Global fjords as transitory reservoirs of labile organic carbon modulated by organo-mineral interactions.

The global carbon cycle is strongly modulated by organic carbon (OC) sequestration and decomposition. Whereas OC sequestration is relatively well constrained, there are few quantitative estimates of its susceptibility to decomposition. Fjords are hot spots of sedimentation and OC sequestration in marine sediments. Here, we adopt fjords as model systems to investigate the reactivity of sedimentary OC by assessing the distribution of the activation energy required to break OC bonds. Our results reveal that OC in fjord sediments is more thermally labile than that in global sediments, which is governed by its unique provenance and organo-mineral interactions. We estimate that 61 ± 16% of the sedimentary OC in fjords is degradable. Once this OC is remobilized and remineralized during glacial maxima, the resulting metabolic CO2 could counterbalance up to 50 ppm of the atmospheric CO2 decrease during glacial times, making fjords critical actors in dampening glacial-interglacial climate fluctuations through negative carbon cycling loops.

Quantification of geogenic carbon in anthropogenic alluvial coal soils of the Susquehanna River.

Alluvial riparian soils act as a filtration system, improving the environmental quality of downstream soils and waters. In areas affected by coal mining, alluvial soils also serve as a modern "sink" of fossil carbon (C). To date, little research has been done on ecosystem services provided by alluvial landscapes (i.e., river islands and tributary deltas) in the retention of coal in coal-mining regions. The objective of this study was to distinguish between and quantify geogenic and neogenetic C in alluvial soils of the North Branch of the Susquehanna River (NBSR). To investigate this, we compared five thermal analysis methods to quantify geogenic (coal) C in soils. Our results indicate that multivariate curve resolution of ramped thermal combustion data provided the most accurate estimate of coal content in soils. Our analysis found that NBSR alluvial soils have accumulated ∼375 Gg of anthropogenic, geogenic C (upper 1 m). In these soils, an average of ∼11% of soil mass is attributable to coal, yet ∼73% of the total soil C is attributable to geogenic C. These soil organic C stocks are substantially greater than locally mapped riparian soils unaffected by coal mining and are greater than regional organic soils (Histosols). Quantification of microbial decomposition of coal in alluvial soils and vulnerability to extreme flood events (potential remobilization) requires further investigation and will be important in determining the fate of this C sink.

Assessment of artificial neural network to identify compositional differences in ultrahigh-resolution mass spectra acquired from coal mine affected soils.

This study assessed the applicability of artificial neural networks (ANNs) as a tool to identify compounds contributing to compositional differences in coal-contaminated soils. An artificial neural network model was constructed from laser desorption ionization ultrahigh-resolution mass spectra obtained from coal contaminated soils. A good correlation (R2 = 1.00 for model and R2 = 0.99 for test) was observed between the measured and predicted values, thus validating the constructed model. To identify chemicals contributing to the coal contents of the soils, the weight values of the constructed model were evaluated. Condensed hydrocarbon and low oxygen containing compounds were found to have larger weight values and hence they were the main contributors to the coal contents of soils. In contrast, compounds identified as lignin did not contribute to the coal contents of soils. These findings were consistent with the conventional knowledge on coal and results from the conventional partial least square method. Therefore, we concluded that the weight interpretation following ANN analysis presented herein can be used to identify compounds that contribute to the compositional differences of natural organic matter (NOM) samples.

Ecosystem services-based soil quality index tailored to the metropolitan environment for soil assessment and management.

The soils in urban greenery provide essential ecosystem services. However, only a few studies have assessed urban soil quality based on a comprehensive view of ecosystem services and soil multi-functionality. In this study, we suggest an urban soil quality index (uSQI) to evaluate soil status in various spatial types of urban greenery. Our objectives are 1) to develop an uSQI incorporating a range of urban soil ecosystem services in metropolitan environments and 2) to test the efficacy of the developed uSQI by applying it to nine different sites. To fully consider ecosystem services provided by the urban soil, a DPSC (drivers and pressures, state, and changes) framework was constructed. Drivers and pressures are influencing factors that continuously alter the state of the urban greenery, eventually leading to changes in ecosystem services and soil functions. The six soil functions considered were physical stability and support, water storage and infiltration, habitat provision, organic matter stabilization, nutrient supply and retention, and pollutant immobilization and decomposition. These functions were measured using ten soil indicators which can be quantified: bulk density, saturated hydraulic conductivity, litter-layer depth, mineral-associated organic matter, clay+silt content, fluorescein diacetate hydrolytic activity, cation exchange capacity, inorganic nitrogen concentration, pH, and concentrations of potentially toxic elements. The uSQI was calculated as the arithmetic mean of the scores of the six soil functions, obtained through the fuzzy logic functions. The uSQI successfully identified the low soil quality sites among nine urban greeneries with different spatial types (point, line, and polygon). In addition, we could examine the degraded soil function of each site and suggest a management guideline using our uSQI. Our novel index can help urban stakeholders evaluate and monitor the soil quality of urban greenery.

Beyond bulk: Density fractions explain heterogeneity in global soil carbon abundance and persistence.

Understanding the controls on the amount and persistence of soil organic carbon (C) is essential for predicting its sensitivity to global change. The response may depend on whether C is unprotected, isolated within aggregates, or protected from decomposition by mineral associations. Here, we present a global synthesis of the relative influence of environmental factors on soil organic C partitioning among pools, abundance in each pool (mg C g-1  soil), and persistence (as approximated by radiocarbon abundance) in relatively unprotected particulate and protected mineral-bound pools. We show that C within particulate and mineral-associated pools consistently differed from one another in degree of persistence and relationship to environmental factors. Soil depth was the best predictor of C abundance and persistence, though it accounted for more variance in persistence. Persistence of all C pools decreased with increasing mean annual temperature (MAT) throughout the soil profile, whereas persistence increased with increasing wetness index (MAP/PET) in subsurface soils (30-176 cm). The relationship of C abundance (mg C g-1  soil) to climate varied among pools and with depth. Mineral-associated C in surface soils (<30 cm) increased more strongly with increasing wetness index than the free particulate C, but both pools showed attenuated responses to the wetness index at depth. Overall, these relationships suggest a strong influence of climate on soil C properties, and a potential loss of soil C from protected pools in areas with decreasing wetness. Relative persistence and abundance of C pools varied significantly among land cover types and soil parent material lithologies. This variability in each pool's relationship to environmental factors suggests that not all soil organic C is equally vulnerable to global change. Therefore, projections of future soil organic C based on patterns and responses of bulk soil organic C may be misleading.

Ecological and Genomic Attributes of Novel Bacterial Taxa That Thrive in Subsurface Soil Horizons.

While most bacterial and archaeal taxa living in surface soils remain undescribed, this problem is exacerbated in deeper soils, owing to the unique oligotrophic conditions found in the subsurface. Additionally, previous studies of soil microbiomes have focused almost exclusively on surface soils, even though the microbes living in deeper soils also play critical roles in a wide range of biogeochemical processes. We examined soils collected from 20 distinct profiles across the United States to characterize the bacterial and archaeal communities that live in subsurface soils and to determine whether there are consistent changes in soil microbial communities with depth across a wide range of soil and environmental conditions. We found that bacterial and archaeal diversity generally decreased with depth, as did the degree of similarity of microbial communities to those found in surface horizons. We observed five phyla that consistently increased in relative abundance with depth across our soil profiles: Chloroflexi, Nitrospirae, Euryarchaeota, and candidate phyla GAL15 and Dormibacteraeota (formerly AD3). Leveraging the unusually high abundance of Dormibacteraeota at depth, we assembled genomes representative of this candidate phylum and identified traits that are likely to be beneficial in low-nutrient environments, including the synthesis and storage of carbohydrates, the potential to use carbon monoxide (CO) as a supplemental energy source, and the ability to form spores. Together these attributes likely allow members of the candidate phylum Dormibacteraeota to flourish in deeper soils and provide insight into the survival and growth strategies employed by the microbes that thrive in oligotrophic soil environments.IMPORTANCE Soil profiles are rarely homogeneous. Resource availability and microbial abundances typically decrease with soil depth, but microbes found in deeper horizons are still important components of terrestrial ecosystems. By studying 20 soil profiles across the United States, we documented consistent changes in soil bacterial and archaeal communities with depth. Deeper soils harbored communities distinct from those of the more commonly studied surface horizons. Most notably, we found that the candidate phylum Dormibacteraeota (formerly AD3) was often dominant in subsurface soils, and we used genomes from uncultivated members of this group to identify why these taxa are able to thrive in such resource-limited environments. Simply digging deeper into soil can reveal a surprising number of novel microbes with unique adaptations to oligotrophic subsurface conditions.

Analyzing Solid-Phase Natural Organic Matter Using Laser Desorption Ionization Ultrahigh Resolution Mass Spectrometry.

Extensive sample preparation procedures are required to analyze natural organic matter (NOM) in soil and sediment samples due to the mineral matrix. The preparation procedure not only requires a large amount of sample (typically more than 50 mg), but NOM extraction is frequently incomplete. In this study, 2-5 µg of solid NOM or 500 µg of unprocessed soil samples were fixed on a metal plate using double-sided adhesive tape and analyzed directly using laser desorption ionization (LDI) and ultrahigh resolution mass spectrometry (UHR-MS). Most of the peaks reported in previous LDI UHR-MS studies using NOM solutions were observed, and an additional ∼2200 unique peaks were found by analyzing the fulvic acids direct solid phase. Differences in the molecular composition of NOM in solid samples were seen clearly with minimum sample preparation. Lignin- and tannin-type molecules were detected in both Elliott soil and topsoil from Kyungpook National University campus. The data presented in this study demonstrate a proof-of-principle that highly sensitive, direct, molecular level analysis of solid-phase NOM from unprocessed soil samples and minimum sample preparation is possible.

Adsorption and Molecular Fractionation of Dissolved Organic Matter on Iron-Bearing Mineral Matrices of Varying Crystallinity.

Iron (Fe)-bearing mineral phases contribute disproportionately to adsorption of soil organic matter (SOM) due to their elevated chemical reactivity and specific surface area (SSA). However, the spectrum of Fe solid-phase speciation present in oxidation-reduction-active soils challenges analysis of SOM-mineral interactions and may induce differential molecular fractionation of dissolved organic matter (DOM). This work used paired selective dissolution experiments and batch sorption of postextraction residues to (1) quantify the contributions of Fe-bearing minerals of varying crystallinity to DOM sorption, and (2) characterize molecular fractionation using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). A substantial proportion of soil SSA was derived from extracted Fe-bearing phases, and FT-ICR-MS analysis of extracted DOM revealed distinct chemical signatures across Fe-OM associations. Sorbed carbon (C) was highly correlated with Fe concentrations, suggesting that Fe-bearing phases are strong drivers of sorption in these soils. Molecular fractionation was observed across treatments, particularly those dominated by short-range-order (SRO) mineral phases, which preferentially adsorbed aromatic and lignin-like formulas, and higher-crystallinity phases, associated with aliphatic DOM. These findings suggest Fe speciation-mediated complexation acts as a physicochemical filter of DOM moving through the critical zone, an important observation as predicted changes in precipitation may dynamically alter Fe crystallinity and C stability.

Elemental Concentrations in Urban Green Stormwater Infrastructure Soils.

Green stormwater infrastructure (GSI) is designed to capture stormwater for infiltration, detention, evapotranspiration, or reuse. Soils play a key role in stormwater interception at these facilities. It is important to assess whether contamination is occurring in GSI soils because urban stormwater drainage areas often accumulate elements of concern. Soil contamination could affect hydrologic and ecosystem functions. Maintenance workers and the public may also be exposed to GSI soils. We investigated soil elemental concentrations, categorized as macro- and micronutrients, heavy metals, and other elements, at 59 GSI sites in the city of Philadelphia. Non-GSI soil samples 3 to 5 m upland of GSI sites were used for comparison. We evaluated differences in elemental composition in GSI and non-GSI soils; the comparisons were corrected for the age of GSI facility, underlying soil type, street drainage, and surrounding land use. Concentrations of Ca and I were greater than background levels at GSI sites. Although GSI facilities appear to accumulate Ca and I, these elements do not pose a significant human health risk. Elements of concern to human health, including Cd, Hg, and Pb, were either no different or were lower in GSI soils compared with non-GSI soils. However, mean values found across GSI sites were up to four times greater than soil cleanup objectives for residential use.

Soil and ecosystem respiration responses to grazing, watering and experimental warming chamber treatments across topographical gradients in northern Mongolia.

Globally, soil respiration is one of the largest fluxes of carbon to the atmosphere and is known to be sensitive to climate change, representing a potential positive feedback. We conducted a number of field experiments to study independent and combined impacts of topography, watering, grazing and climate manipulations on bare soil and vegetated soil (i.e., ecosystem) respiration in northern Mongolia, an area known to be highly vulnerable to climate change and overgrazing. Our results indicated that soil moisture is the most important driving factor for carbon fluxes in this semi-arid ecosystem, based on smaller carbon fluxes under drier conditions. Warmer conditions did not result in increased respiration. Although the system has local topographical gradients in terms of nutrient, moisture availability and plant species, soil respiration responses to OTC treatments were similar on the upper and lower slopes, implying that local heterogeneity may not be important for scaling up the results. In contrast, ecosystem respiration responses to OTCs differed between the upper and the lower slopes, implying that the response of vegetation to climate change may override microbial responses. Our results also showed that light grazing may actually enhance soil respiration while decreasing ecosystem respiration, and grazing impact may not depend on climate change. Overall, our results indicate that soil and ecosystem respiration in this semi-arid steppe are more sensitive to precipitation fluctuation and grazing pressure than to temperature change.

Improved characterization of soil organic matter by thermal analysis using CO2/H2O evolved gas analysis.

Simultaneous thermal analysis [i.e., thermogravimetry (TG) and differential scanning calorimetry (DSC)] is frequently used in materials science applications and is increasingly being used to study soil organic matter (SOM) stability. Yet, important questions remain, especially with respect to how the soil mineral matrix affects TG-DSC results, which could confound the interpretation of relationships between thermal and biogeochemical SOM stability. The objective of this study was to explore the viability of using infrared gas analyzer (IRGA) based CO(2)/H(2)O evolved gas analysis (EGA) as a supplement or alternative to TG-DSC to improve the characterization of SOM. Here, we subjected reference samples and a set of 28 diverse soil samples from across the U.S. to TG-DSC coupled with IRGA-based EGA. The results showed the technical validity of coupling TG-DSC and CO(2)-EGA, with more than 80% of the theoretically evolved CO(2)-C recovered during pure cellulose and CaCO(3) analysis. CO(2)-EGA and DSC thermal profiles were highly similar, with correlation coefficients generally >0.90. Additionally, CO(2)/H(2)O-EGA proved useful to improve the accuracy of baseline correction, detect the presence of CaCO(3) in soils, and identify SOM oxidative reactions normally hidden in DSC analysis by simultaneous endothermic reactions of soil minerals. Overall, this study demonstrated that IRGA-based CO(2)/H(2)O-EGA constitutes a valuable complement to conventional TG-DSC analysis for SOM characterization.

Use of thermal analysis techniques (TG-DSC) for the characterization of diverse organic municipal waste streams to predict biological stability prior to land application.

The use of organic municipal wastes as soil amendments is an increasing practice that can divert significant amounts of waste from landfill, and provides a potential source of nutrients and organic matter to ameliorate degraded soils. Due to the high heterogeneity of organic municipal waste streams, it is difficult to rapidly and cost-effectively establish their suitability as soil amendments using a single method. Thermal analysis has been proposed as an evolving technique to assess the stability and composition of the organic matter present in these wastes. In this study, three different organic municipal waste streams (i.e., a municipal waste compost (MC), a composted sewage sludge (CS) and a thermally dried sewage sludge (TS)) were characterized using conventional and thermal methods. The conventional methods used to test organic matter stability included laboratory incubation with measurement of respired C, and spectroscopic methods to characterize chemical composition. Carbon mineralization was measured during a 90-day incubation, and samples before and after incubation were analyzed by chemical (elemental analysis) and spectroscopic (infrared and nuclear magnetic resonance) methods. Results were compared with those obtained by thermogravimetry (TG) and differential scanning calorimetry (DSC) techniques. Total amounts of CO(2) respired indicated that the organic matter in the TS was the least stable, while that in the CS was the most stable. This was confirmed by changes detected with the spectroscopic methods in the composition of the organic wastes due to C mineralization. Differences were especially pronounced for TS, which showed a remarkable loss of aliphatic and proteinaceous compounds during the incubation process. TG, and especially DSC analysis, clearly reflected these differences between the three organic wastes before and after the incubation. Furthermore, the calculated energy density, which represents the energy available per unit of organic matter, showed a strong correlation with cumulative respiration. Results obtained support the hypothesis of a potential link between the thermal and biological stability of the studied organic materials, and consequently the ability of thermal analysis to characterize the maturity of municipal organic wastes and composts.

Legumes mitigate ecological consequences of a topographic gradient in a northern Mongolian steppe.

Topography should create spatial variation in water and nutrients and play an especially important role in the ecology of water-limited systems. We use stable isotopes to discern how plants respond both to ecological gradients associated with elevation and to neighboring legumes on a south-facing slope in the semi-arid, historically grazed steppe of northern Mongolia. Out of three target species, Potentilla acaulis, Potentilla sericea, and Festuca lenensis, when >30 cm from a legume, all showed a decrease in leaf δ(15)N with increasing elevation. This, together with measures of soil δ(15)N, suggests greater N processing at the moister, more productive, lower elevation, and more N fixation at the upper elevation, where cover of legumes and lichens and plant-available nitrate were greater. Total soil N was greater at the lower elevation, but not lichen biomass or root colonization by AMF. Leaf δ(13)C values for P. acaulis and F. lenensis are consistent with increasing water stress with elevation; δ(13)C values indicated the greatest intrinsic water use efficiency for P. sericea, which is more abundant at the upper elevation. Nearby legumes (<10 cm) moderate the effect of elevation on leaf δ(15)N, confirming legumes' meaningful input of N, and affect leaf δ(13)C for two species, suggesting an influence on the efficiency of carbon fixation. Variation in leaf %N and %C as a function of elevation and proximity to a legume differs among species. Apparently, most N input is at upper elevations, pointing to the possible importance of grazers, in addition to hydrological processes, as transporters of N throughout this landscape.

Experimental warming shows that decomposition temperature sensitivity increases with soil organic matter recalcitrance.

Soil C decomposition is sensitive to changes in temperature, and even small increases in temperature may prompt large releases of C from soils. But much of what we know about soil C responses to global change is based on short-term incubation data and model output that implicitly assumes soil C pools are composed of organic matter fractions with uniform temperature sensitivities. In contrast, kinetic theory based on chemical reactions suggests that older, more-resistant C fractions may be more temperature sensitive. Recent research on the subject is inconclusive, indicating that the temperature sensitivity of labile soil organic matter (OM) decomposition could either be greater than, less than, or equivalent to that of resistant soil OM. We incubated soils at constant temperature to deplete them of labile soil OM and then successively assessed the CO2-C efflux in response to warming. We found that the decomposition response to experimental warming early during soil incubation (when more labile C remained) was less than that later when labile C was depleted. These results suggest that the temperature sensitivity of resistant soil OM pools is greater than that for labile soil OM and that global change-driven soil C losses may be greater than previously estimated.