and inorganic N addition on PyOM and wood C and N mineralization rates, microbial C utilization patterns, and native SOM turnover rates. PyOM charred at 450 °C or its precursor pine wood was incubated in two temperate forest subsoils with contrasting short range order (SRO) clay mineralogy (granite versus andesite parent material). Duplicates of experimental treatments with and without PyOM added were sterilized and abiotic C mineralization was quantified. In a second incubation, PyOM or wood was incubated in granitic soil with and without added NH₄NO₃ (20 kg N ha⁻¹). The fate of ¹³C/¹⁵N-enriched PyOM and wood was followed as soil-respired ¹³CO₂ and total extractable inorganic ¹⁵N. The uptake of ¹³C from PyOM and wood by soil microbial community groups was quantified using ¹³C-phospholipids fatty acids (PLFA). We found that (1) The mean residence time (MRT) of PyOM-C was on a centennial time scale (390–600 yr) in both soil types; (2) PyOM-C mineralization was mainly biologically mediated; (3) Fungi more actively utilized wood-C than PyOM-C, which was utilized by all bacteria groups, especially gram (+) bacteria in the andesite (AN) soil; (4) PyOM-N mineralization was 2 times greater in granite (GR) than in AN soils; (5) PyOM additions did not affect native soil C or N mineralization rates, microbial biomass, or PLFA-defined microbial community composition in either soil; (6) The addition of N to GR soil had no effect on the MRT of C from PyOM, wood, or native SOM. The centennial scale MRT for PyOM-C was 32 times slower than that for the precursor pine wood-C or native soil C, which is faster than the MRT used in ecosystem models. Our results show that PyOM-C is readily utilized by all heterotrophic microbial groups, and PyOM-C and -N may be more dynamic in soils than previously thought.     

Pyrogenic transformation of organic matter in soils of forest bogs

Natural fires on forest bogs significantly affect all the groups and fractions of peat organic matter. The type and intensity of the fires are responsible for the depth of the pyrogenic transformation of peat. In the course of thermal destruction of peat organic matter, humus substances (humic acids in particular) are accumulated, which leads to changes in the type of humus; the humus reserves may increase by 1.5–8 times. Several ways of the formation of humus components related to the intensity of a fire are suggested. The regressive evolution of bog ecosystems caused by fires is a reversible process. The humus status of pyrogenically transformed horizons and their morphology are preserved within the peat deposit as a relic characteristic of the discrete metamorphosis of the soils.     

Selenium retention in the organic matter of Swedish forest soils

Fractions of selenium present in the soil profiles of three Swedish podzols were analysed using a sequential extraction scheme to characterize Se distribution among the organic and inorganic fractions. The process by which selenite deposited from the atmosphere is retained in a podzolic profile rich in organic matter was studied in a column experiment. Selenium present in organic fractions accounted for most of the Se extracted by Na₄P₂O₇/NaOH. All soil organic matter fractions, particularly those in the B horizons, were considerably enriched with Se as compared with plant biomass. The most enriched fraction was that containing hydrophobic fulvates which had C to Se ratios ranging from 33 000 to 80 000. The distribution of Se among the organic fractions differed markedly from that of sulphur. Selenite applied to columns continuously for 67 d was fixed very rapidly upon entering the forest floor layers, with 77% being recovered in the top 2 cm of the forest floor after the experiment. In column leachates from the surface layers, C to Se ratios decreased progressively following Se application. No effect specifically related to Se application was observed for leachates and soil horizons underlying Bs1. The mechanism responsible for the efficient and rapid Se immobilization by organic matter is unknown.     

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

Purpose

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

Materials and methods

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

Results and discussion

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

Conclusions

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