Organic Matter Chemistry Research Articles

Hydrological management affected dissolved organic matter chemistry and organic carbon burial in the Three Gorges Reservoir

With the linkage between dissolved organic matter (DOM) and the characteristics of natural ecosystem assessed extensively, the properties of DOM in reservoirs, the typical human interrupted ecosystems, have been focused on in recent years, which is critical for the understanding of human impacts on watershed ecosystems and carbon cycling. This study aims to analyze the effect of hydrological management on the DOM chemistry and organic carbon burial in Daning River tributary of the world's largest Three Gorges Reservoir (TGR). Based on the application of a combined approach including bulk geochemical analyses, optical spectroscopy, and ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry, various sources of DOM (terrestrial, anthropogenic, and autochthonous sources) were revealed. An increasing trend of terrestrial and recalcitrant DOM was observed along the upstream to downstream transect of Daning River tributary, which was mainly caused by the water intrusion with a higher terrestrial and recalcitrant signature from mainstream to tributary resulted from hydrological management of TGR. Integrated with the analysis of sedimentary organic matter in Daning River tributary in the past decade (after the construction of TGR), our work suggests that organic carbon burial in the reservoir could be enhanced by hydrological management-induced variation in DOM chemistry. Further studies are needed to better constrain the effects of damming reservoirs on carbon cycling considering their booming all over the world.

Three Gorges Reservoir construction induced dissolved organic matter chemistry variation between the reservoir and non-reservoir areas along the Xiangxi tributary

As one of the most dynamic and reactive compound pools, DOM plays a crucial part in various biochemical processes such as element cycling and nutrient export. Although the reservoir DOM has been investigated extensively, the variation of DOM between reservoir area and non-reservoir area induced by reservoir construction is not comprehensively assessed. By the combination of a series of complementary techniques including stable carbon isotope, optical spectroscopy, and ultrahigh-resolution mass spectrometry, here we show that hydrological alterations induced by Three Gorges Reservoir (TGR) construction were responsible for the variation in DOM molecular composition between reservoir area and non-reservoir area in Xiangxi tributary. With water intrusion from mainstream induced by reservoir construction and operation, reservoir area had relatively higher terrestrial input and more recalcitrant DOM than those in the non-reservoir area with limited influence of reservoir operation, whereas, relatively more autochthonous source and higher molecular lability of DOM were observed in the non-reservoir area. The water intrusion from mainstream to tributaries induced by reservoir construction is likely the main factor controlling DOM variation between reservoir area and non-reservoir area, and might devote to the organic carbon burial in the reservoir. This research provides new insight into spatial variations of riverine DOM composition induced by reservoir construction, underlining the important role of the reservoir in DOM cycling.

Soil carbon dynamics during drying vs. rewetting: Importance of antecedent moisture conditions

Soil moisture influences soil carbon dynamics, including microbial growth and respiration. The response of such ‘soil respiration’ to moisture changes is generally assumed to be linear and reversible, i.e. to depend only on the current moisture state. Current models thus do not account for antecedent soil moisture conditions when determining soil respiration or the available substrate pool. We conducted a laboratory incubation to determine how the antecedent conditions of drought and flood influenced soil organic matter (SOM) chemistry, bioavailability, and respiration. We sampled soils from an upland coastal forest, Beaver Creek, WA USA, and subjected them to drying and rewetting treatments. For the drying treatment, field moist soils were saturated and then dried to 75, 50, 35, and 5% saturation. In the rewetting treatment, field moist soils were air-dried and then rewet to 35, 50, 75, and 100% saturation. We measured respiration and water extractable organic carbon (WEOC) concentrations and used 1H-NMR and FT-ICR-MS to characterize the WEOC pool across the treatments. The drying vs. wetting treatment strongly influenced SOM bioavailability, as rewet soils (with antecedent drought) had greater WEOC concentrations and respiration fluxes compared to the drying soils (with antecedent flood). In addition, air-dry soils had the highest WEOC concentrations, and the NMR-resolved peaks showed a strong contribution of protein groups in these soils. Both NMR and FT-ICR-MS analyses indicated increased contribution of complex aromatic groups/molecules in the rewet soils, compared to the drying soils. We suggest that drying introduced organic matter into the WEOC pool via desorption of aromatic molecules and/or by microbial cell lysis, and this stimulated microbial mineralization rates. Our work indicates that even short-term shifts in antecedent moisture conditions can strongly influence soil C dynamics at the core scale. The predictive uncertainties in current soil models may be reduced by a more accurate representation of soil water and C persistence that includes a mechanistic and quantitative understanding of the impact of antecedent moisture conditions.

C-STABILITY an innovative modeling framework to leverage the continuous representation of organic matter

The understanding of soil organic matter (SOM) dynamics has considerably advanced in recent years. It was previously assumed that most SOM consisted of recalcitrant compounds, whereas the emerging view considers SOM as a range of polymers continuously processed into smaller molecules by decomposer enzymes. Mainstreaming this new paradigm in current models is challenging because of their ill-adapted framework. We propose the C-STABILITY model to resolve this issue. Its innovative framework combines compartmental and continuous modeling approaches to accurately reproduce SOM cycling processes. C-STABILITY emphasizes the influence of substrate accessibility on SOM turnover and makes enzymatic and microbial biotransformations of substrate explicit. Theoretical simulations provide new insights on how depolymerization and decomposers ecology impact organic matter chemistry and amount during decomposition and at steady state. The flexible mathematical structure of C-STABILITY offers a promising foundation for exploring new mechanistic hypotheses and supporting the design of future experiments.

The C/N ratio and phenolic groups of exogenous dissolved organic matter together as an indicator for evaluating the stability of mineral-organic associations in red soil

Mineral-organic associations (MOAs) are the basic structural units of soil aggregates and are important reservoirs of nutrients for plants and soil microorganisms, determining the soil structure and fertility. However, the influence of exogenous dissolved organic matter (DOM) chemistry on the stability of MOAs is rarely reported. We first characterized different exogenous DOM through elemental analysis and spectroscopy analysis technologies. Then, a chamber incubation experiment was conducted with DOM addition concentration at 3 g C kg−1 red soil. Principal component analysis, redundancy analysis, and the partial least squares path model were used to better understand the effect of exogenous DOM chemistry on the stability of MOAs. The addition of DOM into the red soil significantly increased not only the organic carbon both in the bulk soil and the soil heavy fraction, but also the soil combined humus and the soil mineral-organic compound quantity. Moreover, the rice straw-derived DOM had the best effect on improving the soil mineral-organic compound quantity/degree (additional), followed by the animal-derived DOM, while the fulvic acid increased it the least. The ratios of elements (C/N ratio, O/C ratio, and H/C ratio), aromaticity (SUVA254), and phenolic C content of exogenous DOM had positively significant contributions to the stability of MOAs. The rice straw-derived DOM had the greatest enhancement on the stability of the MOAs for its higher C/N ratio and phenolic groups content, so the exogenous DOM characteristics could be as an indicator in predicting the stability of the MOAs and evaluating the soil fertility.

Antecedent conditions determine the biogeochemical response of coastal soils to seawater exposure

Coastal landscapes are increasingly exposed to seawater due to sea level rise and extreme weather events. The biogeochemical responses of these vulnerable ecosystems are poorly understood, limiting our ability to predict how their role in local and global biogeochemical cycles will shift under future conditions. Here we evaluate how antecedent conditions influence the biogeochemical response of soil to seawater inundation events based on a 42-day laboratory incubation experiment with soils collected from a natural salinity gradient across a coastal floodplain. We quantified influences of seawater inundation on intact soil cores through high-frequency carbon dioxide (CO2) and methane (CH4) gas fluxes measurements as well as ultrahigh resolution characterization of organic matter chemistry and metabolites. Mean CO2 and CH4 fluxes were higher after inundation compared to control cores for soils that had low in situ electrical conductivity (EC). Soils with low in situ EC also exhibited significant shifts in organic matter profiles after inundation, with surficial soils (0–7.5 cm) becoming more enriched in phenolic compounds, compared to deeper soils (7.5–15 cm). The number of biochemical transformations inferred from mass spectrometry increased significantly after inundation for soils with low in situ EC. Our results suggest that seawater inundation of low-salinity terrestrial environments can lead to increased microbial activity and increasing likelihood of soil carbon release, with sites experiencing infrequent or new seawater exposure likely to be more sensitive to saltwater exposure relative to sites with more frequent exposure. We conclude that the biogeochemical impacts of future seawater exposure will be modulated by antecedent conditions associated with landscape position within coastal watersheds.

Chemical composition of soil organic carbon from mixed aspen‐conifer forests characterized with Fourier transform infrared spectroscopy

AbstractForest species control the quantity and chemistry of organic matter input, which in interaction with the soil physicochemical properties, environmental conditions and microbial community associated with a given ecosystem may result in specific patterns of soil organic carbon (SOC) stabilization and chemistry. The objectives of this study were: (a) to characterize the chemistry of soil organic matter and SOC fractions across the gradient from pure aspen (Populus tremuloidesMichx.) to pure conifer (Abies lasiocarpa(Hook.) Nutt. andPseudotsuga menziesii(Mirbel) Franco) stands in semi‐arid montane forests, and (b) to determine whether the effect of overstory composition on SOC chemistry was patent beyond the influence of site conditions and microbial decomposer community. We used Fourier transform infrared spectroscopy to analyse the chemistry of bulk soil (BS), light fraction (LF) and mineral‐associated SOC (MoM) from mineral soils (0–15 cm) sampled across the natural gradient of aspen and mixed conifer stands from northern and southern Utah. Vegetation overstory had a subtle effect on the MoM fraction, indicating higher proportion of aliphatic C with aspen dominance, whereas there were no differences in LF chemistry between vegetation types. Independently of the vegetation cover type, the MoM fraction was enriched in aliphatic C compared to the LF, although the proportion of polysaccharides and C‐O groups increased in the MoM fraction for plot samples. Differentiation between spectra from soils developed on sedimentary rock and soils developed on basalt, quartzite and limestone, highlighted the influence of parent material and mineralogy on MoM chemistry. The patterns in SOC fractions' chemistry do not allow an affirmation that greater SOC storage under aspen is due to the accumulation of recalcitrant compounds (i.e., aliphatic C) and controlled by litter chemistry. Rather, they suggest that the ensemble of litter chemistry, microbial community and soil properties in aspen stands enhances SOC storage.HighlightsVegetation overstory and site characteristics (e.g., parent material) influence SOC chemistry and stabilization patterns.Light fraction SOC spectra did not differ between forest species in the aspen‐conifer ecotone.The proportion of aliphatic C in mineral‐associated organic carbon (MoM) increased with aspen dominance.The effect of overstory composition on MoM chemistry was patent beyond the influence of site conditions and microbial decomposer community.

Lime and/or Phosphate Application Affects the Stability of Soil Organic Carbon: Evidence from Changes in Quantity and Chemistry of the Soil Water-Extractable Organic Matter.

The mechanisms by which lime and/or phosphate addition impacts the preservation of soil organic matter (OM) are poorly understood. We explored the changes in quantity and chemistry of water-extractable organic matter (WEOM) in the bulk soil and its heavy density fraction (>1.6 g/cm3) of an unmanaged C-rich volcanic soil caused by lime and/or phosphate application. The addition of lime or phosphate caused (i) a significant increase in the WEOM, along with a decrease in its C/N ratio and an increase in its aromaticity, and (ii) changes in the WEOM chemical composition, measured with pyrolysis-gas chromatography/mass spectrometry, this being most impacted by lime application. The combined effect of lime and phosphate addition on the quantity and chemistry of WEOM was larger than the effects of separate lime and phosphate additions. By comparing the response of the bulk soil and the heavy fraction, we infer that phosphate has a greater contribution to the destabilization of vulnerable particulate OM, while lime causes a comparable disruption in the particulate OM and that in the heavy fraction. These findings provide a mechanistic insight into the decreased OM stability after liming and/or P fertilizing Andosols. They have implications for designing climate-smart management practices for these soils.

Soil microbial community composition and function are closely associated with soil organic matter chemistry along a latitudinal gradient

Despite the important roles of microorganisms in soil organic matter (SOM) decomposition and nutrient cycling, how their biomass, community composition and function are driven by a joint effect of edaphic and environmental factors at a large spatial scale remains unclear. Moreover, a mechanistic understanding of the associations among microbial communities and SOM chemistry as a key indicator of substrate availability over large spatial scales has been lacking until now. To fill this knowledge gap, we examined how soil microbial community abundance and composition (via phospholipid fatty acid (PLFA) analysis) and function (via enzyme activities and net N mineralization) were associated with the edaphic and environmental factors from six oriental oak (Quercus variabilis) forest sites arranged across a 11° latitudinal gradient in East China. We found strong relationships between SOM chemistry as indicated by 13C nuclear magnetic resonance (NMR) spectroscopy and microbial community composition and function along this gradient. For example, the ratio of fungi to bacteria (F/B) decreased with increasing carboxyl C percentage; the lower abundance of arbuscular mycorrhizal fungi (AMF) and decreased invertase activity in the southern sites were possibly related to higher ratio of alkyl to O alkyl (A/O); urease activity increased with carboxyl C percentage. Microbial biomass (total PLFAs) had positive relationships with urease activity, net N mineralization rate, and soil pH. Mean annual precipitation (MAP) and pH were important for overall microbial community composition. Overall, our results indicate that SOM chemistry is closely related to microbial community composition and function along the latitudinal gradient and should be considered in future studies. Our study contributes to a better prediction of microbial responses to possible substrate changes brought by future environmental changes, which is urgently needed under the context of accelerating global change.

Plant invasion alters the Michaelis–Menten kinetics of microbial extracellular enzymes and soil organic matter chemistry along soil depth

Microbial extracellular enzymes decompose distinct components of soil organic matter (SOM), thus influencing its stability. However, we lack the knowledge about how the kinetics of individual enzymes vary when multiple substrates change simultaneously. Here we used Japanese knotweed (Polygonum cuspidatum) invasion as a model system to explore how the Michaelis–Menten kinetics (Vmax and km) of microbial extracellular enzymes vary with corresponding SOM components across soil depth (0–5, 5–10, and 10–15 cm). We hypothesized that invasion will increase the Vmax (maximum enzyme activity) and km (substrate concentration at half Vmax) of oxidative enzymes but decrease the Vmax and km of hydrolytic enzymes, and that increasing soil depth will alleviate the invasion effects on the enzyme kinetics. The invasion of knotweed, which input litter rich in recalcitrant compounds, altered soil chemistry including an increase in lignin and fungal biomass compared to the adjacent non-invaded soils. The Vmax of peroxidase, the oxidative enzyme that degrades lignin, increased in the invaded soils (0–5 cm) compared to the non-invaded soils. Among the hydrolytic enzymes, the Vmax of N-acetyl-glucosaminidase which degrades chitin from fungal cell walls increased in the invaded soils (0–5 cm). However, there was no associated change in the km of peroxidase and N-acetyl-glucosaminidase under invasion, suggesting that microbes modified the enzyme production rates, not the types (isozyme) of enzymes under invasion. The Vmax of all enzymes decreased with depth, due to the reduced substrate availability. These results highlight that the addition of relatively recalcitrant substrates due to plant invasion altered the kinetics of microbial extracellular enzymes with implications for SOM chemistry in the invaded soils.

Low-severity fire as a mechanism of organic matter protection in global peatlands: Thermal alteration slows decomposition.

Worldwide, regularly recurring wildfires shape many peatland ecosystems to the extent that fire-adapted species often dominate plant communities, suggesting that wildfire is an integral part of peatland ecology rather than an anomaly. The most destructive blazes are smoldering fires that are usually initiated in periods of drought and can combust entire peatland carbon stores. However, peatland wildfires more typically occur as low-severity surface burns that arise in the dormant season when vegetation is desiccated, and soil moisture is high. In such low-severity fires, surface layers experience flash heating, but there is little loss of underlying peat to combustion. This study examines the potential importance of such processes in several peatlands that span a gradient from hemiboreal to tropical ecozones and experience a wide range of fire return intervals. We show that low-severity fires can increase the pool of stable soil carbon by thermally altering the chemistry of soil organic matter (SOM), thereby reducing rates of microbial respiration. Using X-ray photoelectron spectroscopy and Fourier transform infrared, we demonstrate that low-severity fires significantly increase the degree of carbon condensation and aromatization of SOM functional groups, particularly on the surface of peat aggregates. Laboratory incubations show lower CO2 emissions from peat subjected to low-severity fire and predict lower cumulative CO2 emissions from burned peat after 1-3years. Also, low-severity fires reduce the temperature sensitivity (Q10 ) of peat, indicating that these fires can inhibit microbial access to SOM. The increased stability of thermally altered SOM may allow a greater proportion of organic matter to survive vertical migration into saturated and anaerobic zones of peatlands where environmental conditions physiochemically protect carbon stores from decomposition for thousands of years. Thus, across latitudes, low-severity fire is an overlooked factor influencing carbon cycling in peatlands, which is relevant to global carbon budgets as climate change alters fire regimes worldwide.

Effect of land use on organic matter composition in density fractions of contrasting soils: A comparative study using 13C NMR and DRIFT spectroscopy

13C CP-MAS nuclear magnetic resonance (NMR) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopies were compared for evaluating their potential to characterise the influence of land use change on organic carbon (OC) chemistry of particulate organic matter (POM) and mineral associated OM (MOM) fractions of different soil types. Surface soil samples of Ferralsol, Luvisol, Vertisol and Solonetz were collected from native and crop lands and isolated into different density fractions. NMR and DRIFT showed distinct OC composition for all the soil fractions of two land uses. In NMR spectra, greater proportion of carbohydrate and aromatic C was observed in POM, while MOM fractions were rich in carbohydrate, amino groups and aliphatic C. DRIFT spectra showed greater carboxylic, aromatic C and amide N in MOM than corresponding POM. NMR spectroscopy detected charred aromatic C in both fractions, which was not feasible with DRIFT. The overall effect of land use in both techniques appeared similar on the composition of POM- OC, i.e., increased aromaticity and decreased alkyl C:O-alkyl C ratio. However, differences of land use impact were observed in MOM-OC, e.g., overall decreased aromaticity and increased alkyl C:O-alkyl C for all soils in NMR, and in DRIFT, it varied with soil type (aromaticity: Ferralsol, Vertisol > Luvisol, Solonetz). However, these trends were inconsistent and indistinct among fractions of four soils. Discrepancy in NMR and DRIFT results was ascribed to the sensitivity limitations of the two techniques in characterising soil OM in mineral rich fractions, and sample pre-treatment effect in NMR. We conclude that combination of NMR and DRIFT spectroscopy, preferably supplemented by other techniques e.g., mass spectroscopy and XPS, would improve the proficiency in elucidating small changes in soil OM composition with land use conversion.

Do Fulvic, Humic, and Humin Carbon Fractions Represent Meaningful Biological, Physical, and Chemical Carbon Pools?

Humic fractions have been used to characterize soil organic matter chemistry and to identify different carbon (C) pools. The relevance of alkaline fractionation is questioned because of probable artifacts created by the extraction. Fractionation using water‐soluble organic C (WSOC), soil microbial biomass C (SMBC), permanganate‐oxidizable C (POX‐C), and particulate organic matter C (POM‐C) methodologies are sensitive to C changes. Mineral‐associated organic C (MAOC) includes WSOC and other C fractions that are not all passive C. We measured 15 proficiency soils ranging in texture, mineralogy, pH, and soil organic C (SOC) to isolate C pools with differing turnover times. We define WSOC, SMBC, POX‐C, and fulvic acid (FA) as fast pools. Slow cycling pools include POM‐C and humic acid (HA) fractions, whereas passive pools include MAOC minus WSOC (PMAOC) and humin C (HC). Data showed FA was correlated with POX‐C (r = 0.84) as a fast C pool but was slightly more correlated with PMAOC (r = 0.86), a passive pool. Slow C pools of HA and POM‐C were not strongly correlated (r = 0.49). All alkaline fractions of C showed highest correlations to PMAOC, followed by POX‐C, and all showed poor correlations with POM‐C. The conventional first extraction fraction (A) yielded 70% recovery of the total FA extracted, whereas only 45% of HA was obtain. Acid soils on average had higher total FA. The regression model using physical fractionation was the best (SOC = WSOC + POM‐C + PMAOC, R2 0.98). Only the HA fraction was a significant parameter in the regression of SOC = FA + HA + HC (p = 0.0029).Core Ideas Humic acid C showed the strongest correlation to all C pools except POM‐C. Recovery of fulvic C in conventional fraction was 70% of a total of three fractions. Recovery of humic acid C in conventional fraction was 45% of a total of three fractions. The distribution of fulvic (FA) to humic acid (HA) C changed with acid soils having higher FA. Regression of soil organic C = FA + HA + humin only showed the HA to be significant.

Mineral vs. organic matter supply as a limiting factor for the formation of mineral-associated organic matter in forest and agricultural soils

Physical and chemical interactions between soil organic matter (OM) and minerals is one of the primary mechanisms for stabilizing OM in terrestrial ecosystems. Focusing on OM association with mineral surfaces, this study sought to examine mineral-associated OM from the perspectives of both mineral surface characteristics and organic matter chemistry. The research was conducted at paired-sites under North American Mid-Atlantic Coastal forest and crop production with shared environmental factors. Using carbon (C) and nitrogen (N) 1s micro- X-ray absorption near-edge fine structure (XANES) spectroscopy, we investigated the amounts and types of mineral-associated OM. Mineral specific surface area (SSA) of bulk soil was determined for three conditions: untreated, post OM removal and post iron (Fe) (oxyhydr)oxides removal. Amounts of mineral-associated OM were smaller in the agricultural soil, where greater SSA sourced from clay-sized phyllosilicates and Fe (oxyhydr)oxide minerals did not result in greater OM coverage of the mineral surface area. Although agricultural surface soil showed less abundance of phenolic C, speciation of mineral-associated OM did not differ between comparable horizons. Our results suggest that despite the plow-derived mixing of soil, which increased SSA and secondary minerals available to interact physically and chemically with OM in the plowed layer, the formation of mineral-associated OM in agricultural soil is ultimately limited by available OM.

Organic Functional Group Chemistry in Mineralized Deposits Containing U(IV) and U(VI) from the Jackpile Mine in New Mexico.

We investigated the functional group chemistry of natural organic matter (NOM) associated with both U(IV) and U(VI) in solids from mineralized deposits exposed to oxidizing conditions from the Jackpile Mine, Laguna Pueblo, NM. The uranium (U) content in unreacted samples was 0.44-2.6% by weight determined by X-ray fluorescence. In spite of prolonged exposure to ambient oxidizing conditions, ≈49% of U(IV) and ≈51% of U(VI) were identified on U LIII edge extended X-ray absorption fine structure spectra. Loss on ignition and thermogravimetric analyses identified from 13% to 44% of NOM in the samples. Carbonyl, phenolic, and carboxylic functional groups in the unreacted samples were identified by fitting of high-resolution X-ray photoelectron spectroscopy (XPS) C 1s and O 1s spectra. Peaks corresponding to phenolic and carbonyl functional groups had intensities higher than those corresponding to carboxylic groups in samples from the supernatant from batch extractions conducted at pH 13, 7, and 2. U(IV) and U(VI) species were detected in the supernatant after batch extractions conducted under oxidizing conditions by fitting of high-resolution XPS U 4f spectra. The outcomes from this study highlight the importance of the influence of pH on the organic functional group chemistry and U speciation in mineralized deposits.

Molecular Hysteresis: Hydrologically Driven Changes in Riverine Dissolved Organic Matter Chemistry During a Storm Event

AbstractHydrological events, driven by rainfall, control the amount and composition of dissolved organic matter (DOM) mobilized through river networks. In forested watersheds, the concentration, composition, and reactivity of DOM exported changes as baseflow transitions to storm flow, with major implications to downstream biogeochemistry. Hysteresis describes an observed difference between in‐stream solute concentration/signal and discharge. By studying the relationship between DOM and stream discharge, we refine our understanding of the environmental and hydrological factors that influence the quantity and quality of stream DOM. The main objective of this study was to track hysteretic changes in riverine DOM molecular composition during storm events. Samples were collected from nested sites within the Passumpsic River catchment (Vermont, USA), a tributary of the Connecticut River. High‐resolution monitoring of fluorescent DOM (via in situ sensors) and automated collection of discrete samples captured short‐term, hydrologically driven variations in DOM concentration and composition. Ultrahigh‐resolution mass spectrometry revealed an enrichment in aliphatic compounds at storm onset, while aromatic and polyphenolic compounds were more enriched at peak discharge. Molecular hysteresis patterns were similar across stream orders, indicating that fresh, terrigenous DOM is quickly shunted downstream, through the river network, during pulses of high discharge.

Land use change alters the radiocarbon age and composition of soil and water-soluble organic matter in the Brazilian Cerrado

The Brazilian Cerrado has undergone extensive land-use change in the past century with large areas converted to agriculture and silviculture. Eucalyptus is the dominant planted tree species in Brazil, but the short rotations (6–7 years) and the associated soil disturbances can reduce soil organic matter (SOM), particularly in subsurface soil over long timeframes. SOM is critical to the continued productivity of forests and may be particularly important in Oxisol soils, which underlie much of Brazil. These soils are low in cation exchange sites and rely upon SOM for nutrient and water retention. However, the mechanisms that drive SOM loss under Eucalyptus remain unclear. This study examines both bulk soil and water-soluble organic matter chemistry deep into the soil profile (1.3 m) in paired Eucalyptus and Cerrado forest stands to identify changes in biodegradability, radiocarbon age, and functional group composition that may explain long-term declines in SOM stocks. We hypothesize that changes in litter quality associated with the conversion of Cerrado forest to Eucalyptus plantations increases the content of aliphatic functional groups in water soluble organic matter (WSOM) more quickly than bulk SOM, which drives subsoil accumulation of aliphatic organic moieties through hydrophobic interactions with soil minerals. By utilizing a suite of spectroscopic techniques (1H nuclear magnetic resonance and Fourier transform infrared spectroscopy) alongside isotopic and incubation techniques, this study found substantial shifts toward aliphatic functional group composition in WSOM in both Eucalyptus stands and deeper soil horizons. The radiocarbon age of WSOM was younger than bulk SOM at every depth but still substantially diverged from modern in B horizons. This observation is consistent with the theory that WSOM is in dynamic equilibrium with mineral adsorbed SOM, and that old organic matter replaced on mineral surfaces by fresh inputs may be leached or further degraded by microbes. Radiocarbon age of bulk SOM was younger under Eucalyptus compared to Cerrado forest and the difference between WSOM and bulk SOM radiocarbon age was much smaller in Eucalyptus soils, which together indicate that microbes may be preferentially consuming older SOM after land-use change. The presence of Cerrado understory in Eucalyptus may be important to maintaining SOM cycling; WSOM biodegradability and microbial biomass was greatly reduced in the Eucalyptus site without any understory. Ecotones with Cerrado understory provide diversity in litter types and rooting strategies, while monoculture Eucalyptus, which is commonly found in stand interiors, may reduce the ability for microbes to maintain active nutrient recycling of organic residues.

Chemistry of Natural Organic Matter-The Next Step: Commentary on a Humic Substances Debate.

Natural organic matter (NOM) is ubiquitous on the Earth surface and influences every biogeochemical reaction wherever it is present, and its impact on reactions varies with its abundance, structure, and chemistry. Because of decades of field and laboratory studies conducted on NOM in different environments, we are moving away from treating it as a black box to developing molecular structure-based approaches in investigations of soil, sediment, and aquatic systems and are beginning to make major advances in the detailed understanding of the molecular and structural characteristics of NOM, which in turn are helping in deciphering the biochemical processes involved in its evolution in the environment. Yet, many questions remain: How does NOM exist in different soil and aquatic environments? How should we obtain NOM from a sample, and does the isolated NOM represent the NOM in a sample? Do the geochemical reactions explored with extracted NOM represent the reality? How can we study NOM in situ? Here I present a synopsis of critical perspectives on the state of NOM research and a commentary on the reviews and debate presented in this special section on the NOM nomenclature, extraction procedures, and studies involving NOM in different environmental processes.

Combined Size and Density Fractionation of Soils for Investigations of Organo-Mineral Interactions.

Combined size and density fractionation (CSDF) is a method used to physically separate soil into fractions differing in particle size and mineralogy. CSDF relies on sequential density separation and sedimentation steps to isolate (1) the free light fraction (uncomplexed organic matter), (2) the occluded light fraction (uncomplexed organic matter trapped in soil aggregates) and (3) a variable number of heavy fractions (soil minerals and their associated organic matter) differing in composition. Provided that the parameters of the CSDF (dispersion energy, density cut-offs, sedimentation time) are properly selected, the method yields heavy fractions of relatively homogeneous mineral composition. Each of these fractions is expected to have a different complexing ability towards organic matter, rendering this a useful method to isolate and study the nature of organo-mineral interactions. Combining density and particle size separation brings an improved resolution compared to simple size or density fractionation methods, allowing the separation of heavy components according to both mineralogy and size (related to surface area) criteria. As is the case for all physical fractionation methods, it may be considered as less disruptive or aggressive than chemically-based extraction methods. However, CSDF is a time-consuming method and furthermore, the quantity of material obtained in some fractions can be limiting for subsequent analysis. Following CSDF, the fractions may be analyzed for mineralogical composition, soil organic carbon concentration and organic matter chemistry. The method provides quantitative information about organic carbon distribution within a soil sample and brings light to the sorptive capacity of the different, naturally-occurring mineral phases, thus providing mechanistic information about the preferential nature of organo-mineral interactions in soils (i.e., which minerals, what type of organic matter).

River channel connectivity shifts metabolite composition and dissolved organic matter chemistry

Biogeochemical processing of dissolved organic matter (DOM) in headwater rivers regulates aquatic food web dynamics, water quality, and carbon storage. Although headwater rivers are critical sources of energy to downstream ecosystems, underlying mechanisms structuring DOM composition and reactivity are not well quantified. By pairing mass spectrometry and fluorescence spectroscopy, here we show that hydrology and river geomorphology interactively shape molecular patterns in DOM composition. River segments with a single channel flowing across the valley bottom export DOM with a similar chemical profile through time. In contrast, segments with multiple channels of flow store large volumes of water during peak flows, which they release downstream throughout the summer. As flows subside, losses of lateral floodplain connectivity significantly increase the heterogeneity of DOM exported downstream. By linking geomorphologic landscape-scale processes with microbial metabolism, we show DOM heterogeneity increases as a function of fluvial complexity, with implications for ecosystem function and watershed management.