Soil Organic Matter Stabilization Research Articles

Microbial trait multifunctionality drives soil organic matter formation potential

Soil microbes are a major source of organic residues that accumulate as soil organic matter, the largest terrestrial reservoir of carbon on Earth. As such, there is growing interest in determining the microbial traits that drive soil organic matter formation and stabilization; however, whether certain microbial traits consistently predict soil organic matter accumulation across different functional pools (e.g., total vs. stable soil organic matter) is unresolved. To address these uncertainties, we incubated individual species of fungi in soil organic matter-free model soils, allowing us to directly relate the physiological, morphological, and biochemical traits of fungi to their soil organic matter formation potentials. We find that the formation of different soil organic matter functional pools is associated with distinct fungal traits, and that ‘multifunctional’ species with intermediate investment across this key grouping of traits (namely, carbon use efficiency, growth rate, turnover rate, and biomass protein and phenol contents) promote soil organic matter formation, functional complexity, and stability. Our results highlight the limitations of categorical trait-based frameworks that describe binary trade-offs between microbial traits, instead emphasizing the importance of synergies among microbial traits for the formation of functionally complex soil organic matter.

The Crop Succession Systems Under No-Tillage Alters the Surface Layer Soil Carbon Stock and Stability

The main challenge of the no-tillage system (NTS) is to reconcile productivity, the maintenance of surface residues, and the stabilization of soil organic matter (SOM). To address this challenge, particularly in tropical regions, various cover crops have been tested. The objective of this study was to test the effects of agricultural crop succession systems on the stock and stability of soil organic carbon in different surface layers of the soils. The research was carried out in the state of Goiás, Brazil, in an experiment set up in 2016, designed in randomized blocks with a split-plot scheme (treatments and soil layers), comprising four repetitions (blocks). The treatments (plots) consisted of crops grown in succession to soybean, which were as follows: T1—soybean/corn (Zea mays); T2—soybean/pearl millet (Pennisetum glaucum); T3—soybean/Urochloa ruziziensis (brachiaria); and T4—corn + Urochloa ruziziensis. The subplots represented the following soil layers: 0–5, 5–10, 10–20, and 20–40 cm. We evaluated the biomass dry mass and the soil parameters such as soil density, total porosity, and light organic matter across all layers. The organic carbon, grain size fractionation (mineral-associated organic carbon—MOC; sand-sized carbon—POC), and isotopic composition (δ13C) were determined in the 0–5 and 5–10 cm layers. The highest biomass dry production was observed in the soybean/pearl millet succession, which reduced the soil density and increased the total porosity in the surface layer. The soybean/pearl millet treatment produced high amounts of light organic matter, particularly in the 0–5 cm layer, a result also found for the soybean/brachiaria and soybean/corn + brachiaria systems. The crop successions did not alter the soil carbon stock or stability; however, the surface layer stored the highest amount of carbon, with elevated total organic carbon values and carbon stocks and stability (MOC and POC). Overall, in this study, replacing corn with other crops in succession with soybean did not affect the stock or stability of soil organic carbon. The species grown in succession with soybean contributed to the higher surface carbon stock and stability, promoting the formation of more stable and recalcitrant carbon.

Role of aromatic and non-protonated aromatic carbon in the stability of soil organic matter

Research on soil organic matter (SOM) in its natural state is vital for comprehending the mechanisms governing soil stability and carbon cycling, crucial in addressing global climate change. We utilized solid-state 13C nuclear magnetic resonance (NMR) spectroscopy on HF treated samples, along with a 260-day laboratory mineralization experiment and thermal analysis-programmed pyrolysis (PP) to evaluate SOM biodegradability and thermal stability. To cover the potential range of organic carbon variability, we selected samples from three land uses and soil types, featuring total organic carbon levels from 1% to 39%. Our analysis confirmed the substantial contribution of non-protonated aromatic-carbon not bonded to oxygen to SOM’s biological and thermal stability, constituting approximately 14–21% of soil organic carbon. These components exhibited a strong correlation with SOM stability matrices, such as thermal stability and oxygen index determined by PP. Samples with a higher prevalence of these components also displayed the lowest cumulative carbon mineralization. These findings enhance our understanding of the stable SOM pool, aiding in the identification of sustainable soil management practices to mitigate climate change impacts on soil health and carbon dynamics.

The Use of Integrated Crop–Livestock Systems as a Strategy to Improve Soil Organic Matter in the Brazilian Cerrado

This study aimed to analyze the carbon (C) stock and stabilization of soil organic matter in particulate- and mineral-associated fractions across different land use systems after 32 years of experimentation in the Brazilian Cerrado. The experiment was established in 1991 and was performed in Planaltina-DF. The treatments evaluated included continuous pasture with monoculture grasses; integrated crop–livestock systems under no tillage; continuous cropping under no tillage; minimum tillage; and the preservation of the native Cerrado biome in its original condition. Soil sampling was performed to a depth of 30 cm. Carbon and nitrogen (N) stocks were quantified for the years 2001, 2009, 2013, and 2023, with soil organic matter fractionation performed on samples from 2023. Land use change resulted in significant losses of soil C and N in areas managed with conventional soil preparation practices. Systems that promote plant diversity, such as integrated crop–livestock systems, enhanced soil C and N stocks (72.8 and 5.5 Mg ha−1, respectively) and increased both particulate organic matter and mineral-associated fractions, most of which were in more stabilized forms. Integrated crop–livestock systems are management practices that offer an effective alternative to present methods in terms of combating climate change and supporting ecosystem sustainability.

Protective role of reactive aluminum phases to stabilize soil organic matter against long-term cultivation in the humid tropics under volcanic influence

ABSTRACT Cultivation can cause rapid depletion of soil organic matter (SOM), particularly in the humid tropics. Understanding the factors controlling SOM storage is a key step toward effective soil management for sustainable crop production. While clay + silt content is known to be an important factor for soil organic carbon (SOC) storage, recent studies have shown greater importance of oxalate-extracted Al (Alo; roughly corresponds to short-range-order aluminosilicate minerals and organo-Al complexes) and Fe (Feo). However, the extent to which these mineralogical factors control the response of SOM pool to land-use changes remains unclear, especially in the tropics. This study aimed to clarify the factors regulating SOM content in croplands compared to secondary forests or home gardens in Negros Occidental, Philippines. Along two line transects having a wide range of Alo contents, soil samples were collected at a depth of 0 − 10 cm from sugarcane sites (n = 33; long-term continuous cultivation of >70 years) that were paired with either secondary forests (n = 10) or home gardens (n = 20). Sugarcane sites had significantly lower SOC and total nitrogen (N) contents than secondary forest and home garden sites, whereas there was no clear difference between the latter two land uses. Both SOC and total N contents showed significant positive correlations with Alo content and, to a much less extent, with Feo content, but not with clay + silt content. The slope of the regression line between SOC and Alo was indifferent between sugarcane and the other two land uses, while the intercept was significantly lower in the sugarcane sites. These results suggest the protective role of Alo phases in both agricultural and forest soils and the presence of unprotected SOM (e.g. plant litter) in the latter sites. Furthermore, δ13C analysis suggested that the proportion of forest (C3 plant) derived carbon remaining in the sugarcane soils ranged between 10% and 50% and increased with Alo content. Together, our study showed indirect evidence of Alo-induced stabilization of SOM against long-term cultivation under the humid tropical environment.

Physicochemically protected organic carbon release is the rate-limiting step of rhizosphere priming in paddy soils

Iron oxides affect the stability of soil organic matter (SOM), which in turn affects greenhouse gas emissions in paddy soils. They also regulate the direction and magnitude of the rhizosphere priming effect (RPE) by restricting SOM accessibility and microbial activity. However, the controlling steps and key factors that regulate the RPE magnitude under anoxic conditions are unknown. In this study, we investigated the mechanisms through which Fe(III) reduction affects the RPE using humic acid as an electron shuttle in paddy soils and conducting continuous 13CO2 labeling of rice plants. The RPE, measured via CO2 emission, was approximately 25 % greater in soils with humic acid than in soils without. A rapid increase in the RPE of CH4 emissions after 41 days was attenuated in soils containing humic acid. Root growth and Fe(III) reduction stimulated the total primed CO2 emissions from the rhizosphere independent of the microbial biomass and enzyme activities. Humic acid accelerated Fe(III) reduction, leading to a decrease in Fe-bound organic carbon and an increase in RPE (CO2 emissions). The rhizosphere-primed CO2 emissions decreased with increasing amounts of reactive Fe(III) (oxyhydr)oxides, which protected the SOM from microbial and enzymatic attacks. Biochemical Fe(III) reduction and physical aggregate destruction controlled the abiotic transformation of inaccessible SOM into bioavailable organic carbon, thereby regulating the RPE. The results suggest that the reduction of reactive Fe(III) minerals is the rate-limiting step in the release of the physicochemically protected SOM, which in turn determines the magnitude of rhizosphere priming in paddy soils.

Higher Stability of Soil Organic Matter near the Permafrost Table in a Peatland of Northeast China

Understanding the stability of soil organic matter (SOM) is essential for making accurate predictions regarding carbon release rates. However, there is limited information on the role of chemical composition of dissolved organic matter (DOM) in SOM stability. To address this gap, the peatland soil profile in the discontinuous frozen soil region of Northeast China was selected as the focus of this research, and a comprehensive analysis was conducted on the differences between the molecular composition of DOM and the stability of SOM. The results indicate a significant carbon accumulation phenomenon near the permafrost table. Through analyses using TG-50, δ13C, and δ15N, it was determined that SOM near the permafrost table exhibits high stability, whereas SOM within the permafrost layer demonstrates poor stability. Investigations utilizing UV-vis, 3D-EEM, FT-IR, and 1H-NMR technologies revealed that DOM near the permafrost table is of high quality and highly aromatic. Furthermore, compared to near the permafrost table, humic acid materials in the permafrost layer decreased by 17%, while protein materials increased by 17%. These findings offer a novel perspective on the understanding of SOM stability in peatland soil profiles within discontinuous permafrost regions.

How the effect of earthworms on soil organic matter mineralization and stabilization is affected by litter quality and stage of soil development

Globally soil fauna consumes about half of the annual litter fall. An important question is how this activity affects the mineralization and stabilization of soil organic matter. Here we explore how much earthworms influence the decomposition of litter and the stabilization of organic matter in soils at various stages of soil development (various soil age) that are supplied with litter of various quality. The laboratory mesocosms consist of litter and a mineral layer. The mineral soils originated either from spruce and alder stands growing either on post-mining soils (young soils after about 50 years of soil development) or from soils in the close vicinity of post-mining sites (mature soils with several thousand years of soil development), the mineral soils were supplied by matching litter, the mesocosms were either without earthworms or with two individuals of earthworms. The earthworm effect showed statistically significant interaction with tree and soil age: earthworms increased respiration in both alder soils, but in spruce soils only in mature soil, while the opposite was true for young soils. In general, earthworms promoted the removal of litter from the soil surface and carbon accumulation in the mineral soil. Earthworms promoted C storage in mineral associated organic matter (MAOM), especially in young spruce soils. The results suggested that earthworm activity in young soils which were far from saturation (spruce on post-mining soils) promotes soil C sequestration by promoting C storage in MAOM, whereas earthworms in mature, C saturated soils tend to promote soil respiration. More broadly, earthworms effect on soil depends on stage of soil C saturation.

Effect of Long-Term Fertilization Practices on the Stability of Soil Organic Matter in the Northeast Black Soil Region in China

Soil organic matter (SOM) is an important carbon pool in terrestrial ecosystems and plays a key role in soil functions. Nevertheless, the effects of fertilization practices on the physical, chemical, biological, and comprehensive stability of SOM are still unclear. We carried out a long-term field experiment in the northeast black soil region in China with four different fertilization practices: no fertilizer (CK), single chemical fertilizer (NPK), chemical fertilizer + straw (NPKJ), and chemical fertilizer + organic manure (NPKM). The content of particulate organic matter (POM) and mineral-associated organic matter (MAOM), compound composition of SOM, carbon mineralization characteristics, active soil organic matter (ASOM), and inert soil organic matter (ISOM) were tested. The results showed that the application of fertilizers significantly increased the contents of POM and MAOM to 2.59–4.65 g kg−1 and 32.69–34.65 g kg−1 (p < 0.05), but decreased the MAOM/POM values by 37.8–42.4%, indicating reduced the physical stability of SOM. Fertilization practices increased the contents of aromatic, nitrogen-containing compounds and decreased the oxygen compounds of SOM, representing enhancement of the chemical stability. The contents of ASOM and ISOM increased in fertilization practices, while the biological stability index (BSI) under the NPKJ and NPKM treatments was lower than the CK treatment, suggesting that the biological stability decreased under the manure and straw application. In addition, the comprehensive stability of SOM increased by 26–116% through a reduction in the physical and biological stability, coupled with an increase in the chemical stability. Collectively, our study demonstrated that the application of manure and straw enhanced both the comprehensive stability and content of SOM and reduced the physical and biological stabilities while increasing the chemical stability, which made the largest contribution to the comprehensive stability.

A Review of Analytical Techniques to Characterise Nanomaterial Associations with Minerals, Organic Matter and Organisms

Nanomaterials (NMs) have unique properties and control processes relevant to the fate of contaminants in soils, air, and aquatic systems and within the carbon cycle. Many NMs often occur in association with larger mineral grains, organic matter, or living organisms such as microbes, plants and fungi. The preservation of the spatial, textural, chemical, and mineralogical relations between NMs and minerals, organic matter, and organism (NM‐associations) is of fundamental importance as it provides information about the origin and formation mechanisms of NMs. Here we review analytical approaches and techniques to study NM‐associations at the bulk‐, micro‐, nano‐ and atomic‐scale. We will focus on (a) X‐ray diffraction and mass‐spectroscopy techniques; (2) automatisms within software packages that permit the search of features without operators; (3) preparation and analytical techniques such as the focused‐ion beam technology, transmission electron microscopy and atom probe tomography; (4) nano‐spectroscopic techniques such as tip‐enhanced Raman spectroscopy, synchrotron infrared nanospectroscopy, and nano‐X‐ray fluorescence spectroscopy; (5) ptychographic X‐ray computer tomography. This review paper concludes with selected new perspectives such as (a) the characterisation of NM‐precursors, (b) the role of NM‐associations in the stabilisation of soil organic matter and (c) the interaction of NM‐associations in wildfire smoke with contaminants from other sources.

Nutrient removal efficacy and microbial dynamics in constructed wetlands using Fe(III)-mineral substrates for low carbon-nitrogen ratio sewage treatment.

This study evaluated the roles of two common sources of Fe(III)-minerals-volcanic rock (VR) and synthetic banded iron formations from waste iron tailings (BIF-W)-in vertical flow-constructed wetlands (VFCWs). The evaluation was conducted in the absence of critical environmental factors, including Fe(II), Fe(III), and soil organic matter (SOM), using metagenomic analysis and integrated correlation networks to predict nitrogen removal pathways. Our findings revealed that Fe(III)-minerals enhanced metabolic activities and cellular processes related to carbohydrate decomposition, thereby increasing the average COD removal rates by 10.7% for VR and 5.90% for BIF-W. Notably, VR improved nitrogen removal by 1.70% and 5.40% compared to BIF-W and the control, respectively. Fe(III)-mineral amendment in bioreactors also improved the retention of denitrification and nitrification bacteria (phylum Proteobacteria) and anammox bacteria (phylum Planctomycetes), with increases of 3.60% and 3.20% using VR compared to BIF-W. Metagenomic functional predictionindicated that the nitrogen removal mechanisms in VFCWs with low C/N ratios involve simultaneous partial nitrification, ANAMMOX, and denitrification (SNAD). Network-based analyses and correlation pathways further suggest that the advantages of Fe(III)-minerals are manifested in the enhancement of denitrification microorganisms. Microbial communities may be activated by the functional dissolution of Fe(III)-minerals, which improves the stability of SOM or the conversion of Fe(III)/Fe(II). This study provides new insights into the functional roles of Fe(III)-minerals in VFCWs at the microbial community level, and provides a foundation for developing Fe-based SNAD enhancement technologies.

EDDS application destabilizes soil organic matter in phytoremediation: Insights from quantity and molecular composition of dissolved organic matter

The stability of soil organic matter (SOM) is crucial for metal transport and carbon cycling. S,S-ethylenediaminedisuccinic acid (EDDS) is widely used to enhance phytoremediation efficiency for heavy metals in contaminated soils, yet its specific impacts on SOM have been underexplored. This study investigates the effects of EDDS on SOM stability using a rhizobox experiment with ryegrass. Changes in soil dissolved organic matter (DOM) quantity and molecular composition were analyzed via Fourier transform ion cyclotron resonance mass spectrometry. Results showed that the use of EDDS increased the uptake of Cu, Cd and Pb by ryegrass, but simultaneously induced the destabilization and transformation of SOM. After 7 days of EDDS application, dissolved organic carbon (DOC) and nitrogen (DON) concentrations in rhizosphere soils increased significantly by 3.44 and 10.2 times, respectively. In addition, EDDS reduced lipids (56.3%) and proteins/amino sugars-like compounds (52.1%), while increasing tannins (9.11%) and condensed aromatics-like compounds (24.4%) in the rhizosphere DOM. These effects likely stem from EDDS's dual action: extracting Fe/Al from SOM-mineral aggregates, releasing SOM into the DOM pool, and promoting microbial degradation of bioavailable carbon through chain scission and dehydration. Our study firstly revealed that the application of EDDS in phytoremediation increased the mineralization of SOM and release of CO2 from soil to the atmosphere, which is important to assess the carbon budget of phytoremediation and develop climate-smart strategy in future.

Применение биоугля на основе отработанных грибных блоков в условиях агротемно-серых почв

Recycling of organic waste from mushroom production will improve their environmental safety and optimize the applied agricultural technologies. The aim of the work was to assess the environmental performance of the agricultural technology of using a soil fertilizing material – biochar obtained from spent lignin-cellulose mushroom blocks. It was found that the introduction of biochar from mushroom production waste materials into Gray-Luvic Phaeozems Hortic helps stabilize soil organic matter and increase the productivity of lawn grasses. It was noted that the quality and agroecological efficiency of biochar obtained from spent mushroom blocks based on cereal straw are significantly higher than biochar from spent mushroom blocks based on sunflower husk. Keywords: SOIL ORGANIC MATTER, CO2 EMISSION, SUNFLOWER HUSK, LAWN GRASSES, OYSTER MUSHROOM

Reducing the Sodium Adsorption Ratio Improves the Soil Aggregates and Organic Matter in Brackish-Water-Irrigated Cotton Fields

The assessment of soil health relies on key parameters such as soil aggregates and organic matter content. Therefore, examining the impact of irrigation water ion composition and variations in salinity on soil aggregates and organic matter is imperative, which is key to developing a theoretical basis for the sustainable utilization of saline water resources, particularly in extremely arid regions. This experiment was conducted to investigate the impact of different irrigation water salinity treatments (T3: 3 g/L, T5: 5 g/L, and T7: 7 g/L) on the root zone soil of cotton fields. Each salinity treatment included three variations of the sodium adsorption ratio (SAR) at S10: 10 (mmol/L)1/2, S15: 15 (mmol/L)1/2, and S20: 20 (mmol/L)1/2. Local freshwater irrigation served as the control, resulting in a total of 10 treatments. Our findings show that the soil Ca2+ and Mg2+ content increased with higher irrigation water salinity but decreased with increasing irrigation water SAR. The relative macroaggregate stability and the content of water-stable macroaggregates and soil organic matter (SOM) decreased as the irrigation water salinity and SAR increased. In comparison to T3S20, T5S10 did not improve the soil Na+ content but significantly increased the soil Ca2+ content by 147.76%, while the water-stable aggregate and SOM saw a notable increase of 7.66% and 9.86%, respectively. Reducing the SAR in brackish water lessens its negative impact on soil aggregates in cotton fields. This is primarily because Ca2+ counteracts the dispersive effect of high Na+ concentrations and promotes aggregate formation. Irrigation water with a salinity of 3 g/L and an SAR of 10 (mmol/L)1/2 positively affected the stabilization of soil aggregates and organic matter.

Mechanisms of soil organic carbon and nitrogen stabilization in mineral-associated organic matter – insights from modeling in phase space

Abstract. Understanding the mechanisms of plant-derived carbon (C) and nitrogen (N) transformation and stabilization in soil is fundamental for predicting soil capacity to mitigate climate change and support other soil functions. The decomposition of plant residues and particulate organic matter (POM) contributes to the formation of mineral-associated (on average more stable) organic matter (MAOM) in soil. MAOM is formed from the binding of dissolved organic matter (ex vivo pathway) or microbial necromass and bioproducts (in vivo pathway) to minerals and metal colloids. Which of these two soil organic matter (SOM) stabilization pathways is more important and under which conditions remains an open question. To address this question, we propose a novel diagnostic model to describe C and N dynamics in MAOM as a function of the dynamics of residues and POM decomposition. Focusing on relations among soil compartments (i.e., modeling in phase space) rather than time trajectories allows isolating the fundamental processes underlying stabilization. Using this diagnostic model in combination with a database of 36 studies in which residue C and N were tracked into POM and MAOM, we found that MAOM is predominantly fueled by necromass produced by microbes decomposing residues and POM. The relevance of this in vivo pathway is higher in clayey soils but lower in C-rich soils and with N-poor added residues. Overall, our novel modeling in phase space proved to be a sound diagnostic tool for the mechanistic investigation of soil C dynamics and supported the current understanding of the critical role of both microbial transformation and mineral capacity for the stabilization of C in mineral soils.

Control of landscape position on organic matter decomposition via soil moisture during a wet summer

Sustainable cropland management requires preservation of soil organic matter (SOM). In spite of in depth understanding gained from ample field and laboratory studies, we have a poor understanding of landscape scale spatial variation of fresh organic matter (OM) decomposition and its conversion into soil organic carbon (SOC). Particularly, local topographic position may be expected to co-control these processes via soil hydrology. In this study, we sought to identify if such control is significant by setting up a field experiment with two contrasting positions across 10 gently sloping cropland fields covering three different soil texture groups, i.e. loamy sand, (sandy) loam and silt loam. We wanted to link OM decomposition to within-field differences in soil moisture, whilst keeping variation in other soil and management factors minimal. Specifically, mesocosms with 13C enriched ryegrass (the OM source) were incorporated in the fields for ten weeks and afterwards, soil was separated into > 500 µm, 53 – 500 µm and < 53 µm sized fractions. Overall, we found that lower located positions were wetter than higher positions with average differences of 11 %, 20 % and 16 % in water-filled pore space for the loamy sand, (sandy) loam and silt loam soil, respectively. Mineralization of added OM was surprisingly independent of landscape position, even though moisture conditions appeared wetter than optimal at the low but not at the high landscape positions. Remaining ryegrass residues > 500 µm did follow local topography-driven gradients in soil moisture with higher amounts in low landscape positions. In other words, drier conditions at high landscape positions improved coarse OM decomposition, with consequently more ryegrass-carbon (C) ending up in finer soil fractions (< 500 µm). Additionally, soil texture affected decomposition of the smallest fraction (< 53 µm) with a stabilizing effect for finer-textured (silt loam) soils. We conclude that, despite significant contrasts in moisture conditions between landscape positions, within-field spatial variability of OM mineralization was overall limited during the observed wet summer period. Nevertheless, landscape position affected the quality of remnant unmineralized C, with relatively more conversion of freshly added OM into OM associated with silt and clay at the drier higher positions, potentially improving the long-term stability of SOM. Likewise observations under different weather conditions are needed to evaluate the necessity of precise modelling of local soil hydrology for predicting SOC stock evolution on the landscape scale.

Variations in source-specific soil organic matter components across 32 forest sites in China

Forest soils store substantial amounts of carbon in various soil organic matter (SOM) components due to high plant litter inputs and active microbial turnover. However, the variations in plant- and microbial-derived SOM components in surface and subsurface forest soils across a wide geographic scale remain poorly understood. This study investigated the SOM components from aboveground and belowground plant inputs and fungal and bacterial necromass in surface (soil0–5 cm) and subsurface (soil5–10 cm) soils across 32 forest sites in China and analyzed their relationships with climate and edaphic factors. Compared to soil0–5 cm, soil5–10 cm exhibited lower soil organic carbon content and cutin biomarker concentration but higher concentrations of fungal necromass carbon and lignin phenols. Higher mean annual precipitation led to higher concentrations of cutin and suberin biomarkers in soil0–5 cm and soil5–10 cm, respectively. Higher soil organic carbon content was associated with lower plant-derived lignin biomarkers, higher lignin oxidation degrees, and increased microbial necromass-derived amino sugars across sites, highlighting the pivotal role of microbial necromass in SOM stabilization. Additionally, both fungal and bacterial necromass decreased with increasing mineral weathering across sites. These insights improve the understanding of environmental drivers of source-specific carbon storage in forest soils.Graphical

Molecular Composition of Humic Acids and Soil Organic Matter Stabilization Rate of the First Arctic Carbon Measurement Supersite “Seven Larches”

In the framework of the implementation of the all-Russian climatic project “Carbon measurement test sites”, reference soils of “Seven Larches” carbon supersite, which is a benchmark and the only monitoring site in the Artistic latitudes, were investigated. The morphological structure of representative soils was specified, and it was found that soils are classified as Cryosols of different types (Histic, Stagnic or Gleyic). The basic physico-chemical characteristics of the soils were studied. By means of elemental analysis and 13C NMR spectroscopy, the composition and molecular structure of humic acids from organic and mineral soil horizons were studied. The surface (organic) soil horizons are characterized by high values of H:C molar ratios (1.2–1.3), which indicates a lower degree of the molecular structure aromaticity of humic acids from organic soil horizons. Analysis of the molecular structure of humic acids by 13C NMR spectroscopy showed that humic acids of the studied soils are characterized by the predominance of non-substituted aliphatic (0–47 ppm) and aromatic (108–164 ppm) fragments. Mineral soil horizons are characterized by higher stabilization of organic matter (with lower SOC content—0.5–0.9%) and higher hydrophobicity of humic acid molecules. Comparison of the obtained results with previously published data on the structural and elemental composition of humic acids isolated from soils of similar genesis and geographical location did not reveal any significant differences between the data obtained by us and previously published data. Thus, for “Seven Larches” carbon supersite “reference” parameters of elemental composition of humic acids, their molecular composition and degree of stabilization of soil organic matter were identified.

Mycorrhizal associations of temperate forest seedlings mediate rhizodeposition, but not soil carbon storage, under elevated nitrogen availability.

Tree-mycorrhizal associations are associated with patterns in nitrogen (N) availability and soil organic matter storage; however, we still lack a mechanistic understanding of what tree and fungal traits drive these patterns and how they will respond to global changes in soil N availability. To address this knowledge gap, we investigated how arbuscular mycorrhizal (AM)- and ectomycorrhizal (EcM)-associated seedlings alter rhizodeposition in response to increased seedling inorganic N acquisition. We grew four species each of EcM and AM seedlings from forests of the eastern United States in a continuously 13C-labeled atmosphere within an environmentally controlled chamber and subjected to three levels of 15N-labeled fertilizer. We traced seedling 15N uptake from, and 13C-labeled inputs (net rhizodeposition) into, root-excluded or -included soil over a 5-month growing season. N uptake by seedlings was positively related to rhizodeposition for EcM- but not AM-associated seedlings in root-included soils. Despite this contrast in rhizodeposition, there was no difference in soil C storage between mycorrhizal types over the course of the experiment. Instead root-inclusive soils lost C, while root-exclusive soils gained C. Our findings suggest that mycorrhizal associations mediate tree belowground C investment in response to inorganic N availability, but these differences do not affect C storage. Continued soil warming and N deposition under global change will increase soil inorganic N availability and our seedling results indicate this could lead to greater belowground C investment by EcM-associated trees. This potential for less efficient N uptake by EcM-trees could contribute to AM-tree success and a shift toward more AM-dominated temperate forests.

Compost derived from olive mill cake: Effects on isohumic soil quality based on humic acids characterization

The compost effects on soil organic matter (SOM) stability were evaluated. Manure at 10 % ratio and compost at 10 %, 20 % and 40 % ratios (v/v) were added to the soil and their effects were compared to unamended control soil after 90-days of greenhouse-experiment. Humic acids (HA) and fulvic acids (FA) were extracted from two different soil-sample layers at 0–15 and 15–30 cm depth. The CHA/CFA ratio and the humification parameters were determined, and the soil-HA were characterized by spectroscopic methods (E4/E6 and FTIR). The humification parameters progress with time were affected by the amendment concentration. After 90 days, the treated soils HA’ FTIR spectra showed an increase in aromatic carbon polycondensation and O-containing groups reflecting the high degrees of molecular associations and humification of soil HA. Compared to 10 % manure application and 40 % compost ratio use, the applications of 10 % and 20 % compost ratios induced higher humification level and highly oxidized HA structure. Moreover, changes in the HA compositional and functional groups were noticed at the upper layer which exhibited higher reactivity compared to the lower layer which displayed more humified SOM. Through the humification process, the HA fraction was improved to reach more stable and complex macromolecules, where aromatic structures were bio-converted into highly functionalized compounds.