Composition Of Soil Organic Matter Research Articles

Impact of Climate on Soil Organic Matter Composition in Soils of Tropical Volcanic Regions Revealed by EGA-MS and Py-GC/MS.

Soil organic matter (SOM) crucially influences the global carbon cycle, yet its molecular composition and determinants are understudied, especially for tropical volcanic regions. We investigated how SOM compounds change in response to climate, vegetation, soil horizon, and soil properties and the relationship between SOM composition and microbial decomposability in Tanzanian and Indonesian volcanic regions. We collected topsoil (0-15 cm) and subsoil (20-40 cm) horizons (n = 22; pH: 4.6-7.6; SOC: 10-152 g kg-1) with undisturbed vegetation and wide mean annual temperature and moisture ranges (14-26 °C; 800-3300 mm) across four elevational transects (340-2210 m asl.). Evolved gas analysis-mass spectrometry (EGA-MS) documented a simultaneous release of SOM compounds and clay mineral dehydroxylation. Subsequently applying double-shot pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) at 200 and 550 °C, we detailed the molecular composition of topsoil and subsoil SOM. A minor portion (2.7 ± 1.9%) of compounds desorbed at 200 °C, limiting its efficacy for investigating overall SOM characteristics. Pyrolyzed SOM closely aligns with the intermediate decomposable SOM pool, with most pyrolysates (550 °C) originating from this pool. Pyrolysates composition suggests tropical SOM is mainly microbial-derived and subsoil contains more degraded compounds. Higher litter inputs and attenuated SOM decomposition due to cooler temperatures and lower soil pH (<5.5) produce less-degraded SOM at higher elevations. Redundancy analyses revealed the crucial role of active Al/Fe (oxalate-extractable Al/Fe), abundant in low-temperature/high-moisture conditions, in stabilizing these less-degraded components. Our findings provide new insights into SOM molecular composition and its determinants, critical for understanding the carbon cycle in tropical ecosystems.

The Use of Spectroscopic Methods to Study Organic Matter in Virgin and Arable Soils: A Scoping Review

The use of modern spectroscopic methods of analysis, which provide extensive information on the chemical nature of substances, significantly expands our understanding of the molecular composition and properties of soil organic matter (SOM) and its transformation and stabilization processes in various ecosystems and geochemical conditions. The aim of this review is to identify and analyze studies related to the application of nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopy techniques to study the molecular composition and transformation of organic matter in virgin and arable soils. This article is mainly based on three research questions: (1) Which NMR spectroscopy techniques are used to study SOM, and what are their disadvantages and advantages? (2) How is the NMR spectroscopy technique used to study the molecular structure of different pools of SOM? (3) How is ESR spectroscopy used in SOM chemistry, and what are its advantages and limitations? Relevant studies published between 1996 and 2024 were searched in four databases: eLIBRARY, MDPI, ScienceDirect and Springer. We excluded non-English-language articles, review articles, non-peer-reviewed articles and other non-article publications, as well as publications that were not available according to the search protocols. Exclusion criteria for articles were studies that used NMR and EPR techniques to study non-SOM and where these techniques were not the primary methods. Our scoping review found that both solid-state and solution-state NMR spectroscopy are commonly used to study the structure of soil organic matter (SOM). Solution-phase NMR is particularly useful for studying soluble SOM components of a low molecular weight, whereas solid-phase NMR offers advantages such as higher 13C atom concentration for stronger signals and faster analysis time. However, solution-phase NMR has limitations including sample insolubility, potential signal aggregation and reduced sensitivity and resolution. Solid-state NMR is better at detecting non-protonated carbon atoms and identifying heterogeneous regions within structures. EPR spectroscopy, on the other hand, offers significant advantages in experimental biochemistry due to its high sensitivity and ability to provide detailed information about substances containing free radicals (FRs), aiding in the assessment of their reactivity and transformations. Understanding the FR structure in biopolymers can help to study the formation and transformation of SOM. The integration of two- and three-dimensional NMR spectroscopy with other analytical methods, such as chromatography, mass spectrometry, etc., provides a more comprehensive approach to deciphering the complex composition of SOM than one-dimensional techniques alone.

No-tillage farming for two decades increases plant- and microbial-biomolecules in the topsoil rather than soil profile in temperate agroecosystem

The impact of conservation tillage on the composition, stability, and origin (plant- or microbial-derived) of soil organic matter (SOM) within soil profiles remains unclear. In this study, we characterized the impact of different tillage practices on the molecular composition and status of SOM. These tillage practices included conventional tillage (CT) and conservation tillage such as rotary tillage (RT) and no-tillage (NT). Our investigation spanned a two-decade period within a temperate agroecosystem. Soil samples were collected down to a 30 cm profile, and four biomarkers, namely free lipids, lignin phenols, neutral sugars, and amino sugars, in conjunction with 13C NMR techniques were used to quantify SOM characteristics. The results revealed that conservation tillage (especially NT) led to improvements in both the quantity and composition of SOM when compared to CT in the topsoil (0–5 cm), but not in deeper layers (5–30 cm). More precisely, the NT topsoil, as opposed to CT, exhibited enhancements in two categories: (1) an increase in plant-derived compounds such as long-chain lipids (≥ C20) and steroids by 51%, lignin phenols by 106%, and plant-derived neutral sugars by 61%, and (2) an augmentation in microbial-derived biomolecules, which includes microbial-derived neutral sugars by 49% and microbial necromass carbon (MNC) by 94%. Furthermore, NT, relative to CT, increased the contribution of MNC to soil organic carbon in topsoil (up to 64%, mainly fungal necromass). This highlighted the predominant role of microbial-derived biomolecules in SOM formation. Similarly, RT treatments enhanced the microbial-derived neutral sugars by 18%, plant-derived neutral sugars by 14%, and lignin phenols by 43% relative to CT in the topsoil. Collectively, our study suggests that NT practice could be considered an effective strategy for enhancing SOM accumulation and persistence by fostering the accrual of plant- and microbial-derived biomolecules in the topsoil.

Long-term restoration with organic amendments is clearer evidenced by soil organic matter composition than by changes in microbial taxonomy and functionality

Here, a degraded soil, located in a semi-arid Mediterranean region, was characterized 17 years after organic amendment with sludge or compost (differing in their stabilization degree) for restoration purposes. To do this, (i) soil physicochemical properties and plant cover, (ii) soil organic matter (SOM) content and composition, (iii) soil basal respiration and enzymatic activities, and (iv) abundance, taxonomic composition, and functionality (shotgun metagenomics) of microbial communities were studied. Increased SOM and nutrient contents were found in soil from amended plots with respect to the control, with no differences between amendment types. This is explained by the lasting effects of organic amendments and the higher plant cover. Thermal and pyrolytic analyses showed that the restoration process enriched soil mainly with SOM of intermediate recalcitrance and of high chemical diversity. SOM composition did not differ between amendment types. Increased microbial abundances and activities were found in the amended plots, without differences between compost and sludge. Shotgun metagenomics showed that microbial communities changed in taxonomic and functional terms between amended and unamended plots, but these differences were rather limited. The taxonomic differences between treatments were mainly driven by increasing abundances of Actinobacteria and decreasing abundances of Proteobacteria in soil from amended plots. Soil microbial communities in amended plots showed some functional adaptation to the increased nutrient contents and predominant nutrients forms. This, together with the higher microbial abundances detected, explained the conspicuous soil enzymatic activities reported in amended plots. The effectiveness of the studied soil restoration process was confirmed here from an integrative perspective.

Soil metabolomics - current challenges and future perspectives

Soil is an extremely complex and dynamic matrix, in part, due to the wide diversity of organisms living within it. Soil organic matter (SOM) is the fundamental substrate on which the delivery of ecosystem services depends, providing the metabolic fuel to drive soil function. As such, studying the soil metabolome (the diversity and concentration of low molecular weight metabolites), as a subset of SOM, holds the potential to greatly expand our understanding of the behaviour, fate, interaction and functional significance of small organic molecules in soil. Encompassing a wide range of chemical classes (including amino acids, peptides, lipids and carbohydrates) and a large number of individual molecules (ca. n = 105 to 106), the metabolome is a resultant (indirect) output of several layers of a biological hierarchy, namely the metagenome, metatranscriptome and metaproteome. As such, it may also provide support and validation for these “multi-omics” datasets. We present a case for the increased use of untargeted metabolomics in soil biochemistry, particularly for furthering our fundamental understanding of the functions driving SOM composition and biogeochemical cycling. Further, we discuss the scale of the challenge in terms of metabolite extraction, analysis and interpretation in complex plant-soil-microbial systems. Lastly, we highlight key knowledge gaps which currently limit our use of metabolomic approaches to better understand soil processes, including: (i) interpretation of large untargeted metabolomic datasets; (ii) the source, emission and fate of soil-derived volatile organic compounds (VOCs), (iii) assessing temporal fluxes of metabolites, and (iv) monitoring ecological interactions in the rhizosphere. While the application of metabolomics in ecosystem science is still in its relative infancy, its importance in understanding the biochemical system in relation to regulation, management and underpinning the delivery of ecosystem services is key to further elucidating the complex links between organisms, as well as the fundamental ability of the biological community to process and cycle key nutrients.

Fire Impacts on the Soil Metabolome and Organic Matter Biodegradability.

Global wildfire activity has increased since the 1970s and is projected to intensify throughout the 21st century. Wildfires change the composition and biodegradability of soil organic matter (SOM) which contains nutrients that fuel microbial metabolism. Though persistent forms of SOM often increase postfire, the response of more biodegradable SOM remains unclear. Here we simulated severe wildfires through a controlled "pyrocosm" approach to identify biodegradable sources of SOM and characterize the soil metabolome immediately postfire. Using microbial amplicon (16S/ITS) sequencing and gas chromatography-mass spectrometry, heterotrophic microbes (Actinobacteria, Firmicutes, and Protobacteria) and specific metabolites (glycine, protocatechuate, citric cycle intermediates) were enriched in burned soils, indicating that burned soils contain a variety of substrates that support microbial metabolism. Molecular formulas assigned by 21 T Fourier transform ion cyclotron resonance mass spectrometry showed that SOM in burned soil was lower in molecular weight and featured 20 to 43% more nitrogen-containing molecular formulas than unburned soil. We also measured higher water extractable organic carbon concentrations and higher CO2 efflux in burned soils. The observed enrichment of biodegradable SOM and microbial heterotrophs demonstrates the resilience of these soils to severe burning, providing important implications for postfire soil microbial and plant recolonization and ecosystem recovery.

Study in a long-term laboratory experiment of the potential susceptibility to mineralization of organic matter in post-agrogenic light gray soils

Relevance. Since 2021, work has begun in Russia to assess the fertility of unused arable land and its involvement in agricultural circulation. Changes in the farming system of post-agrogenic soils can lead to uncontrolled mineralization of newly formed soil organic matter (SOM) under fallow vegetation and significant CO2 emissions into the atmosphere. Studies of the nature of SOM accumulation under fallow vegetation and assessment of its potential susceptibility to mineralization are relevant, since they can become the basis for the development of agrotechnical methods for returning unused lands to arable circulation with the maximum preservation of their fertility.Methods. In a long-term laboratory incubation experiment, we studied the dynamics of changes in the intensity of basal (BR) and substrate-induced respiration (SIR) in postagrogenic soils to assess the potential susceptibility to SOM mineralization with a change in land use. Layered samples (0–10 and 10–20 cm) were used from the old arable horizons of two fallow plots with different humus conditions. The results of the incubation experiment were compared with the results of assessing the quantitative content and qualitative composition of SOM.Results. In the 0–10 cm layer, the respiration rates are higher than in the 10–20 cm layer. The results of the assessment of the respiration intensity are consistent with the assessment of the qualitative composition of SOM. The accumulation of SOM occurs mainly in the upper part of postagrogenic soils due to mobile easily oxidized organic compounds of a fulvic nature. When developing agrotechnical methods for returning fallow lands to arable circulation, it is necessary to focus primarily on basic processing technologies that ensure maximum preservation of potentially easily mineralized material accumulated in the upper layer.

Spatial variation of soil organic matter and metal mobility in wetland soils: Implications for biogeochemical processes in lateritic landscape

The evolution of the lateritic landscape over time changes the path of pedogenesis and soil hydrology. The occurrence of upland wetlands in the savannah flat plateau transforms well-drained lateritic soils into hydric soils enriched in organic matter. We investigated the composition of soil organic matter (SOM) produced in a wetland and their link with the mineral constituents of soil and their contribution to the mobility of Si, Fe, and Al in the soil. Six tranches were opened in a soil catena located from the plateau toward the center of the wetland, representing different hydromorphic features. Soil morphology was described, and chemical, physical and mineralogical analyses were performed to evaluate soil properties. Chemical fractionation of Soil Organic Matter (SOM) was performed and total organic carbon (TOC), dissolved organic carbon (DOC), particulate organic carbon (POC), and water-extractable organic matter (WEOM) were determined in the soil horizons. Metal complexing properties of DOM were determined using an excitation-emission matrix of fluorescence. The occurrence of Fe eluviation in the superficial horizons, Fe2O3 enrichment in the mottled horizons, and ferricrete formation in the center of the wetland were observed. Geochemical transfer assessment and the variation of oxides within the horizons showed that Fe2O3 and SiO2 were more mobile than Al2O3, whose variability in the gley horizon reflected the mobility of Fe2O3 and SiO2. The stability constants of Al and Cu showed an increase with the depth for WEOM and SOM. The pattern of the organic carbon fraction varied according to the level of hydromorphy and soil depth. It was also observed an increase in the C/N ratio with depth, possibly related to the reduced bioavailability of DOC due to its adsorption with soil solids.

Organic soils in Southeastern Brazilian highlands: formation and relations to vegetation history

Well-drained organic soils form in distinctive highland environments in contrast to wetland organic soils. Similar to the concept of endemic soils, they are restricted to specific geographic areas due to a unique combination of soil-forming factors. In this contribution, we aim to improve the pedological knowledge of the Brazilian highlands, focusing on well-drained organic soils. The objective was to propose a soil formation scheme for the study area, combining traditional techniques and paleoenvironmental reconstruction (phytoliths, isotypes, dating). Organic soils were formed by litter accumulation in two pedoenvironments: (1) directly on bedrock in the high-elevation grasslands, by the processes of addition and transformation of the litter, which constitute the parent material from these soils; and (2) in upper-montane forests, with organic horizons formed above mineral horizons. In both conditions, the cold and humid climate and vegetation (organisms) are the main soil-forming factors, reducing organic matter decomposition, and influencing the physical and chemical soil properties. Phytolith records and isotopic composition of soil organic matter indicated four environmental phases: Phase I (before ∼18,9k yr BP). A cooler and drier climate than present, with predominance of subtropical grassland vegetation and fire events; Phase II (∼18,9k yr BP to ∼11,1k yr BP). A slightly increase of humidity, decrease of fire events, and expansion of forest formations, with rare presence of Araucaria angustifolia; Phase III (from ∼11,1k years BP) increase in humidity, the establishment of current climate conditions (cold and humid climate, typical of Southeastern Brazilian highlands), and significant presence of Araucaria in upper-montane forests; Phase IV (present) environmental changes related to increased fire events, reduction of Araucaria and increase of Bambusoid, Arecaceae and C4 plants, possibly resulted from human activities. This contribution enhances the understanding of well-drained organic soils in the Southeastern Brazilian highlands, showing their unique formation under cold and humid climates, influenced by vegetation history and environmental phases, as revealed through phytolith records, isotopic composition, and dating.

Chemical composition and thermal stability of topsoil organic carbon: Influence of cropping system and tillage practices

AbstractAgricultural management practices play a significant role in regulating the potential for soil organic carbon (SOC) sequestration. The objective of this study was to determine the effects of cropping systems and tillage practices on the chemistry and thermal stability of topsoil SOC in a long‐term field study in Ontario, Canada. The cropping system is based on rotations including corn, alfalfa, cereals, soybeans and a red clover cover crop. Tillage practices of conventional (moldboard plow, CT) and conservation (no‐till, NT) were applied to each cropping system. A 130‐day laboratory incubation was conducted to measure the potentially mineralizable SOC. The thermal stability and molecular structure of SOC were investigated using thermal analysis‐programmed pyrolysis (PP) and solid‐state 13C cross polarization/total sideband suppression magic angle spinning nuclear magnetic resonance (CP/TOSS MAS NMR) spectroscopy, respectively. The SOC stocks were larger under NT practices and the crop rotations incorporating alfalfa and cover crops. Under NT practices, an abundance of aromatic‐C components was observed, however, soil under CT showed an abundance of aliphatic‐C compounds (p &lt; 0.001), with a higher alkyl/O‐alkyl‐C ratio, indicating a higher degree of SOC decomposition. Soil under rotations that included soybeans demonstrated a significant increase in aliphatic‐C components, whereas those with cover cropping exhibited an enrichment in O‐alkyl‐C groups (p &lt; 0.05), representing the presence of more resistant and easily decomposable SOC constituents, respectively. The results demonstrated that the thermal stability of SOC in CT systems was higher than that of NT practices (p &lt; 0.05), while NT practices and crop rotations including cover crops are better capable of conserving the labile pool of SOC. Our findings confirmed the correlations among the parameters that characterize both the labile and stable pools of SOC as determined by the methods employed in this study. These results demonstrated that agricultural management practices significantly influence the chemical composition and thermal stability of soil organic matter (SOM), which can have significant impacts on soil health and C sequestration potential.

Mechanism of interaction between ascorbic acid and soil iron-containing minerals for peroxydisulfate activation and organophosphorus flame retardant degradation

Soil constituents may play an important role in peroxydisulfate (PDS)-based oxidation of organic contaminants in soil. Iron-containing minerals (Fe-minerals) have been found to promote PDS activation for organics degradation. Our study found that ascorbic acid (H2A) could enhance PDS activation by soil Fe-minerals for triphenyl phosphate (TPHP) degradation. Determination and characterization analyses of Fe fractions showed that H2A could induce the reductive dissolution of solid Fe-minerals and the increasing of oxygen vacancies/hydroxyl groups content on Fe-minerals surface. The increasing of divalent Fe (Fe(II)) accelerated PDS activation to generate reactive oxygen species (ROS). Electron paramagnetic resonance (EPR) and quenching studies showed that sulfate radicals (SO4•−) and hydroxyl radicals (HO•) contributed significantly to TPHP degradation. The composition and content of Fe-minerals and soil organic matter (SOM) markedly influenced ROS transformations. Surface-bond and structural Fe played the main role in the production of Fe(II) in reaction system. The high-concentration SOM could result in ROS consumption and degradation inhibition. Density functional theory (DFT) studies revealed that H2A is preferentially adsorbed at α-Fe2O3(012) surface through Fe–O–C bridges rather than hydrogen bonds. After absorption, H atoms on H2A may further be migrated to adjacent O atoms on the α-Fe2O3(012) surface. With the transformation of H atoms to the α-Fe2O3(012) surface, the Fe–O–C bridge is broken and one electron is transferred from the O to Fe atom, inducing the reduction of trivalent Fe (Fe(III)) atom. MS/MS2 analysis, HPLC analysis, and toxicity assessment demonstrated that TPHP was transformed to less toxic 4-hydroxyphenyl diphenyl phosphate (OH-TPHP), diphenyl hydrogen phosphate (DPHP), and phenyl phosphate (PHP) through phenol-cleavage and hydroxylation processes, and even be mineralized in reaction system.

Grazing intensity and nitrogen fertilization timing to increase soil organic carbon stock and nitrogen in integrated crop-livestock systems

ABSTRACT Integrated crop-livestock systems (ICLS) foster synergistic relationships to increase nitrogen (N) cycling and soil organic carbon (SOC) accrual in agricultural setups. This study evaluated how the grazing intensity and N fertilization (rates and timing) affect both SOC and N fractions, and soil organic matter chemical composition in an ICLS managed under no-tillage in an Oxisol, six years after initiation. The ICLS was compared to a nearby pasture (PA) and a native forest (NF). The treatments consisted of two grazing intensities: Low Sward Height (LH) and High Sward Height (HH) were maintained with high and low stocking rates, respectively. The HH varied between 0.20 and 0.60 m, and LH between 0.10 and 0.30 m according to the plant forage species throughout the experiment. Fertilization using 200 kg ha -1 N-urea, not splitting up, was conducted at two timings, either at the winter pasture establishment (autumn), about 35 days after sowing or during the summer cash crop cycle (spring). Total N amount per year, including both phases, pasture and cash crop was the same for all treatments. The SOC and N contents were assessed in soil and particulate organic matter (POM), while carbon (C) and N stocks were specifically determined in the soil. Soil organic matter composition was characterized by FTIR. The combination of HH and N fertilization during the pasture phase increased the content of C from 36.1 to 39.9 ± 0.7 g C kg -1 and of N from 2.7 to 3.2 ± 0.1 g N kg -1 . The SOC stocks varied from 37.3 to 41.1 ± 0.7 Mg C ha -1 , and the N stocks from 2.1 to 3.3 ± 0.1 Mg N ha -1 at 0.0-0.10 m soil layer. The SOC content of the POM and the soil organic matter chemical composition determined by FTIR were mainly affected by the grazing intensity. The HH led to an increased in C content within the POM fraction, reaching values of 51.6 ± 1 and 49.2 g C kg -1 , respectively to N crop fertilization and N pasture fertilization. Land-use changed how organic functional groups were stored in soil organic matter fractions. The NF had a greater abundance of aliphatic and phenol in the MAOM, while pasture and ICLS systems had greater aliphatic in the POM fraction. In ICLS, SOC accrual was positively associated with more recalcitrant organic functional groups of phenol, aromatic, and carbonyl C-O. The HH increases SOC accrual, while N-fertilization on pasture ensures adequate nutrition of plants and animals during the winter ICLS phase, at the same time as providing greater residual N for subsequent cash crops through enhanced catalyzed by ruminants. Therefore, grazing and fertilization management strategies should be considered to promote sustainable agriculture intensification with ICLS.

Holocene and late Pleistocene bioclimatic change inferred from δ13C of organic carbon in buried alluvial soils in the tallgrass prairie, northeastern Kansas

The tallgrass prairie of the Central Plains is one of the largest North American biomes, yet little is known about the history of late-Quaternary bioclimatic change in the region because settings that are favorable for the preservation of traditional paleoenvironmental proxies, such as pollen and plant macrofossils, are rare. However, the valleys of large streams contain thick soil-stratigraphic sequences that span the Holocene and terminal Pleistocene and are archives of other paleoenvironmental proxies. To better understand late-Quaternary bioclimatic change in the tallgrass prairie, we analyzed the stable carbon isotope (δ13C) composition of soil organic matter (SOM) preserved in buried soils in floodplain, terrace, and alluvial fan deposits in the Big Blue River valley of northeast Kansas. A combination of eight cores and three outcrops at six localities were investigated, and 27 radiocarbon ages provide a robust numerical chronology. Based on our analysis, the tallgrass prairie has experienced significant bioclimatic changes over the past 27,000 years. From the LGM to the beginning of the late Holocene (ca. 25,000–4500 cal yr BP), there is an overall increase in C4 plant contributions to SOM, suggesting general warming for that period. However, warming and C4 grass expansion occurred in a non-linear fashion, with minor variations in the abundance of C4 vs C3 plants reflecting either small fluctuations in temperatures during general warming trends or other environmental factors, such as landscape disturbance, periods of drought, or the seasonal distribution of precipitation. The δ13C record also suggests that a major warming event occurred in the region between ca. 3000–1300 cal yr BP, when the contribution of C4 biomass to SOM exceeded 95 %. This climatic event does not appear to be an anomaly, as multiple paleoenvironmental proxies indicate a brief but exceedingly warm and dry interval occurred from 3300 to 2800 cal yr BP.

Soil water repellency in the Brazilian neotropical savanna: first detection, seasonal effect, and influence on infiltrability.

Soil water repellency (SWR) has been detected worldwide in various biomes and climates. However, this phenomenon has not been shown yet in the Brazilian neotropical savanna. The present study addressed the following questions: (a) Does SWR occur in the Brazilian neotropical savanna? If so, (b) does it exhibit seasonality? (c) Does it influence infiltration? To do that, we selected two similar study areas covered by similar soils (oxisol) and vegetation (netropical savanna). We performed water repellency and infiltration tests in both areas during the transition from dry to wet season. Our results indicate that SWR occurs in soils of the Brazilian neotropical savanna only during the dry season and influence water infiltration in the dry season. The likely cause of SWR might be related to the chemical composition of soil organic matter since neotropical savanna plants produce hydrophobic substances as a survival strategy, especially during the dry season.

Microbial successional pattern along a glacier retreat gradient from Byers Peninsula, Maritime Antarctica

The retreat of glaciers in Antarctica has increased in the last decades due to global climate change, influencing vegetation expansion, and soil physico-chemical and biological attributes. However, little is known about soil microbiology diversity in these periglacial landscapes. This study characterized and compared bacterial and fungal diversity using metabarcoding of soil samples from the Byers Peninsula, Maritime Antarctica. We identified bacterial and fungal communities by amplification of bacterial 16 S rRNA region V3–V4 and fungal internal transcribed spacer 1 (ITS1). We also applied 14C dating on soil organic matter (SOM) from six profiles. Physico-chemical analyses and attributes associated with SOM were evaluated. A total of 14,048 bacterial ASVs were obtained, and almost all samples had 50% of their sequences assigned to Actinobacteriota and Proteobacteria. Regarding the fungal community, Mortierellomycota, Ascomycota and Basidiomycota were the main phyla from 1619 ASVs. We found that soil age was more relevant than the distance from the glacier, with the oldest soil profile (late Holocene soil profile) hosting the highest bacterial and fungal diversity. The microbial indices of the fungal community were correlated with nutrient availability, soil reactivity and SOM composition, whereas the bacterial community was not correlated with any soil attribute. The bacterial diversity, richness, and evenness varied according to presence of permafrost and moisture regime. The fungal community richness in the surface horizon was not related to altitude, permafrost, or moisture regime. The soil moisture regime was crucial for the structure, high diversity and richness of the microbial community, specially to the bacterial community. Further studies should examine the relationship between microbial communities and environmental factors to better predict changes in this terrestrial ecosystem.

Soil conditioners promote the formation of Fe-bound organic carbon and its stability

The close association of soil organic carbon (SOC) with Fe oxides is an important stabilization mechanism for soil organic matter (SOM) against biodegradation. Soil conditioners are of great importance in improving soil quality and soil health. Yet it remains unclear how different conditioners would affect the fractionation of SOC, particularly the Fe-bound organic carbon (Fe-OC). Field-based experiments were conducted in farmland to explore the fractionation of organic carbon (OC) and Fe oxides under the effects of three different soil conditioners (mineral, organic, and microbial conditioners). The results showed that all soil conditioners increased the total OC and Fe-OC contents, with the contribution of Fe-OC to total OC increasing from 1.57% to 2.99%. The low OC/Fe molar ratio indicated that surface adsorption played a crucial role in soil Fe-OC accumulation. Nuclear magnetic resonance (NMR) results suggested that soil conditioner altered the composition of SOM, accelerating O-alkyl C degradation and increasing recalcitrant alkyl C and aromatic C sequestration. Scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) analysis indicated that all conditioners promoted the association of OC and Fe oxides. Furthermore, comprehensive analysis of 13C isotope and synchrotron radiation-based Fourier transform infrared (SR-FTIR) spectroscopy showed that the mineral conditioner enhanced the association of microbial-derived OC and Fe oxides, whereas the organic conditioner increased the association of plant-derived OC with Fe oxides. These findings provide important insights into the potential mechanisms through which soil conditioners regulate the stability of OC and guide agricultural management.

Distinct dynamics of plant- and microbial-derived soil organic matter in relation to varying climate and soil properties in temperate agroecosystems

Global environmental change can substantively alter soil carbon storage and dynamics in agroecosystems. However, investigations of soil organic matter (OM) composition associated with various environmental factors are still limited, and this hinders the understanding of soil carbon biogeochemistry in agricultural settings. Soil samples were collected at two times (time 0 and 8 or 10 years) from 10 agricultural sites across Canada and New Zealand with varying soil properties (i.e., soil texture, pH) and climates (MAT, mean annual temperature; MAP, mean annual precipitation). The soils were analyzed for organic carbon, nitrogen and soil OM composition using molecular-level techniques that included targeted compound-specific and solid-state 13C nuclear magnetic resonance analyses. Soil carbon contents were similar over time and were not correlated with environmental factors. Molecular-level characterization of soils found that the preservation and degradation of specific soil OM components differed. Simple sugars and microbial-derived compounds (i.e., fungi-derived ergosterol and microbial-derived lipids) persist less with time in soils compared to other OM components. However, the increased cutin- and suberin-derived compounds at most sites and similar alkyl carbon contents with time suggested that cutin- and suberin-derived compounds were longer-lived compared to other soil OM compounds. Correlation analyses indicated that temporal differences in fungal-derived ergosterol were positively correlated with silt content (r = 0.78). An inverse correlation was observed between lignin degradation and silt content (r = −0.75). Cutin- and suberin-derived compounds were negatively correlated with MAT (r = −0.69); when the analysis was restricted to only Canadian sites, cutin- and suberin-derived compounds and their degradation were correlated with MAP and MAT (climate variables). Further, partial correlation analysis revealed that the correlations between soil texture and the temporal shifts in soil OM composition were indirectly regulated by MAP but not MAT. Overall, distinct environmental constraints were observed for specific OM components in agroecosystems, suggesting that the preservation mechanisms are not uniform across a wide range of soils under different climates and soil properties.

Combining thermal analyses and wet-chemical extractions to assess the stability of mixed-nature soil organic matter

In this study, we combined Rock-Eval® analysis, analytical pyrolysis, and wet-chemical extractions, assisted by Fourier transform infrared spectroscopy (FTIR), and measurements of soil heterotrophic respiration. Our objective was to assess the biological and thermal stability of mixed-nature soil organic matter (SOM) derived from grass litter and kerogen. We studied a Technosol constructed with Ca, Mg and kerogen-rich waste rock (black shales), which has been under pasture cultivation for 20 years. We compared the results with those from black shales and an adjacent natural soil under long-term pasture cultivation, both used as endmembers representing kerogen and plant-derived SOM, respectively. Analytical pyrolysis and FTIR analyses revealed a mixed composition of SOM in the Technosol. Predominantly, polysaccharides, lignin, lipids and N-compounds were originated from plant-derived SOM, while (poly)aromatic and most aliphatic compounds were traced back to kerogen. Rock-Eval analysis showed that 58% of SOM in Technosol was kerogen-derived, which was poorly accessible to soil microbiota, as evidenced by heterotrophic respiration. In addition, an important portion of plant-derived SOM (>70%) was only released during the Rock-Eval oxidation stage. The impact of chemical recalcitrance of kerogen compounds on short-term biological stability was remarkable and demonstrated a strong correlation with the thermal indices derived from the Rock-Eval pyrolysis stage. Conversely, the parameters from Rock-Eval oxidation stage showed a positive correlation with the amount of SOM involved in mineral-organic associations, particularly with Ca2+ and Mg2+ (i.e., cation bridging). Thus, to disentangle the contribution of chemical recalcitrance and mineral-organic associations to SOM stability, we recommend the assessment of all Rock-Eval thermograms (from pyrolysis and oxidation stages) in combination with chemical and biological assessments, especially when studying soils containing mixed-nature SOM.

Mercury mobilization in shrubland after a prescribed fire in NE Portugal: Insight on soil organic matter composition and different aggregate size

Soils constitute the major reservoir of mercury (Hg) in terrestrial ecosystems, whose stability may be threatened by wildfires. This research attempts to look at the effect of prescribed fire on the presence of Hg in a shrubland ecosystem from NE Portugal, delving into its relationship with soil aggregate size and the molecular composition of soil organic matter (SOM). During the prescribed fire, on average 347 mg Hg ha−1 were lost from the burnt aboveground biomass of shrubs and 263 mg Hg ha−1 from the combustion of the soil organic horizon. Overall, Hg concentration and pools in the mineral soil did not show significant changes due to burning, which highlights their role as long-term Hg reservoirs. The higher Hg concentrations found in smaller aggregates (<0.2 mm) compared to coarser ones (0.5–2 mm) are favored by the higher degree of organic matter decomposition (low C/N ratio), rather than by greater total organic C contents. The Hg-enriched finest fraction of soil (<0.2 mm) could be more prone to be mobilized by erosion, whose potential arrival to water bodies increases the environmental concern for the Hg present in fire-affected soils. The SOM quality (molecular composition) and the main organic families, analyzed by Fourier-transform infrared spectroscopy in combination with multivariate statistical analysis, significantly conditioned the retention/emission of Hg in the uppermost soil layers. Thus, before the fire, Hg was strongly linked to lipid and protein fractions, while Hg appeared to be linked to aromatic-like compounds in fire-affected SOM.

Short-term nitrogen deposition changes chemical composition of litter and soil organic matter in a Moso bamboo forest

To investigate the effects of short-term nitrogen (N) deposition on organic matter composition of litter and soil in Moso bamboo (Phyllostachys edulis) forests, we established a N-addition treatments (50 kg N·hm-2·a-1) to simulate the ambient and N deposition in a subtropical Moso bamboo forest from July 2020 to January 2022. We analyzed the organic matter composition of Moso bamboo leaf/root litter and soil by using pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) technique. The results showed that short-term N deposition significantly increased the relative content of soil phenols by 50.9%, while significantly decreased fatty acids by 26.3%. The rela-tive content of alkanes & alkenes and lignin in leaf litter was significantly increased by 51.9% and 33.5%, respectively, while that of phenols and polysaccharides significantly decreased by 52.2% and 56.3%. In root litter, eleva-ted N significantly decreased the relative content of polycyclic aromatic hydrocarbons by 16.6%. Moreover, the relative content of fatty acids in soil organic matter was significantly positively correlated with the relative content of poly-saccharides in leaf litter. The relative content of phenols in soil organic matter was significantly positively correlated with the relative content of lignin, and negatively correlated with the relative content of polysaccharides in leaf litter. Our results demonstrated that short-term N deposition did not change the concentration of total organic carbon, total nitrogen, and C/N of the soil, leaf litter, and root litter, but significantly altered the chemical composition of organic matter. In addition, the changes in chemical composition of organic matter in soil under short-term N deposition were affected by the composition of organic matter in leaf litter.