Formation Of Organic Matter Research Articles

Using the triple proxy δ13C–radiocarbon–major and trace elements to understand stalagmite stable carbon composition in Madagascar

Stalagmite δ13Cc is one of the most complicated and difficult proxies to interpret in paleoclimate and paleoenvironmental studies because of its sensitivity to various local, regional, and global factors. The significance of the large δ13Cc shift (~12‰, vs. VPDB) in several stalagmites from Anjohibe Cave, in northwestern Madagascar, in the late Holocene remains a subject of scientific debate. Although it was assumed to reflect C3 to C4 vegetation cover change caused by anthropogenic activities, recent investigations inside the cave, at different locations, revealed a wide δ13Cc range of ~10‰ (vs. VPDB), a value that is comparable in its significance to the 12 ‰ temporal variability mentioned above.In this paper we combine δ13C, radiocarbon (14C), and major and trace elements (MTE) as a proxy to provide additional insights about the non-vegetation drivers of δ13Cc in Madagascar. Our study uses two models, the CaveCalc model and the Fohlmeister model, to help our interpretation of the drip water evolution while it percolates down to the cave and precipitates CaCO3. The radiocarbon results, with values >100 pMC, demonstrate that the bomb peak, i.e., the pronounced increase in atmospheric 14C signals, are clearly recorded in these modern stalagmites. This suggests that the samples are younger than 1950 CE, and the soil carbon cycling has been fast without significant formation of aged soil organic matter. These radiocarbon results further suggests that stalagmites from Anjohibe Cave might be of use for future efforts to reconstruct past atmospheric 14C concentrations. Drip water geochemistry, δ13Cc and MTE, combined with two model simulations (CaveCalc and Fohlmeister model) suggests that the great δ13Cc range is best explained by the combined effects of the DIC residence time in the epikarst that affect the water-rock interaction and the rate of prior carbonate precipitation. This study therefore demonstrated how combined modelling and multi-proxy approaches can advance our understanding of the control mechanisms of δ13Cc for paleoclimate research.

Heterogeneous characteristics and controlling factors of oil bearing of the Chang 7 shale in the Ordos Basin

Oil-bearing evaluation is the key issue of shale oil geologic exploration, which affects the mobility and effective development of the remaining oil in the shale system. To study the oil content and its controlling factors of shale in Chang 7 member of the Yanchang Formation in the Ordos Basin, we take the core of well H317 as the main research object, analyze the oil-bearing characteristics of shale by conventional rock pyrolysis and multistep pyrolysis experimental methods, and analyze the change characteristics of the elements by handheld X-ray fluorescence. The results indicate that the organic heterogeneity of Chang 7 shale in well H317 is obvious. It can be divided into upper shale and lower shale with 2467 m as the boundary. The upper shale has low total organic carbon (TOC), poor kerogen type, and slightly low oil content, but strong mobility. The lower shale has high TOC, good kerogen type, and good oil content, but poor mobility. The content and type of organic carbon in shale have obvious control effect on oil bearing and mobility. Differences in paleosedimentary environment control the formation of original sedimentary organic matter. During the lower shale deposition period, the climate was humid and the terrigenous input was relatively weak, high paleoproductivity and strong reducing water effectively preserved the original sedimentary organic matter, and finally formed an organic-rich interval with high TOC. Early diagenesis further altered the abundance and type of original organic matter.

On the provenance of GEMS, a quarter century post discovery

The provenance of GEMS (glass with embedded metal and sulfides) in cometary type interplanetary dust particles is investigated using analytical scanning transmission electron microscopy and secondary ion mass spectrometry. We review the current state of knowledge and closely examine the densities, elemental compositions and distributions, iron oxidation states, and isotopic compositions of a subset of GEMS in chondritic porous interplanetary dust. We find that GEMS are underdense with estimated densities that are 35–65% of compositionally equivalent crystalline aggregates. GEMS low densities result in a lower contribution to the bulk compositions of IDPs than has been assumed based on their volume fraction. We also find that element/Si ratios, assumed to be primary (indigenous), are instead perturbed by contamination and secondary alteration, including pulse heating during atmospheric entry. Fe in pyrrhotite inclusions was oxidized and Mg, S, Ca, and Fe were depleted relative to lithophile Al and Si, resulting in reduction in element/Si ratios. Because they trap outgassing elements, Fe-rich oxide rims that formed on the surface of GEMS are serendipitous “witness plates” to the changes in composition that accompany atmospheric entry. As a result of alteration, GEMS elemental compositions cannot reliably inform about their provenance. Except for highly anomalous oxygen isotope ratios measured in some large GEMS grains that indicate a contribution from circumstellar dust, oxygen isotope compositions are generally poor indicators of provenance. Prior work indicates that most GEMS fall close to the terrestrial oxygen isotope composition, which, however, does not exclude a presolar interstellar origin. Nitrogen isotopic compositions are more diagnostic. Elevated 15N/14N ratios indicate that GEMS accreted in conjunction with formation of organic matter by ion–molecule reactions in a cold (<50 K) presolar environment like the extreme outer nebula or interstellar medium. Considering all observations, we conclude that GEMS are most likely processed interstellar silicates.

Sedge replacement by grasses accelerates litter decomposition and decreases organic matter formation in alpine meadow soils

AbstractThe dominance of sedges in Tibetan meadows is decreasing with the increasing abundance of grasses, mainly due to anthropogenic activities, nitrogen deposition, and climate warming. We investigated the effects of this vegetation shift on litter decomposition (in the field), mineralization to CO2, and litter carbon (C) incorporation into microorganisms (under laboratory conditions) under sedges and grasses at community and species levels. Litter C incorporation into soil organic carbon (SOC) was estimated using 13C labelling of the vegetation community. After one year of field incubation, a smaller proportion of shoot litter C was lost in sedges than in grasses at the species level and smaller proportions of root C were lost under sedges at community and species levels. Roots decomposed markedly slower than shoots across communities or species under sedges and grasses. Smaller proportions of the shoot and root C from sedges were incorporated into SOC than from grasses, and root C was less incorporated into the SOC than shoot C across communities. Over the 38‐day laboratory incubation, total microbial respiration and assimilation of litter C changed similarly to the C loss between litter types under the field condition. Across community and species levels, litter C loss in the field and incorporation of litter C into microorganisms under laboratory conditions all increased with initial phosphorus content in the litter and decreased with lignin content. Considering greater allocation of photosynthates belowground by sedges than grasses, we demonstrated that the ongoing replacement of sedges by grasses in the vast Tibetan alpine meadows is unfavorable for the maintenance of SOC storage.

Litterfall and Associated Macrozoobenthic of Restored Mangrove Forests in Abandoned Aquaculture Ponds

Mangrove restoration projects are now widely established, aiming to regain the carbon benefit of the mangrove ecosystem that is essential for climate change mitigation. This study aimed to investigate mangrove litter as the source of carbon in restored mangrove forests in Perancak Estuary, Bali, Indonesia, which previously experienced substantial mangrove loss due to shrimp aquaculture development. We assessed the production and decomposition of mangrove litter and associated macrozoobenthic biodiversity in restored forests with plantation age ≥14 years and intact mangrove forests as the reference. The monthly production of three groups of mangrove litter (leaf, reproductive, and wood) was assessed over 12 months. A leaf litter decomposition experiment was performed to inspect the interspecific and disturbance history variation in organic matter formation among four major mangrove species: Rhizophora apiculata, Bruguiera gymnorhiza, Avicennia marina, and Sonneratia alba. Our results showed that annual litterfall production from restored and intact mangroves in Perancak Estuary were 13.96 and 10.18 Mg ha−1 year−1, which is equivalent to approximately 6282 and 4581 kg C ha−1 year−1 of annual litterfall carbon sink, respectively. Although restored mangroves had significantly higher plant litterfall production than intact mangroves, no significant difference was detected in leaf litter decomposition and macrozoobenthic biodiversity between these forest types.

Quantitative analysis of paleoenvironment of Qingshankou Formation in northern Songliao Basin, Northeastern China

The Qingshankou Formation in the Songliao Basin of northeastern China is an important layer for unconventional oil exploration. To solve the problem of the main controlling factors of shale oil enrichment, to quantitatively analyze the paleosalinity, paleoclimate, paleoredox environment, provenance, and lake productivity, and to clarify the relationship between sedimentary environment and total organic carbon, we have analyzed quantitative reconstruction of the sedimentary environment by using comprehensive analysis of multiple indicators with major and trace elements, fossils, and biomarkers. The results are as follows: the paleosalinity index w(Sr)/ w(Ba) ratio is 0.4–4, the w(B)/ w(Ga) value is 0.7–8.5, and the w(Pr)/ w(Ph) value is 0.8–2.8, which mainly stand for fresh brackish to saline water. The paleoclimate CIW′ (Chemical index of weathering, CIW = [A12O3/(AI203 + CaO+Na20)] × 100 [molecular proportions]) value is 90–100, indicating a warm and humid climate. The redox index w(V)/ w((V + Ni)) value is 0.54–0.89, which mainly belongs to the dysoxic environment. The provenance input of terrigenous debris index w(Ti)/ w(Al) value is 0.026–0.034, mainly standing for low terrigenous debris input. The production index CAL (enrichment degree of Cu) value is 1–3, belonging to high production. We conclude that the humid climate, the fresh brackish to saline and dysoxic water, and the high lake production provide favorable geologic conditions for biological reproduction and then provide a foundation for the formation, enrichment, and preservation of organic matter. The quantitative reconstruction of the paleoenvironment of the Qingshankou Formation has theoretical significance for Gulong shale oil exploration and development.

The Volcanic Impacts on the Formation of Organic-Rich Shales From the Freshwater to Saline Lakes: Cases Study in the Ordos and the Junggar Basins

High organic matter abundance is necessary for large-scale hydrocarbon accumulation and enrichment in shale oil. Comparing conventional marine basins with lacustrine ones, the formation of organic matter in the latter usually changes rapidly with strong heterogeneity, which brings great challenges to identify sweet spot prediction and resource evaluation. In addition, water columns with different redox conditions also affect the burial and preservation of organic matter during the deposition of lacustrine shales, especially under the impact of ephemeral critical geological events such as volcanic activities. Therefore, determining the properties and the influencing factors on the water columns is one of the key scientific issues in revealing the differential enrichment of organic matter in such basins. By comparing the petrological and geochemical characteristics of organic-rich shales between a typical freshwater and a saline lacustrine basin, this study analyzes the depositional environment and water column properties during the burial and preservation of organic matter in the Ordos and Junggar basins. The results demonstrate that volcanic activity intensifies the degree of hypoxia during the formation of organic-rich matter, which in turn affects organic matter preservation. The sulfate reduction index (SRI) indicates that the organic-rich shale of the Chang seven Member (Ch7) of Yanchang Formation in the Ordos Basin and the Lucaogou Formation (P2l) in the Jimsar Sag of Junggar Basin in freshwater-saline environments have a certain intensity of sulfate reduction (BSR) (SRI &amp;lt;1.375). The organic matter consumed by BSR is lower than the preserved organic matter, resulting a higher TOC content. However, excessive volcanic activity or the input of hydrothermal fluids caused strong BSR (SRI &amp;gt;1.375) of the Fengcheng Formation (P2f) in saline lake of the Mahu Sag in Junggar Basin. This indicates that a large amount of organic matter was degraded and consumed, causing more iron oxides (and other iron-containing minerals) dissolve gradually to form pyrite, reducing the TOC content. This study enabled us to understand the shale oil enrichment, sweet spot prediction and evaluation, and propose a better theoretical and practical model for hydrocarbon accumulation in shale formations, to guide future exploration efforts.

Dissolved organic matter derived from aquatic plants in constructed wetlands: Characteristics and disinfection byproducts formation

Constructed wetlands are used to improve water quality in the drinking water sources, but aquatic plants planted in constructed wetlands may be a source of disinfection by-product (DBP) precursors. This study explored the characteristics of aquatic plant-derived dissolved organic matter (DOM) and DBP formation in micro-polluted raw water treated by constructed wetlands. A simulated surface flow wetland (SSFW) planted with the typical aquatic plants Lythrum salicaria and Acorus calamus was studied. Raw water, control samples, effluent, and Suwannee River natural organic matter (SRNOM) were analyzed. via ultraviolet-visible (UV-Vis) spectroscopy, three-dimensional excitation emission matrix (3D-EEM) fluorescence combined with a parallel factor (PARAFAC) analysis, liquid chromatography-organic carbon detection (LC-OCD), principal component analysis (PCA), and other detection and analysis methods. A DBP formation potential (DBPFP) index was also constructed. Treatment by the SSFW increased the effluent dissolved organic carbon (DOC), UV254, trihalomethane formation potential (THMFP), haloacetic acid formation potential (HAAFP) by 72.2%, 171.4%, 83.7% and 88.6%, respectively. Planting aquatic plants in surface wetlands will negatively affect the water quality of micro-polluted raw water, with the plants being an important source of increased DBP precursors in wetland effluent. The effluent properties were similar to those of SRNOM, and had a higher degree of aromatization and larger molecular weight than raw water. The increased DBP precursors were mainly fulic acid and humus with higher aromatic and molecular weight, which derived from the growth and metabolism of aquatic plants. Humic substances (HS) accounted for the highest proportion of DOM in this study, with the average molecular weight of HS increasing from 648 to 698 g·mol−1 as the DOM content increased. Results from this study are particularly applicable to the establishment of constructed wetlands planted with aquatic plants in the drinking water sources, and the DOM derived from aquatic plants in constructed wetlands may be important precursors in the subsequent water disinfection process.

Technosol rehabilitation strategies drive soil physico-chemical properties and fauna diversity on a former coking plant area

Anthropic activities such as mining, resource extraction or transformation profoundly modify ecosystems and may lead to Technosol formation. Post-industrial areas are examples of potentially degraded lands, due to soil contamination with metals or organic pollutants, as well as soil structure damage. Soil biodiversity being closely involved in many soil functions such as organic matter decomposition, formation and stabilization of soil structure, the recovery of degraded soil functions partly depends on soil fauna colonization. However, the relationship between Technosol abiotic parameters and soil fauna colonization is still to be disentangled. In an attempt to fill this gap, we studied a former coking plant area in north-eastern France, composed of Technosols resulting from coking plant embankments or thermally-treated industrial soils and compared them with two local soils considered as references. We hypothesized that the taxonomic and trophic diversity of Technosol-dwelling invertebrates would be more diverse and abundant in the soils with the higher soil physico-chemical quality (i.e higher fertility and lower pollutions levels). To test this hypothesis, we studied four Technosols that were settled following two different rehabilitation strategies within the same post-industrial area and we compared them with two local soils of reference using nested ANOVAs as well as multivariate analyses of soil abiotic parameters and soil fauna community indices, both within the soil and on its surface.The results on physico-chemical analyses showed that the substrates used for Technosol rehabilitation were impoverished in clay content when compared with local soils of reference (4.1 to 7.8-fold) and enriched in sandy materials. The use of coking plant by-products for Technosol implementation have led to poor chemical quality, with low nutrient content but high organic carbon content (41 to 51%) resulting from the use of coal and correlated with the higher lead concentration in the area. The use of thermally-treated industrial soil has led to more fertile Technosols with significantly lower lead content. Meso and macrofauna densities did not vary significantly between the Technosols and the local reference soils. Both Technosol-dwelling fauna trophic and taxonomic community compositions were impacted by the rehabilitation strategy. Few to no earthworms were found in Technosols (mean up to 16 ind.m−2) compared to the local soils of reference (mean of 118.4 and 201.6 ind.m−2). Conversely, Technosols resulting from coking plant embankments and thermally-treated industrial soils were dominated by epigeic soil fauna with an intense activity of soil surface macro-saprophages and micro-arthropods, as well as Formicidae. Our results suggest that the use of technogenic elements in the rehabilitation of post-industrial area led to the development of specific soil invertebrate communities, different from the reference. The gap between the high level of activity of epigeic organisms that we found on Technosols and the low trophic diversity of the litter and soil-dwelling communities suggest that the soil fauna community on a former coking plant is driven rather by soil physico-chemical properties than by colonization constraints.

Effects of Microbial-Mineral Interactions on Organic Carbon Stabilization in a Ponderosa Pine Root Zone: A Micro-Scale Approach

Soil microbial communities affect the formation of micro-scale mineral-associated organic matter (MAOM) where complex processes, including adhesion, aggregate formation, microbial mineral weathering and soil organic matter stabilization occur in a narrow zone of large biogeochemical gradients. Here we designed a field study to examine carbon stabilization mechanisms by using in-growth mesh bags containing biotite that were placed in a ponderosa pine root zone for 6 months and compared to the surrounding bulk soil. We sought to determine the composition of the microbial community in the mesh bags compared to the surrounding soils, analyze the direct interactions between microbes and biotite, and finally identify the nature of the newly formed MAOM within the mesh-bags. Our results revealed that minerals in the mesh bags were colonized by a microbial community that produced organic matter in situ. The 16S rRNA gene sequencing and ITS2 region characterization showed phylogenetic similarity between the mesh bag and bulk soil archaea/bacteria and fungi microbiomes, with significant differences in alpha- and beta-diversity and species abundances. Organic matter pools in the mesh bags, analyzed by Fourier transform ion cyclotron resonance mass spectrometry, contained protein- (peptides) and lipid-like compounds while the bulk soil OM was comprised of lignin-like and carboxyl-rich alicyclic molecules. These results support that the newly formed biotite associated organic compounds have a microbial signature in the mesh bags. High-resolution electron microscopy documented strongly adhered organic compounds to biotite surfaces, formation of microaggregates, elemental uptake at the microbe (organic matter)-mineral interface, and distortion of biotite layers. Overall, this study shows the direct and indirect involvement of soil microbial communities from the root zone of ponderosa pine in the formation of MAOM, soil organic carbon stabilization, microaggregation, and mineral weathering at micro- and nano-scales.

Long-Term Compost Amendment Spurs Cellulose Decomposition by Driving Shifts in Fungal Community Composition and Promoting Fungal Diversity and Phylogenetic Relatedness.

ABSTRACTCellulose is the most abundant polysaccharide in plant biomass and an important precursor of soil organic matter formation. Fungi play a key role in carbon cycling dynamics because they tend to decompose recalcitrant materials. Here, we applied [12C]cellulose and [13C]cellulose to distinguish the effects of application of compost, nitrogen-phosphorus-potassium (NPK) fertilizer, and no fertilizer (control) for 27 years upon cellulose decomposition via RNA-based stable isotope probing (RNA-SIP). The loss ratio of added cellulose C in compost soil was 67.6 to 106.7% higher than in NPK and control soils during their 20-day incubation. Dothideomycetes (mainly members of the genus Cryptococcus) dominated cellulose utilization in compost soil, whereas the copiotrophic Sordariomycetes were more abundant in NPK and unfertilized soils. Compared with NPK and control soils, compost application increased the diversity of 13C-assimilating fungi. The 13C-labeled fungal communities in compost soil were more phylogenetically clustered and exhibited greater species relatedness than those in NPK and control soils, perhaps because of stringent filtering of narrow-spectrum organic resources and biological invasion originating from added compost. These changes led to an augmented decomposition capacity of fungal species for cellulose-rich substrates and reduced cellulose C sequestration efficiency. The RNA-SIP technique is more sensitive to responses of fungi to altered soil resource availability than DNA-SIP. Overall, long-term compost application modified fungal community composition and promoted fungal diversity and phylogenetic relatedness, accelerating the decomposition of substrate cellulose in soil. This work also highlights the RNA-SIP technique’s value for comprehensively assessing the contributions of active fungi to the substrate decomposition process.

Bacterial Transformation and Processing of Diatom-Derived Organic Matter: A Case Study for Skeletonema dohrnii.

Bacterial transformation and processing of phytoplankton-derived organic matter are extremely important for the formation of ubiquitous organic matter (OM) in aquatic ecosystems. Heterotrophic bacteria convert OM into biomass and recycle inorganic components, contributing to the production of microbial food webs. While phytoplankton-derived organic matter is commonly studied, the transformation and processing of dissolved OM (DOM) and lysate OM (LOM) by culturable epiphytic bacteria remains poorly understood. In this study, cultivable epiphytic bacteria from the marine diatom, Skeletonema dohrnii, were isolated, purified, and identified. Three bacteria, Roseobacteria sp., Marinobacter sp., and Bacillus sp., were selected to study the transformation and processing of S. dohrnii-derived DOM and LOM using excitation-emission matrix (EEM) fluorescence methods, and bacterial abundance, dissolved organic carbon (DOC) concentration, and transparent exopolymer particle (TEP) content were measured. Meanwhile, the bacterial transformation of DOM and LOM was further evaluated by the fluorescence index, biological index, β/α, and humification index. The primary fluorophores, peak A (humic-like), peak C (humic-like), peak M (humic-like), peak B (protein-like), and peak T (tryptophan-like), were present in the sample. The fluorescence of DOM and LOM was dominated by protein-like signal that became increasingly humic-like over time, suggesting that more complex molecules (e.g., recalcitrant OM) are being produced. The fluorescence of DOM and LOM was dominated by a protein-like signal that became increasingly humic-like over time, suggesting that epiphytic bacteria produced more complex molecules. Results showed that the bacteria utilized LOM more rapidly than DOM. While the three bacteria transformed OM to different degrees, all were able to facilitate microbial reprocessing of OM into refractory OM.

Characteristics and Aging of Traffic-Emitted Particles with Sulfate and Organic Compound Formation in Urban Air

Traffic is a major source of anthropogenic aerosol in urban atmosphere. In this study, aerosol particles were measured with a TEM-EDX system at the roadside of a main road in the northwestern part of Beijing, China, under clear and hazy conditions. Soot, organic, sulfur-rich (S-rich), mineral, and metal particles, as well as the mixtures, were frequently encountered in aerosols. Under hazy conditions, S-rich particles coated with organic matter (S-OM particles) accounted for most of the total particles (15% to 24%), followed by soot particles (18% to 21%), organic particles (17% to 21%), non-mixed S-rich particles (10% to 18%), and S-rich particles with soot-, mineral-, or metal-inclusions (here referred to as S-inclusion particles) (11% to 15%). Under clear conditions, non-mixed S-rich and organic particles were dominant components, while mineral and soot particles were secondary components, among which, ~14% of the total particles had a sulfate core or OM coating; inclusions of mixture particles were often mixed with sulfate cores. In the sulfate core–OM shell structure particles, the ratio of core diameter to the whole particle diameter was ~0.52 under hazy conditions and ~0.60 under clear conditions, indicating a substantial sulfate and organic formation on the particles. Soot particles accounted for 18% to 21% of the total particles. The relative growth of aged soot particles was higher under hazy conditions than under clear conditions. In sum, particles from traffic emissions on a main urban road aged with the formation of sulfate and organic matter.

Nitrogen increases early‐stage and slows late‐stage decomposition across diverse grasslands

Abstract To evaluate how increased anthropogenic nutrient inputs alter carbon cycling in grasslands, we conducted a litter decomposition study across 20 temperate grasslands on three continents within the Nutrient Network, a globally distributed nutrient enrichment experiment We determined the effects of addition of experimental nitrogen (N), phosphorus (P) and potassium plus micronutrient (Kμ) on decomposition of a common tree leaf litter in a long‐term study (maximum of 7 years; exact deployment period varied across sites). The use of higher order decomposition models allowed us to distinguish between the effects of nutrients on early‐ versus late‐stage decomposition. Across continents, the addition of N (but not other nutrients) accelerated early‐stage decomposition and slowed late‐stage decomposition, increasing the slowly decomposing fraction by 28% and the overall litter mean residence time by 58%. Synthesis. Using a novel, long‐term cross‐site experiment, we found widespread evidence that N enhances the early stages of above‐ground plant litter decomposition across diverse and widespread temperate grassland sites but slows late‐stage decomposition. These findings were corroborated by fitting the data to multiple decomposition models and have implications for N effects on soil organic matter formation. For example, following N enrichment, increased microbial processing of litter substrates early in decomposition could promote the production and transfer of low molecular weight compounds to soils and potentially enhance the stabilization of mineral‐associated organic matter. By contrast, by slowing late‐stage decomposition, N enrichment could promote particulate organic matter (POM) accumulation. Such hypotheses deserve further testing.

The Role of Input Litter Quality and Quantity on Soil Organic Matter Formation and Sequestration in Rehabilitated Mine Soil

Several anthropogenic factors including mining cause land degradation. The negative impacts of coal mining in accentuating the greenhouse effect may include carbon dioxide (CO 2 ) emission from fell biomass and mineralization of soil organic matter (SOM) exposed by land disturbance. SOM content declines drastically in soils disturbed by mining. Thus soil restoration and reestablishment of vegetation cover on disturbed land could lead to carbon (C) sequestration. The present paper reviewed the research on carbon sequestration related to litter substrate quality and quantity, including litter decomposition, litter nutrients release and molecular property of C sequestered. The present paper aimed to clarify the relationship between soil organic matter formation, litterfall, and litter quality, which will help to understand forest succession in restored mine soil and soil organic matter formation.

INFLUENCE OF VEGETATION ON SOIL CARBON STOCKS IN FORESTS (REVIEW)

Existing estimates of carbon stocks in taiga and coniferous-broadleaf forests show that almost half of the total organic carbon in these ecosystems is accumulated in forest soils. Vegetation as the main source of organic matter in the soil interacts with soil biota, which processes plant litter, and, with abiotic environmental factors, determines the processes of formation and accumulation of soil organic matter. Changes in the composition of vegetation are the driver of the dynamics of soil carbon stocks; however, insufficient attention has been paid to the analysis of this issue. This review analyzes the main ways of transferring carbon from the vegetation pool to the soil pool and the influence of three main predictors of vegetation that affect the carbon stock in soils: the amount and quality of litter of individual plant species (species identity) and the structural diversity of the plant community; it identifies gaps in knowledge and proposes some ways of developing this field of research.

The role of plant input physical-chemical properties, and microbial and soil chemical diversity on the formation of particulate and mineral-associated organic matter

Soil organic matter (SOM) is a fundamental resource to humanity for the many ecosystem services it provides. Increasing its stocks can significantly contribute to climate change mitigation and the sustainability of agricultural production. Elucidating the mechanisms and drivers of the formation of the main components of SOM, particulate (POM) and mineral associated (MAOM) organic matter, from the decomposition of plant inputs is therefore critical to inform management and policy designed to promote SOM regeneration. We designed a two-tiered laboratory incubation experiment using 13C and 15N labeled plant material to investigate the effects of the physical nature (i.e., structural versus soluble) of plant inputs as well as their chemical composition on (1) the pathways of SOM formation, (2) the soil microbial community and chemical diversity, and (3) their interaction on the stabilization efficiency of litter-derived C in POM and MAOM, in a topsoil and a subsoil. We found that: i) the physical nature of the plant input (structural vs soluble) drove both the pathways and efficiencies of SOM formation; ii) POM formation from the decomposition of structural residues increased in efficiency the more decomposed were the residues, and linearly with soil microbial and chemical diversity, the latter only for subsoil; ii) more input-derived C and N were retained in subsoil because of both higher stabilization in MAOM and POM, and slower residue decay. Our results also confirm the importance of direct sorption of soluble inputs to silt- and clay-sized minerals for the formation of MAOM in bulk soils. Taken together these finding suggest that the highest potential for SOM accrual is in subsoils characterized by higher C saturation deficit, from the separate addition of decomposed residues and soluble plant inputs.

Irrigation effects on the formation of soil organic matter from aboveground plant litter inputs in semiarid agricultural systems

In semiarid agricultural systems, irrigation may increase soil organic matter (SOM) due to higher biomass production. However, aboveground production tends to increase more than belowground biomass, and in no-till systems SOM responses to irrigation would strongly depend on the fate of crop residues left on the soil surface. Litter decomposition and SOM formation can also be affected by irrigation due to changes in water dynamics, but little is known about the impacts of irrigation on the pathways of SOM formation. We used dual 13C and 15N labeled maize litter to quantify the effect of irrigation on the contribution of aboveground crop residues to SOM formation in semiarid, no-till agricultural systems. We incubated the litter in a continuous maize cropping system with two treatments, dryland and irrigated, and measured litter decomposition and its contribution to different SOM pools. Irrigation had a larger effect on litter C and N losses than on the formation of new SOM. After 13 months the C and N losses from the litter layer were 24% higher under irrigation, but there were no treatment differences in the litter-derived C and N recovered in SOM. Most of the litter-derived organic matter (OM) was found in the mineral associated OM pool (MAOM), but irrigation did not affect the amount of litter-derived OM found as MAOM or as particulate organic matter. Irrigation increased the amount of litter-derived OM found in macroaggregates, and this observation was more related to a higher aggregation than to aggregate enrichment in the irrigated treatment. Our results suggest that a smaller proportion of aboveground crop residues will form SOM in irrigated systems compared to dryland conditions and may help to partially explain why irrigation has a stronger relative effect on crop productivity and C inputs than on SOC stocks in semiarid agricultural systems.

Influences of microwave irradiation on pore, fracture and methane adsorption of deep shale

In order to better understand the feasibility of enhancing shale gas recovery by microwave irradiation, the influences of microwave irradiation on the pore, fracture, and methane (CH4) adsorption pertaining to shale gas production of a deep shale were fully addressed. A combination of multiple characterization methods was used, including pyrolysis, X-ray diffraction, back-scattered scanning electron microscope, probe molecule adsorption, steady-state permeability, X-ray computed tomography, and gravimetric method for measuring CH4 adsorption. Results show that microwave irradiation has no obvious impact on micropore but decreases mesoporous surface area, and volume of the shale matrix. It particularly reduces the adsorption pores for accommodating CH4 with the diameter of less than 8 nm. The nanoscale pore structure evolution is mainly associated with alterations in organic matter and inorganic minerals of the deep shale after microwave irradiation. Microwave irradiation induces the decomposition, cracking, shift, stripping and formation of minerals, as well as the formation of organic matter. Furthermore, it significantly increases fractures paralleling to bedding planes of the deep shale due to thermal stress generated within various minerals. These newly formed fractures dramatically increase the porosity and permeability of the deep shale. Finally, microwave irradiation significantly decreases the maximum CH4 adsorption capacity of the deep shale by 16.38%. In summary, this study put forward a novel method for enhancing shale gas production via simultaneously promoting CH4 migration and reforming reservoirs under controlled microwave field.

Geochemical interactions among water, minerals, microbes, and organic matter in formation of speleothems in volcanic (lava tube) caves

Volcanic (lava tube) caves at Lava Beds National Monument (N. CA, USA) provide a valuable terrestrial analog for volcanic caves on Mars and the Moon. Terrestrial volcanic caves host a diverse microbial life, liquid water, and a variety of secondary mineral deposits (speleothems) with diverse morphologies and chemical compositions. Speleothems may preserve records of past and present microbial life and signatures of paleoenvironmental changes in terrestrial volcanic caves. Distinguishing between speleothems via chemical processes and microbially-mediated processes in terrestrial volcanic caves will provide valuable insights for future exploration of martian volcanic caves. To elucidate the formation of speleothems, we studied the chemical makeup (inorganic and organic) of cave waters in seven volcanic caves of variable ages, temperature, moisture content, light intensity, and frequency of human visitation. Cave water was characterized by stable isotopic composition (δ18O and δ2H), concentrations of major and trace elements, cations, anions, and characteristics of dissolved organic matter (DOM). A forward reaction model (PHREEQC) was used to test possible pathways for secondary mineral precipitation that formed these speleothems. The source of cave water was primarily regional meteoric precipitation that entered the caves through cave openings or through the cave overburden and fractured basalt walls as indicated by cave floor puddle water line δ2H = 8.32*δ18O + 9.55 parallel to the global meteoric water line (GMWL, δ2H = 8.3*δ18O + 10). A line formed by cave ceiling drip water δ2H = 3.39*δ18O – 44.77 intersecting the GMWL indicated that the water may be undergoing evaporation within the caves. Silicate weathering was found to be a primary process resulting in cave water enriched in Si (22 ± 7 mg/L), and contained trace levels of Al, Fe, Zn, Li, Sr, Cu, B, V, Ba, Cr and Mn. Geochemical calculations indicated that cave waters were undersaturated with respect to both amorphous silica (SiO2am) and calcite (CaCO3) which were the major components of speleothems observed within the caves. Results of a forward reaction model showed that evaporation of cave waters could lower the solubility of SiO2am and CaCO3 by increasing their saturation and ultimately precipitate these two secondary minerals forming the speleothems. The cave water DOM was characterized by high concentrations of dissolved organic carbon (DOC, 12 ± 8 mg/L) with a molar C/N ratio ranging from 2 to 22. The DOM was found to be aromatic (SUVA254, 1.2–2.9 L/mg.m), terrestrially derived and humic-like (humification index, 7–26) and contained molecules of 100 Da and 5000 Da approximate molecular weight (AMU). Our results indicated that the terrestrially derived carbonaceous organic matter transported into the caves was not utilized for heterotrophic microbial metabolisms as DOC was accumulated over dissolved inorganic carbon (DIC). Both findings suggest that with minimal heterotrophy, chemo-litho-autotrophy may be important pathways that cycle the elements within these volcanic caves with low light conditions. Together, this study proposes a potential pathway of speleothem precipitation through the interaction of water, dissolved mineral constituents, and microbial life where dissolved ions are concentrated in cave drip water through cyclic condensation-vaporization processes. This work is part of a multi-disciplinary project Biologic Resource Analog in Low Light Environments (BRAILLE) funded by the NASA PSTAR Program (NNH16ZDA001N), which focuses on studying volcanic caves as terrestrial analogs for the Moon and Mars.