Soil Organic Matter Degradation Research Articles

Controls on Soil Organic Matter Degradation and Subsequent Greenhouse Gas Emissions Across a Permafrost Thaw Gradient in Northern Sweden

Warming-induced permafrost thaw could enhance microbial decomposition of previously stored soil organic matter (SOM) to carbon dioxide (CO2) and methane (CH4), one of the most significant potential feedbacks from terrestrial ecosystems to the atmosphere in a changing climate. The environmental parameters regulating microbe-organic matter interactions and greenhouse gas (GHG) emissions in permafrost peatlands are however still largely unknown. The objective of this work is to understand controls on SOM degradation and its impact on GHG emissions across the Stordalen Mire, a thawing peat plateau in Northern Sweden. Here, we applied high-resolution mass spectrometry to characterize SOM molecular composition in peat soil samples from the active layers of a Sphagnum-dominated bog and rich fen sites in the Mire. Microbe-organic matter interactions and GHG emissions across the thaw gradient were controlled by aboveground vegetation and soil pH. An increasingly high abundance of reduced organic compounds experiencing greater humification rates due to enhanced microbial activity were observed with increasing thaw, in parallel with higher CH4 and CO2 emissions. Bog SOM however contained more Sphagnum-derived phenolics, simple carbohydrates, and organic- acids. The low degradation of bog SOM by microbial communities, the enhanced SOM transformation by potentially abiotic mechanisms, and the accumulation of simple carbohydrates in the bog sites could be attributed in part to the low pH conditions of the system associated with Sphagnum mosses. We show that Gibbs free energy of C half reactions based on C oxidation state for OM can be used as a quantifiable measure for OM decomposability and quality to enhance current biogeochemical models to predict C decomposition rates. We found a direct association between OM chemical diversity and δ13C-CH4 in peat porewater; where higher substrate diversity was positively correlated with enriched δ13C-CH4 in fen sites. Oxidized sulfur-containing compounds, produced by Sphagnum, were further hypothesized to control GHG emissions by acting as electron acceptors for a sulfate-reducing electron transport chain, inhibiting methanogenesis in peat bogs. These results suggest that warming-induced permafrost thaw might increase organic matter lability, in subset of sites that become wetlands, and shift biogeochemical processes toward faster decomposition with an increasing proportion of Creleased as CH4.

Decoupled richness of generalist anaerobes and sulphate‐reducing bacteria is driven by pH across land uses in temperate soils

AbstractSulphate‐reducing bacteria (SRB) represent a key biological component of the global sulphur (S) cycle and are common in soils, where they reduce SO42− to H2S during the anaerobic degradation of soil organic matter. The factors that regulate their distribution in soil, however, remain poorly understood. We sought to determine the ecological patterns of SRB richness within a nationwide 16S metabarcoding dataset. Across 436 sites belonging to seven contrasting temperate land uses (e.g., arable, grasslands, woodlands, heathland and bog), SRB richness was relatively low across land uses but greatest in grasslands and lowest in woodlands and peat‐rich soils. There was a shift in dominant SRB taxa from Desulfosporosinus and Desulfobulbus in arable and grassland land uses to Desulfobacca in heathland and bog sites. In contrast, richness of other generalist anaerobic bacterial taxa found in our dataset (e.g., Clostridium, Geobacter and Pelobacter) followed a known trend of declining richness linked to land‐use productivity. Overall, the richness of SRBs and anaerobes had strong positive correlations with pH and sulphate concentration and strong negative relationships with elevation, soil organic matter, total carbon and carbon‐to‐nitrogen ratio. It is likely that these results reflect the driving influence of pH and competition for optimal electron acceptors with generalist anaerobic bacteria on SRB richness.Highlights Sulphate‐reducing bacteria (SRB) are key but rare soil biota that may compete with other anaerobes As UK sulphur deposition rates fall, local populations of SRB may also decline in soils Sulphate concentrations were higher in arable and wooded sites, not at higher elevation as expected SRB richness was lower than generalist anaerobes, with peaks in grasslands and a drop in lowland woods.

Soil Organic Matter Degradation in Long-Term Maize Cultivation and Insufficient Organic Fertilization.

Soil organic matter carbon (CSOM) compounds degradation was observed in long-term field experiments with silage maize monoculture. Over a period of 26 years, the content of carbon in topsoil decreased by 22% in control unfertilized plots compared to 25% and 26% in treatments fertilized annually with mineral nitrogen. With annual wheat straw application (together with mineral N), the content of CSOM decreased by 8%. Contrary to that, the annual application of farmyard manure resulted in a CSOM increase of 16%. The ratio of carbon produced by maize related to total topsoil CSOM content ranged between 8.1–11.8%. In plots with mineral N fertilization, this ratio was always higher than in the unfertilized control plots. With the weaker soil extraction agent (CaCl2), the ratio of carbon produced by maize was determined to be 17.9–20.7%. With stronger extraction agent (pyrophosphate) it was only 10.2–14.6%. This shows that maize produced mostly unstable carbon compounds. Mineral N application resulted in stronger mineralization of original and stable organic matter compared to the unfertilized control. However, the increase of maize-produced carbon content in fertilized plots did not compensate for the decrease of “old” organic matter. As a result, a tendency to decrease total CSOM content in plots with mineral N applied was observed.

Application of biofertilizer containing Bacillus subtilis reduced the nitrogen loss in agricultural soil

Nitrogen losses caused by excess fertilizer application in agriculture are one of the main sources of non-point pollution. Biofertilizer has been considered as an effective alternative to synthetic nitrogen fertilizer, but the effectiveness and mechanism for controlling non-point pollution remains unclear. In this study, the effect of biofertilizer containing Bacillus subtilis on nitrogen loss in agricultural soil was investigated. Compared with the application of urea alone, the strategy of substituting 50% urea with biofertilizer reduced the nitrogen loss from farmland soil by 54%. Moreover, this strategy also increased nitrogen use efficiency by 11.2% and achieved a 5.0% increase in crop yield. Application of biofertilizer decreased the abundance of bacterial amoA gene and increased the abundance of narG, nirS, nirK, and nosZ genes in soil. This implied a decrease in nitrification and an increase in denitrification. Thus, it reduced the accumulation of NO3−-N in soil and greatly reduced nitrogen runoff and leaching loss. In addition, biofertilizer decreased the abundance of nitrogen-fixing gene nifH by up to 2 times. Application of biofertilizer also increased the abundance of Bacteroidetes and Chloroflexi, which play important roles in degradation of soil organic matter. In conclusion, the biofertilizer regulated the microbial nitrogen transformation process in soil and reduced nitrogen loss from agroecosystems.

Winter soils of Mongolian forests have viable ectomycorrhizas and soil enzymatic activity

In forests in Mongolia, tree roots and ectomycorrhizal fungi must survive several months of soil freezing in winter. To investigate the ectomycorrhizal community after winter, we collected fine roots of Scots pine (Pinus sylvestris) and Siberian pine (Pinus sibirica) and associated soil from Nukht forest in the Bogd-Khan National Reserve, Mongolia. Soil samples were collected from frozen soil at the end of April 2016. We described the ectomycorrhizal community, and determined on ectomycorrhizal roots tips and in soils the potential activity of enzymes involved in the degradation of soil organic matter. In order to assess the temperature sensitivity of enzyme activity, potential soil enzyme activities were assayed at temperatures from 5 to 20 °C. We detected 24 different ectomycorrhizal morphotypes associated with Pinus sylvestris and Pinus sibirica, and 18 morphotypes were identified to taxa. The two Pinus species had dissimilar ectomycorrhizal communities, and only 2 ectomycorrhizal fungal taxa were common to both species. Most ectomycorrhizal taxa had measurable activity of at least one extracellular enzyme. A high contribution to the community extracellular enzyme activity was shown for both abundant and less abundant taxa. Among the eight tested soil enzymes, only the activity of leucine amino peptidase showed consistent higher Q10 values at 5–15 °C than at 10–20 °C, suggesting that the enzyme is adapted to colder temperatures. Total soil N was the strongest factor explaining differences in soil enzyme potential activity. A positive relationship was found between soil N and the soil potential enzyme activity of acid phosphatase. We suggest that viable ectomycorrhizas during winter provide an advantage to Pinus sibirica and Pinus sylvestris in acquiring nutrients as soil thaws in spring.

Assessments of Humic Substances Application and Deficit Irrigation in Triploid Watermelon

Soil organic matter degradation and water limitation caused by intense farming activities are some of the major threats affecting agricultural production. Accordingly, the concepts of sustainable agricultural systems with optimized irrigation and improved soil quality can be adapted to address these issues. During this 2-year field study, two management factors—humic substances (HS) as organic inputs (HS vs. control) and deficit irrigation as the irrigation method (50% vs. 100% based on evapotranspiration)—were evaluated based on triploid watermelon (Citrullus lanatus cv. Fascination) yield and soil property changes. HS application increased watermelon early yield by 38.6% and total yield by 11.8% compared with the control; the early yield mainly increased under deficit irrigation. Compared with full irrigation, deficit irrigation increased water use efficiency (WUE) without significantly affecting total yield. In addition, HS application significantly increased the soil organic carbon (SOC) content, which was found to be positively correlated with crop WUE. These results indicate that soil organic inputs with HS and deficit irrigation are valuable strategies to establish sustainable systems for watermelon production, which will not only increase yield and WUE but also significantly improve soil quality and save irrigation water.

Humic Acid Transformation by the Fungus Cerrena unicolor Growing on Cellulose and Glucose

Microbial degradation of lignocellulose and soil organic matter is an important process controlling CO2 flow into the atmosphere, which is mostly carried out by fungi. The alkali-soluble fraction, which is represented mostly by the so-called humic acids (HA), dark-colored compounds of phenolic nature, constitute ~30‒70% of soil Corg. This is the first report on efficient degradation of soil HA under cellulolytic conditions; cellobiose dehydrogenase (CDH), an oxidative enzyme of the cellulolytic complex, was shown to play the key role in this process. Growth of a wood-decomposing fungus Cerrena unicolor on cellulose (CDH and laccase production) and glucose (laccase production) resulted in HA decolorization by 60 and 20%, respectively. HA depolymerization in the presence of CDH and its polymerization in the presence of laccase were shown by gel filtration experiments with the culture liquid and pure enzyme preparations. HA degradation in the presence of CDH was inhibited by radical scavengers, indicating the radical nature of this process (probably the Fenton’s reaction). HA depolymerization by CDH decreased in the presence of laccase, while HA polymerization by laccase decreased in the presence of CDH. Thus, the interaction of these enzymes affects HA transformation under cellulolytic conditions. These results improve our understanding of the mechanisms of microbial degradation of the aromatic components of soil humus and of the ecological functions of wood-decomposing fungi grown under cellulolytic conditions.

Impact of No-Till on physicochemical properties of Vertisols in Chaouia region of Morocco

Conservation agriculture (CA) relies on low soil disturbance, mulching, and crop rotation, and these characteristics present CA as a good candidate to control soil degradation and preserve soil fertility. Therefore, agricultural scientists promote it as an efficient technique to sustain agricultural production. Conventional tillage (CT) dominates many semi-arid regions of Morocco, like Chaouia. However, crop/livestock management worsens degradation of soil organic matter and thus soil fertility. Since the eighties’, controlled experimental trials tried to promote No-Till (NT) system in these regions. But it is still experiencing a low level of adoption. This on-farm research study aimed to evaluate NT effect on some Vertisols' physicochemical properties of this region. Analysis of variance only found a significant NT effect on soil organic matter (SOM), but factorial analysis provided evidence of a behavior of its effect on several physicochemical properties such as active limestone (CaCO3), total nitrogen (TN), nitrate (NO3-), calcium (Ca2+), potassium (K+) and cation exchange capacity (CEC). Furthermore, pH, Electrical Conductivity (EC) and sodium (Na+) did not show any significant difference between the two tillage treatments. This study also found that continuous cereal cropping with no mulching management mostly explains this low improvement in soil quality. This last approach, reduce CA to NT process. To promote CA in these regions, more efforts are still needed for a satisfactory up-scaling and a sustainable soil fertility conservation.

Origin of dissolved gas (CO2, O2, N2, alkanes) in pore waters of a clay formation in the critical zone (Tégulines Clay, France)

Understanding weathering processes in clay formations is an issue of primary importance for the preservation of our natural environment. Reactive-transport modeling used to simulate weathering of clay formations has indicated that reactive gases (CO2 and O2) are major parameters in controlling weathering processes.The Lower Cretaceous Tégulines marine-clay formation outcropping in the area of Brienne-le-Chateau (north-eastern France) has been investigated in the context of a sub-surface waste repository. We developed gas monitoring (CO2, O2, N2, alkanes) of core samples from two boreholes that entirely crosscut the Tégulines Clay formation, to define the consequences of weathering and oxidation processes on gases dissolved in pore waters. We discuss amounts of gas and the carbon isotopic composition of CO2 in terms of pore water chemistry including dissolved-inorganic carbon (DIC) and alkalinity, mineral reactivity, organic-matter degradation and oxygen diffusion. Degassing of samples conditioned under He atmosphere provided evidence of very high CO2 production in the soil (0–30 cm), and high CO2 degassing associated with a high oxygen level in the first 2–10 m of the clay. The CO2 degassing increase observed in weathered clay relative to preserved clay resulted from calcite dissolution due to pyrite oxidation and organic matter degradation. The δ13C of CO2 indicates that organic matter degradation was a major source of CO2 at shallow depths and down to 10–12 m, which is the maximum depth at which we observed fossil roots. Then the CO2 degassing decreased down to a constant value in preserved clay, where the carbonate system and the mineral assemblage control dissolved carbonates in pore waters. The profile of the δ13CCO2 also provides evidence of progressive CO2 diffusion of organic origin from the underlying Greensands aquifer in the lower part of Tégulines Clay up to ~40 m in the AUB230 borehole.As a first step toward understanding interactions between Tégulines Clay and near surface waters or water at the Greensands interface, we developed a reactive-transport model to simulate in one dimension weathering processes under ambient temperature, constrained by geochemical reactions in soil (organic matter degradation) and in the clay (pyrite oxidation and calcite dissolution), exchange, DIC and pore water chemistry. The simulation was carried out for 10 kyrs, assuming that weathering and soil formation began after the last glacial maximum. The DIC profile cannot be simulated without considering evaporation processes in agreement with the isotopic data. This type of approach combining a complete field dataset (reactive-gas concentrations, δ13C of CO2, major-ion concentrations, δ18O and δD of pore waters) and reactive-transport modeling is necessary for better understanding of chemical weathering processes in the critical zone.

Monitoring in situ microbial activities in wet or clayey soils by a novel microbial-electrochemical technology

Abstract Measuring microbial respiration is the most common method used to determine soil biological (microbial) activities, soil organic matter content, and general soil health; however, these methods are usually time-consuming, consumables heavy, and are less sensitive to soils with excessive moisture content or low permeability (such as clay). While soil respiration methods rely on carbon dioxide (CO2) released from microbial metabolism, microbial electrochemical technologies (MET) assess soil microbial activity levels by quantifying electrons that are released from the microbial degradation of soil organic matter. An MET device was designed and constructed as an alternative tool for measuring soil microbial activities and to validate its effectiveness in clayey soil. The results indicated that the electrical potentials (voltage drops across a resistive load) measured by the MET device correlated with soil CO2 emissions and microbial activity levels. Overall, MET offers an alternative and cost-effective method for determining soil microbial activity levels without using consumables. It can also overcome the limitations associated with wet or clayey soils, further validated through ongoing and future studies and field applications.

Lipids source and degradation as revealed by molecular biomarkers in soils after acid-pretreatment: A case of a plantain soil under long-term cultivation

The extractable lipids are important components in soil organic matter (SOM) which were used to trace the sources and degradation of SOM. The protection of lipids by soil mineral have been suggested through organic solvents. But, the extraction efficiency of some lipid compounds was low. This study applied a mild acid treatment to firstly remove most of the reactive mineral particles, and without altering SOM chemical structures in 10% HF/1M HCl (1:4 w: v). Based on the obtained lipid biomarker information, we observed that the lipid extraction efficiency significantly increased by organic solvents on after removal of active minerals. The acid treatment increased the scientific to quantitative the amount of lipids. The minerals showed significant differences in the selective protection to different components of lipids. In this study, the amount range of protected n-alkanoic acids is 73~85%, n-alkanol 41~62% and n-alkanes 26~46%. After the vegetation was replaced, the increased alkenoate and alkane in soil input by the plant tissues of plantain directly, and the alkanols probably input by the hydrolysis of wax esters. Under the interference of man-made tillage activities, the C content in 0-20 cm decreased, suggesting that cultivated activities may enhance SOM degradation and accelerate SOM turnover. Understanding SOM behaviour in this area will provide important information for soil management and to evaluate carbon cycling in human-affected ecological systems.

Effects of titanium dioxide nanoparticles and sheep manure biochar on the behavior of methylene blue organic contaminant in sandy loam and loam soils

The present study aimed to evaluate the effects of titanium dioxide nanoparticles (T: 1 and 3%), sheep manure biochar (B: 1 and 3%), and their combination (T0.5% + B0.5% and T1.5% + B1.5%) on the degradation/sorption of methylene blue (MB: 100, 200, and 300 mg/kg soil) in sandy loam (SL) and loam (L) soils with variable light radiation (ultraviolet(UV) and visible(VS)). According to the results, the application of T (especially with UV radiation exposure), B, and their combination significantly reduced the MB concentration in the soils compared to the control treatment (without T and B). In addition, the combined application of T and B (T1.5% + B1.5%) was significantly more efficient than the other treatments in the reduction of the MB extraction in the studied soils. However, B and T application decreased the soil MB concentration, but due to the competition between the ions in the soils and MB for adsorption on the B and T surfaces, insufficient water in the soils to form appropriate amounts of degradable hydroxyl radicals, low absorption of UV radiation by T (due to the high thickness of the soil layer), and consumption of large amounts of T for soil organic matter degradation, the efficacy of these amendments decreased. Moreover, lack of using specialized biochar for this cationic contaminant (modification of the B functional groups) diminished the efficiency of this adsorbent in the soil, which requires further investigations.

Degradasi bahan organik di beberapa perkebunan teh di Jawa Barat

The tea production and the quality were over the years, especially in West Java, Indonesia. This had been affected by several factors including the aged of tea plantations, declining soil fertility, and soil degradation among other factors. Andisol is the most suitable soil and dominant in Indonesia for tea plantation. The objectives of this study were to evaluate the impact of long-term tea cultivation on soil organic matter degradation in 6 years (2011 and 2017) in tea plantations in Bandung, Bogor, Cianjur and Garut, West Java. Data analysis used the independent sample t-test with SPSS 16.0 at a significant level of 95%. This study used a quantitative descriptive method by comparing the levels of organic matter and macronutrients from 89 samples taken in 2011 and 2017. Soil samples were taken at a depth of 35-40 cm with a distance of 25cm of the tea plant. Conclusion of the study, that the levels of organic matter in the regions of Bandung, Cianjur, and Garut are not significantly different between 2011 and 2017; while in the Bogor area the level of organic matter in 2011 was 5.41% and decreased in 2017 was 4.40% (a decrease of 18.72%). However, based on C-organic data in 2011 and 2017 nutrient degradation has occurred in all locations. Decreasing organic matter can cause a low decrease in soil fertility and productivity of tea crops. Provision of organic matter and proper fertilization must be done to maintain soil fertility and productivity of tea plants.

Chemical structure of soil organic matter

Selective preservation belongs among the important stabilization mechanisms of soil organic matter (SOM). Conceptually, it is based on non-covalent intermolecular interactions of organic molecules, which leads to a decrease in the Gibbs’ energy of the SOM structure. Earlier works suggested that this stabilization of SOM physical structure is supported also by water molecules that form clusters bridging SOM moieties. This article reports results suggesting that water is connected also to stabilization of SOM chemical structure. We analyzed the dynamics and composition of gases evolved during drying of 33 mineral soils, which were exposed to 40% relative humidity prior to the analysis. It was observed that moisture elimination occurring below 100 °C is accompanied by evolution of a small amount of low molecular mass gases representing typical degradation products of organic materials. In particular, analyses revealed the evolution of CO, HCN, NO, and probably traces of NH3 and CO2, which implied degradation of N-containing molecules. The peak temperature of evolved CO correlated with the amount of adsorbed water. The amount of evolved CO positively correlated with the amount total organic C and N contents and clay content. On the contrary, the amount of CO evolved during degradation did not correlate with the amount of CO2 produced during incubation of analyzed soils either at short or longer incubation times. Evolution of gases started and culminated simultaneously with drying. The analysis of soils exposed to higher relative humidity levels resulted in a shift of the CO peaks to higher temperatures. Therefore, the results suggested a possible causality between water desorption at elevated temperature and SOM degradation processes.

Elucidating molecular level impact of peat fire on soil organic matter by laser desorption ionization Fourier transform ion cyclotron resonance mass spectrometry.

In this work, laser desorption ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry (LDI-FTICRMS) was used to investigate the molecular composition of a peat fire and laboratory heated soil organic matter (SOM). SOM isolated from soils obtained from unburned and burned sites at Central Kalimantan, Indonesia, were analyzed with LDI-FTICRMS. About 7500 peaks were found and assigned with molecular formulas for each mass spectrum. SOM isolated from fire-affected soil sites are relatively more abundant in low oxygenated classes (e.g., O1-O5) and thermally stable compounds, including condensed hydrocarbon and nitrogen heterocyclic compounds. Abundances of highly condensed hydrocarbon compounds with carbon number > 30 were increased for the fire-affected SOM. In vivo heating experiments were conducted for SOM extracted from unburned sites, and the prepared SOMs were analyzed with LDI-FTICRMS. Overall, the same trend of change at the molecular level was observed from both the laboratory heated and the peat fire-affected SOM samples. In addition, it was observed that heat caused the degradation of SOM, generating lignin and tannin-type molecules. It was hypothesized that they were formed by thermal degradation of high molecular weight SOM. All the information presented in this study was obtained by consuming ~ 5μg of sample. Therefore, this study shows that LDI-FTICRMS is a sensitive analytical technique that is effective in obtaining molecular level information of SOM. Graphical abstract.

Influence of Long-term Chemical fertilizers and Organic Manures on Soil Fertility - A Review

The Effects of Chemical Fertilizers and organic manure on soil fertility focuses primarily on the behavior of nitrogen (N) and phosphorus (P) in soil because these two nutrients are the main nutrients that limit crop yields and they are also the nutrients of particular concern for environmental quality. Besides, potassium (K), sulfur (S), macronutrients (primary and secondary), micronutrient and other elements, salts, and sodium, soil pH, EC. CEC and organic matter are covered. Organic fertilizer improves physical and biological activities of soil but they have comparatively low in nutrient content, so larger quantity is required for plant growth. However, inorganic fertilizer is usually immediately and fast containing all necessary nutrients that are directly accessible for plants. But continuous use of inorganic fertilizers alone causes soil organic matter degradation, soil acidity, and environmental pollution. So the combined application of inorganic fertilizer and organic manure has an alternative system for the sustainable and cost-effective management of soil fertility. The objective of the present review is to assess the effect of long-term chemical fertilizers and organic manure on soil fertility. The study revealed that the appropriate application of inorganic fertilizers along with organic manure increases soil fertility than the values obtained by organic or inorganic fertilizers separately.

Reservoir CO2 evasion flux and controlling factors of carbon species traced by δ13CDIC at different regulating phases of a hydro-power dam

As the world largest hydropower reservoir, the Three Gorges Reservoir (TGR) significantly impacted on the carbon cycle since reservoirs are sources of carbon sink. This study was carried out to investigate the effects of damming on the carbon cycle. δ13CDIC and δ13CDOC were used to trace the origin of dissolved organic (DOC) and inorganic carbon (DIC). The estimated CO2 evasion flux in two regulating phases (discharge and recharge) with averages of 111 mg/m2 h and 264 mg/m2 h, respectively. At the basin scale, average CO2 flux was about 188 mg/m2 h and varies from −158 mg/m2 h to 1092 mg/m2 h. The highest average pCO2 (1294 ppmv) was observed during the discharge period, which was oversaturated than atmospheric equilibrium value; hence, the TGR act as a considerable sink of atmospheric carbon. The δ13CDIC varies from −8.95‰ to 0.00‰ with mean −1.87‰; these enrich isotope values indicated that metabolic process (photosynthesis and respiration) and the rapid kinetics of carbonate weathering by soil CO2 control the pCO2. The low pCO2 of reservoir water caused the rapid dissolution of CO2 from the atmosphere during the recharge period. The δ13CDOC varies between −30.64‰ to −23.05‰, which is similar to the values of C3 vegetation; thus, the source of DOC would be the degradation of soil organic matter. Overall, this study revealed the δ13CDIC signature coupled with soil CO2 dissolution and admixture of atmospherically equilibrated waters resulting in the sink of atmospheric CO2 of the reservoir and impoundment of the dam alters the carbon cycle and aquatic carbon budget in TGR. The findings of this study provide a global image on the contribution of reservoirs to the carbon cycle and aquatic carbon budget. Coupling with isotope signatures and elemental concentrations, investigation of the biogeochemical cycle of the carbon can be effectively traced.

A benchmark for soil organic matter degradation under variably saturated flow conditions

Soil contains the largest terrestrial pool of organic matter, and the cycling of organic carbon in soils plays a crucial role in controlling atmospheric carbon dioxide (CO2) and global climate change. Although considerable progress has been made in previous modeling studies on the fate of soil organic matter (SOM), only a few models used a process-based approach for investigating these strongly coupled and complex soil systems, which involve SOM oxidation, transient water flow, and mass transport processes in aqueous and gaseous phases. Typically, physically based models for water flow, as well as solute and gas transport, are not coupled with state-of-the-art SOM degradation models. Reactive transport models (RTMs) provide a flexible framework for implementing different SOM degradation concepts and integrating biogeochemical processes with water flow and mass transport. Given the complex nature of carbon cycling in soils coupled with flow and mass transport, code intercomparison using well-defined benchmarks is in many cases the only practical method of model verification. The benchmark presented in this manuscript focuses on SOM oxidation under variably saturated flow conditions. The benchmark consists of three problems characterized by increasing complexity. The problems were solved using two different reactive transport codes, namely HP1 and MIN3P-THCm. The first supporting problem introduces a batch-type simulation to assess kinetic networks of SOM degradation. In the second supporting problem, transient water flow, solute transport, gas generation, and diffusive gas transport are considered. The principal problem combines the kinetic networks of SOM degradation with reactive transport under variably saturated flow conditions, including CO2 transport from soils to the atmosphere. Simulation results for the benchmark problems demonstrate an overall excellent agreement between the two codes, building confidence in the ability of RTMs to simulate complex C-cycling in dynamic environments.

Lignin chemistry of wetland soil profiles in two contrasting basins of the Louisiana Gulf coast

Recent studies indicate that loss of soil organic matter (SOM) from coastal wetlands can contribute to the hypoxia in the northern Gulf of Mexico along the Louisiana coast. In this study, coastal marsh soil profiles of two contrasting basins were investigated for lignin composition in order to assess organic matter source and degradation status under these different wetland formations. The Atchafalaya Basin is undergoing land building whereas the Barataria Basin is experiencing land loss. Lignin monomers were extracted using alkaline CuO oxidation followed by gas chromatography–mass spectrometry characterization. Marsh soil profiles from the Barataria Basin showed strong lignin storage with two-fold higher lignin contents (sum of vanillyl, syringyl, and cinnamyl phenols, Λ8) than those in the Atchafalaya Basin. Source SOM inputs in the Barataria Basin were non-woody angiosperms, whereas in the Atchafalaya Basin inputs were non-woody angiosperms and some gymnosperm inputs. Principal component analysis (PCA) showed that different soil environmental factors dominated the status of soil organic matter degradation. Soil pH was negatively related to the lignin degradation in the Atchafalaya Basin, whereas high total N contents inhibited lignin degradation in the Barataria Basin. Increasing electrical conductivity inhibited organic matter degradation in the low salinity wetlands of the Atchafalaya Basin, but positively influenced lignin decomposition in the higher salinity marsh soil profiles of Barataria Basin. Despite SOM in the Barataria Basin being more degraded than in the Atchafalaya, the nearly 10-fold greater amount of organic carbon (C) in the Barataria, coupled with net land loss, indicates that it is greater source of oxygen-consuming organic C, thus contributes more to the hypoxia in the Gulf.

Phenolic Compounds as Unambiguous Chemical Markers for the Identification of Keystone Plant Species in the Bale Mountains, Ethiopia.

Despite the fact that the vegetation pattern and history of the Bale Mountains in Ethiopia were reconstructed using pollen, little is known about the former extent of Erica species. The main objective of the present study is to identify unambiguous chemical proxies from plant-derived phenolic compounds to characterize Erica and other keystone species. Mild alkaline CuO oxidation has been used to extract sixteen phenolic compounds. After removal of undesired impurities, individual phenols were separated by gas chromatography and were detected by mass spectrometry. While conventional phenol ratios such as syringyl vs. vanillyl and cinnamyl vs. vanillyl and hierarchical cluster analysis of phenols failed for unambiguous Erica identification, the relative abundance of coumaryl phenols (>0.20) and benzoic acids (0.05—0.12) can be used as a proxy to distinguish Erica from other plant species. Moreover, a Random Forest decision tree based on syringyl phenols, benzoic acids (>0.06), coumaryl phenols (<0.21), hydroxybenzoic acids, and vanillyl phenols (>0.3) could be established for unambiguous Erica identification. In conclusion, serious caution should be given before interpreting this calibration study in paleovegetation reconstruction in respect of degradation and underground inputs of soil organic matter.