Organic Carbon Stability Research Articles

Evidence for preservation of organic carbon interacting with iron in material displaced from retrogressive thaw slumps: Case study in Peel Plateau, western Canadian Arctic

In northern high latitudes, rapid warming is set to amplify carbon-climate feedbacks by enhancing permafrost thaw and biogeochemical transformation of large amounts of soil organic carbon. However, between 30 % and 80 % of permafrost soil organic carbon is considered to be stabilized by geochemical interactions with the soil mineral pool and thus less susceptible to be emitted as greenhouse gases. Quantification of the nature of and controls on mineral-organic carbon interactions is needed to better constrain permafrost-carbon-climate feedbacks, particularly in ice-rich environments resulting in rapid thaw and development of thermokarst landforms. On sloping terrain, mass wasting features called retrogressive thaw slumps are amongst the most dynamic forms of thermokarst. These multi-decadal disturbances grow due to ablation of an ice-rich headwall, and their enlargement due to warming of the Arctic is mobilizing vast stores of previously frozen materials. Here, we investigate headwall profiles of seven retrogressive thaw slumps and sediments displaced from these mass wasting features from the Peel Plateau, western Canadian Arctic. The disturbances varied in their headwall height (2 to 25 m) and affected land surface area (<1 to > 30 ha). We present total and water extractable mineral element concentrations, mineralogy, and mineral-organic carbon interactions in the headwall layers (active layer, permafrost materials above an early Holocene thaw unconformity, and Pleistocene-aged permafrost tills) and in displaced material (suspended sediments in runoff and material accumulated on the debris tongue). Our data show that the main mechanism of organic carbon stabilization through mineral-organic carbon interactions within the headwall is the complexation with metals (mainly iron), which stabilizes 30 ± 15 % of the total organic carbon pool with higher concentrations in near-surface layers compared to deep permafrost. In the displaced material, this proportion drops to 18 ± 5 %. In addition, we estimate that up to 12 ± 5% of the total organic carbon is stabilized by associations to poorly crystalline iron oxides, with no significant difference between near-surface layers, deep permafrost and displaced material. Our findings suggest that the organic carbon interacting with the sediment mineral pool in slump headwalls is preserved in the material mobilized by slumping and displaced as debris. Overall, up to 32 ± 6 % of the total organic carbon displaced by retrogressive thaw slumps is stabilized by organo-mineral interactions in this region. This indicates that organo-mineral interactions play a significant role in the preservation of organic carbon in the material displaced from retrogressive thaw slumps over years to decades after their development resulting in decadal to centennial scale sequestration of this retrogressive thaw slump-mobilized organic carbon interacting with the soil mineral pool.

Effects of acid rain and root exclusion on soil organic carbon in Cunninghamia lanceolata and Michelia macclurei plantations.

Acid rain is one of most serious environment problems in China. The types of acid rain have gradually transformed from sulfuric acid rain (SAR) to mixed acid rain (MAR) and nitric acid rain (NAR) in recent years. Roots are one source of soil organic carbon and play an important role in the formation of soil aggregates. However, the changes in acid rain type and the effect of root removal on soil organic carbon in forest ecosystems are poorly understood. In this study, we removed roots and simulated acid rain with different types (SO42-:NO3- ratio of 4:1, 1:1, and 1:4) for three years in Cunninghamia lanceolata (CP) and Michelia macclurei (MP) plantations to analyze the changes of soil organic carbon and physical properties and to measure the size and mean weight diameter (MWD) of aggregates. Results showed that root removal in C. lanceolata and M. macclurei remarkably reduced soil organic carbon pool by 16.7 % and 21.5%, and soil recalcitrant carbon by 13.5% and 20.0%, respectively. Root removal substantially decreased the MWD and proportion and organic carbon content of soil macroaggregates in M. macclurei, but not in C. lanceolata. Acid rain did not affect soil organic carbon pool and soil aggregate structures. Our results indicated that roots promote soil organic carbon stabilization and that their contribution to the stability of soil organic carbon varies with forest types. Moreover, soil organic carbon stabilization is not affected by different types of acid rain in the short term.

Carbon stability and morphotype composition of biochars from feedstocks in the Mekong Delta, Vietnam

This study investigates carbon stability of biochars produced from wood, herbaceous, and fruit agricultural waste products from southern Vietnam. The biochars were produced by pyrolysis to 500 °C, 700 °C, and 900 °C at a heating rate of 10 °C/min. Organic geochemistry and petrology have well-established measurable parameters that define degree of preservation of organic carbon in the Earth's crust. These parameters are utilized, comparatively, to infer the organic carbon stability in biochars relative to that preserved in the geological carbonaceous rocks. The stability of carbon was assessed by measuring incident-light random reflectance (%Ro) to determine the degree of thermally induced aromatization/ordering of the biochar carbon molecules. Additionally, the organic carbon content of the biochars was quantitatively separated into reactive and non-reactive carbon fractions based on thermodynamic stability of CH and CO bonds. The results show increasing carbon stability with increasing pyrolysis temperature and indicate high stability has been achieved at 700 °C and 900 °C where the biochars have the highest total organic carbon (TOC) contents and lowest Hydrogen Index values. Occurrence of a small quantity of readily degradable, labile free hydrocarbons (soluble organic matter; SOM) is due to condensation of the thermally generated hydrocarbons on the surfaces of the biochars. This fraction is minimized in the biochars produced at 700 °C and 900 °C due to volatilization of the free hydrocarbons at higher pyrolysis temperatures. The relatively substantial content of labile hydrocarbons associated with particulate organic matter (POM) in the 500 °C produced biochars is attributed to the incomplete pyrolyzation of the feedstock at this temperature. This fraction declines to near zero at 700 °C and 900 °C as complete carbonization is achieved at these higher temperatures. The overall carbon budget within the biochar shows that irrespective of the biochar type, over 97% of the TOC consists of highly refractory, residual carbon that geochemically is considered to possess long-term stability (“inert”). This is further underpinned by a pronounced increase in biochar mean random reflectance from <2.33% at 500 °C to >4.35% when pyrolyzed at 700 °C, indicating a highly polyaromatized condensed carbon structure. Rice husk is an exception as it yields a mean reflectance of 4.55% at 500 °C suggesting that pyrolysis temperature is not the only factor controlling %Ro and hence carbon stability. Furthermore, the biochar compositions show a relationship between feedstock and pyrolysis temperature. The biochar morphotype composition changes significantly with increasing temperature, but the analyzed biochars are all dominated by fusinoid/solid (primarily fusinoid) morphotypes with the highest initial quantity recorded at 500 °C. This quantity decreases with increasing pyrolysis temperature while porous morphotypes (tenuinetwork, crassinetwork, mixed porous, mixed dense) increase. Variations in morphotype composition observed between the biochars is attributed to the structural nature of the feedstocks.

Effect of Biochar on the Stability of Soil Organic Carbon in the Context of Global Warming

Since the industrial revolution, the mean global temperature has increased by more than 0.6°C, and this has led to serious ecoenvironmental problems, such as melting ice sheets, retreating glaciers, species extinction and extreme weather-climate events, which have substantially affected human social life. The issue of how to reduce global warming has become a worldwide concern. Soil is one of the primary carbon emission sources, and maintaining the stability of soil organic carbon (SOC) plays a crucial part in reducing soil carbon emission. Soil-biochar has great potential as a natural carbon sequestration solution. Biochar produces positive/negative stimulation effects on soil organic carbon when applied to soil. In this review, the direct and indirect effects of biochar on the soil environment are described, with a focus on priming effects and influencing factors of biochar. In terms of direct effects, biochar and its surface dissolved organic carbon can directly replenish the organic carbon source required by the soil carbon cycle. In terms of indirect effects, biochar can influence SOC mineralization by affecting the species, number, and activity of microorganisms in soil. Soil properties, vegetation type, biochar raw material, and pyrolysis temperature as well as their aging effects are the key factors affecting soil-biochar carbon sequestration. This review will contribute to the understanding of the contribution of biochar to the reduction of soil emissions.

The Impact of Profile Genesis and Land Use of Histosol on Its Organic Substance Stability and Humic Acid Quality at the Molecular Level

This study is designed to evaluate soil organic matter (SOM) quality indicators: molecular indicators of dissolved organic matter (DOM) and hydrophobicity of humic acid (HA), distribution of quantity in humified and labile fractions of histosols during renaturalization. The aim is to determine the differences in the qualitative composition of humic acids at the molecular level, which are decided by the previous tillage and genesis, and to evaluate the impact of anthropogenization on the peat soil according to hydrophobicity, as well as to estimate the impact of soil genesis and removing peat layer. Soil samples were taken from the three Sapric Histosol (according to WRB2022) profiles and the 0–30 cm layer in three field replicates (Lithuania, Radviliskis mun.). Our study suggested that in the differently managed drained Sapric Histosol under renaturalization, the most significant changes occurred in the topsoil layer (0–30 cm), in which an increase in the content of SOM particles 106–2 µm in size. It is expedient to grow perennial grasses and legumes to maintain the soil organic carbon stability mobile humic acids to mobile fulvic acids ratio (MHA:MFA 0.83 to 0.86). An evaluation of the quality of HA (E4:E6) revealed their highest maturity in the unfertilized perennial grasses (3.88) and crop rotation (3.87) with grasses. The highest concentrations of hydrophilic groups (ratio of the C=O to O-H) were found in Sapric Histosol under deciduous hardwood forest (12.33). The lowest hydrophilicity (9.25 and 9.36) was of the crop rotation Sapric Histosol with removed peat layer. The most sustainable use of drained Sapric Histosol in the context of the sustainability and quality of its humus substances should be associated with the formation of perennial grass and clover grassland and the cultivation of deciduous hardwood. Therefore, the horizon forms on the top part of the profile, which protects deeper Histosolic material layers from its mineralization.

Soil organic carbon stability of vegetation restoration during 11-year-old grassland succession

Soil organic carbon stability of vegetation restoration during 11-year-old grassland succession

Tradeoffs of microbial life history strategies drive the turnover of microbial-derived organic carbon in coastal saline soils.

Stable soil organic carbon (SOC) formation in coastal saline soils is important to improve arable land quality and mitigate greenhouse gas emissions. However, how microbial life-history strategies and metabolic traits regulate SOC turnover in coastal saline soils remains unknown. Here, we investigated the effects of microbial life history strategy tradeoffs on microbial carbon use efficiency (CUE) and microbial-derived SOC formation using metagenomic sequencing technology in different salinity soils. The results showed that high-salinity is detrimental to microbial CUE and microbial-derived SOC formation. Moreover, the regulation of nutrients stoichiometry could not mitigate adverse effects of salt stress on microbial CUE, which indicated that microbial-derived SOC formation is independent of stoichiometry in high-salinity soil. Low-salinity soil is dominated by a high growth yield (Y) strategy, such as higher microbial biomass carbon and metabolic traits which are related to amino acid metabolism, carbohydrate metabolism, and cell processes. However, high-salinity soil is dominated by stress tolerance (S) (e.g., higher metabolic functions of homologous recombination, base excision repair, biofilm formation, extracellular polysaccharide biosynthesis, and osmolytes production) and resource acquisition (A) strategies (e.g., higher alkaline phosphatase activity, transporters, and flagellar assembly). These trade-offs of strategies implied that resource reallocation took place. The high-salinity soil microbes diverted investments away from growth yield to microbial survival and resource capture, thereby decreasing biomass turnover efficiency and impeding microbial-derived SOC formation. Moreover, altering the stoichiometry in low-salinity soil caused more investment in the A-strategy, such as the production of more β-glucosidase and β-N-acetyl-glucosaminidase, and increasing bacterial chemotaxis, which thereby reduced microbial-derived SOC formation. Our research reveals that shift the microbial community from S- and A- strategies to the Y-strategy is important to increase the microbial CUE, and thus enhance SOC turnover in coastal saline soils.

Measuring the stability of soil organic carbon in Arenosols in the Senegalese Groundnut Basin

Soil organic carbon (SOC) contributes to agrosystem productivity. Understanding how farming practices implemented by smallholders affect the levels and distribution of SOC in carbon (C) pools with different stabilities is essential in sub-Saharan Arenosols where SOC mineralization is intense. The stability of SOC was studied by thermal (Therm-C), physical (particulate organic matter >50 μm, POM-C and fine soil fractions <50 μm, FF-C), chemical (permanganate-oxidizable carbon, POX-C) and biological (mineralizable C, Min-C) approaches. Soil samples were collected at depths of 0–10 and 10–30 cm in cultivated fields (out- or home-fields) without any input, with millet residues, amended with manure, or with household organic wastes. Globally, average SOC contents were low (<6 g C kg-1). The variability in SOC and C pool contents was sensitive to field management. The different approaches to measuring the stability of SOC did not measure the same fraction of SOC. POM-C and Therm-C were correlated and both explained Min-C similarly, thus suggesting that in these sandy soils, POM-C or Therm-C probably measured comparable properties of the stability of C. The lack of relationships between POX-C and other pools suggested that POX-C encompassed a different nature of SOC while providing complementary information on the biogeochemical stability of SOC.

Role of silt and clay fractions in organic carbon and nitrogen stabilization in soils of some old fruit orchards in the Nile floodplain, Sohag Governorate, Egypt

PurposeStudy the effect of both silt and clay fractions on stabilizing organic carbon and total nitrogen in soils of some old cultivated fruit orchards in Sohag, Egypt.MethodsEleven surface soil samples (0–15 cm) were collected. Silt (63–2 μm) and clay (< 2 μm) fractions were separated from soils and analyzed for cation exchange capacity (CEC), organic carbon (OC), total nitrogen (TN), specific surface area (SSA), X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopic analysis were determined using the standard analysis methods.ResultsThe silt and clay were responsible for 77.49 to 93.34% of soil CEC. The SSA of clay was twice the silt, and the phyllosilicates (%) average was 29.83% and 62.70% in silt and clay. Other minerals, such as quartz, feldspar, and carbonate minerals, were detected in these fractions. The phyllosilicate species of clay were smectite-illite mixed layers (44.4–88.9%), Smectite (1.0–6.4% except one sample recorded 45.7%), Kaolinite (3.7–10.5%) and kaolinite/smectite mixed layers (2.0–8.2%). The average OC-silt and clay were 6.79 and 8.96 g OC kg−1 soil (38.01 and 50.28% of soil OC). On average, the total nitrogen (TN) in silt and clay particles was 0.817 and 1.156 g N kg−1 soil (35.88 and 50.66% of soil TN). The OC and N content significantly and positively correlated with CEC, SSA, and phyllosilicates in the fine fractions.ConclusionResults showed that the soil's capacity to sequester OC and N is affected significantly by its fine particles' content, characteristics, and mineralogy.

The impact of agricultural management on soil aggregation and carbon storage is regulated by climatic thresholds across a 3000 km European gradient.

Organic carbon and aggregate stability are key features of soil quality and are important to consider when evaluating the potential of agricultural soils as carbon sinks. However, we lack a comprehensive understanding of how soil organic carbon (SOC) and aggregate stability respond to agricultural management across wide environmental gradients. Here, we assessed the impact of climatic factors, soil properties and agricultural management (including land use, crop cover, crop diversity, organic fertilization, and management intensity) on SOC and the mean weight diameter of soil aggregates, commonly used as an indicator for soil aggregate stability, across a 3000 km European gradient. Soil aggregate stability (-56%) and SOC stocks (-35%) in the topsoil (20 cm) were lower in croplands compared with neighboring grassland sites (uncropped sites with perennial vegetation and little or no external inputs). Land use and aridity were strong drivers of soil aggregation explaining 33% and 20% of the variation, respectively. SOC stocks were best explained by calcium content (20% of explained variation) followed by aridity (15%) and mean annual temperature (10%). We also found a threshold-like pattern for SOC stocks and aggregate stability in response to aridity, with lower values at sites with higher aridity. The impact of crop management on aggregate stability and SOC stocks appeared to be regulated by these thresholds, with more pronounced positive effects of crop diversity and more severe negative effects of crop management intensity in nondryland compared with dryland regions. We link the higher sensitivity of SOC stocks and aggregate stability in nondryland regions to a higher climatic potential for aggregate-mediated SOC stabilization. The presented findings are relevant for improving predictions of management effects on soil structure and C storage and highlight the need for site-specific agri-environmental policies to improve soil quality and C sequestration.

Climate warming alters the relative importance of plant root and microbial community in regulating the accumulation of soil microbial necromass carbon in a Tibetan alpine meadow.

Climate warming is predicted to considerably affect variations in soil organic carbon (SOC), especially in alpine ecosystems. Microbial necromass carbon (MNC) is an important contributor to stable soil organic carbon pools. However, accumulation and persistence of soil MNC across a gradient of warming are still poorly understood. An 8-year field experiment with four levels of warming was conducted in a Tibetan meadow. We found that low-level (+0-1.5°C) warming mostly enhanced bacterial necromass carbon (BNC), fungal necromass carbon (FNC), and total MNC compared with control treatment across soil layers, while no significant effect was caused between high-level (+1.5-2.5°C) treatments and control treatments. The contributions of both MNC and BNC to soil organic carbon were not significantly affected by warming treatments across depths. Structural equation modeling analysis demonstrated that the effect of plant root traits on MNC persistence strengthened with warming intensity, while the influence of microbial community characteristics waned along strengthened warming. Overall, our study provides novel evidence that the major determinants of MNC production and stabilization may vary with warming magnitude in alpine meadows. This finding is critical for updating our knowledge on soil carbon storage in response to climate warming.

Combining Rock-Eval® thermal analysis and thermochemolysis to evaluate the influence of forest management on soil organic matter quality and stability

The effect of forest management practices on carbon quality is poorly documented. To assess changes in the quality and stability of soil organic carbon (SOC) of a temperate forest upon human activities, we investigated soil from forests (i) developed following natural regeneration after clearcutting 20 and 40 years ago, (ii) developed following afforestation on an abandoned crop area 40 years ago and (iii) in an area where regular clear-cut (with wood residues input) was conducted 40 years ago. Topsoil and subsoil layers were collected (0–20 cm and 50–80 cm). Soil organic matter (OM) was characterized by elemental analysis (total carbon and total nitrogen), thermal analysis (Rock-Eval®) and thermochemolysis (i.e., Py-GC/MS in the presence of tetramethylammonium hydroxide (TMAH)). In addition, a size fractionation to separate the labile coarse fraction (50–2000 μm) from the fine fraction (<50 µm) was performed. These fractions were analyzed by thermal analysis.Despite no measurable differences in carbon and nitrogen contents, the characterization of the OM by thermal analysis, and the relative quantification of OM compounds revealed differences in the composition in OM for the topsoil layers. The thermal analysis clearly distinguished sites with inputs of woody residues (higher HI) with a higher relative contribution of lignin and cutin/suberin compounds. However, the OM thermal stability seems mainly controlled by the organo-mineral interactions rather than chemical composition. Combination of Rock-Eval® thermal analysis and Py-GC/MS suggests that thermal stability cannot be used as an indicator of stability in specific contexts where pedogenetic processes are deeply modified by regular and extensive anthropogenic inputs of woody residues.

The efficiency of the hard wood origin biochar addition on the PAHs bioavailability and stability in sediment

Polluted sediments with organic pollutants like PAHs represent a potential danger to environment, human health and potential obstacle to water management. Removal of polluted sediment provides the potential for reuse of nutrients from sediment, for crop production or for materials in building industry. The purpose of this study was to determine the efficacy of using hard wood biochar for the immobilization of bioaccessible polycyclic aromatic hydrocarbons (PAHs) in historically polluted sediment. The main question is would carbon materials' presence (in this case biochar) have influence on PAH bioavailability and their stability in sediment. This is important because the world trend is to go for sediment reuse in agriculture purposes, and biochar is proven to be good amendment for increasing soil organic carbon (SOC) stabilization and increasing soil carbon stock. The manuscript provides a detailed consideration of the supersorption performance of the biochar and PAH sequestration in different types of PAHs with aging period up to 6 months in ex situ treatment. The efficiency of biochar to sequester the PAHs was evaluated by assessing the bioavailable fraction (Frap) using desorption method with Amberlite XAD4 resin assistance. In untreated sediment, Frap ranged from 22% up to 42% for 2–4 rings, and around 9% for 5–6 rings PAHs. 180-days amendment of biochar led to a further decrease in the bioavailable portion of PAHs. The results of this laboratory study shown that biochar produced from hard wood gave promising results for binding and further stabilizing PAH in historically polluted sediments.

Elucidating the composition of organic matter in water-repellent forest soils using analytical pyrolysis combined with gas chromatography–mass spectrometry (Py-GC–MS)

Soil water repellency (SWR) is an inability of soil to spontaneously absorb water. It affects the spatial distribution of soil moisture, may cause formation of preferential flow, increase surface runoff and the risk of water erosion, or affect nutrient availability for crops. Although there is a general agreement among published data that SWR is caused by hydrophobic components of soil organic matter (SOM), more comprehensive information is needed. Additionally, the link between SWR occurrence and SOM genesis has not been studied sufficiently. In this work, we analyzed SOM in water-repellent soils by pyrolysis combined with gas chromatography and mass spectrometry (Py-GC–MS). The samples were taken from two forest ecosystems in Slovakia: relatively dry pine–oak forest, located in the Záhorská nížina lowland (Záhorie), and spruce forest in the High Tatras with a colder and more humid climate. The results showed that the composition of SOM at both sites is dominated by partially decomposed residues of plant biomass. Whereas most compounds detected in the Záhorie samples were identified as pyrolysis products of lignin, the pyrolysates of the High Tatras soils originated mainly from polysaccharide-like substances. Although lauric (12:0), myristic (14:0) and palmitic (16:0) fatty acids were detected in pyrograms of water-repellent samples, the results suggest that the residues of lignin decomposition may contribute to SWR as well. Pyrolysates such as methylguaiacol, vinylguaiacol, dimethylphenol, or the methyl ester of vanillic acid are examples of less polar lignin-like moieties. Py-GC–MS data, as well as other soil properties, suggest that SWR at both sites is associated with the accumulation of certain SOM components in the topsoil. Acidic pH, higher C/N ratio and coarse soil texture prevent organic carbon stabilization in the studied soils. As a result, SOM is in great part composed of accumulated particulate residues with lignin-like origin and lipidic coatings, which contribute to SWR development during drier conditions.

Microbial necromass carbon in estuarine tidal wetlands of China: Influencing factors and environmental implication

Microbial necromass is an important component of the stable soil organic carbon (SOC) pool. However, little is known about the spatial and seasonal patterns of soil microbial necromass and their influencing environmental factors in estuarine tidal wetlands. In the present study, amino sugars (ASs) as biomarkers of microbial necromass were investigated along the estuarine tidal wetlands of China. Microbial necromass carbon (C) contents were in the range of 1.2–6.7 mg g−1 (3.6 ± 2.2 mg g−1, n = 41) and 0.5–4.4 mg g−1 (2.3 ± 1.5 mg g−1, n = 41), which accounted for 17.3–66.5 % (44.8 % ± 16.8 %) and 8.9–45.0 % (31.0 % ± 13.7 %) of the SOC pool in the dry (March to April) and wet (August to September) seasons, respectively. At all sampling sites, fungal necromass C predominated over bacterial necromass C as a component of microbial necromass C. Compared to bacterial necromass C, fungal necromass C showed a stronger connection with ferrous oxides (Fe2+) and total Fe concentrations. Both fungal and bacterial necromass C contents revealed large spatial heterogeneity and declined in the estuarine tidal wetlands with the increase in latitude. Statistical analyses showed that the increases in salinity and pH in the estuarine tidal wetlands suppressed the accumulation of soil microbial necromass C.

Soil Organic Carbon Stabilization Is Dominated by Non‐Sorptive Process Among the Subsoils From Different Parent Material

AbstractSoil parent material can strongly influence soil's physical and chemical properties, significantly affecting soil organic carbon (SOC) stabilization. However, the importance of different soil parent materials on the amount and composition of subsoil carbon in subtropical soils remains poorly qualified. Subsoil SOC (below A horizon) is chosen for this study because of its higher proportion of stable carbon than surface soils in A horizon. Here, we investigated the dependence of soil mineral‐associated organic carbon (MAOC) and its three mineral‐stabilized fractions on soil physical and chemical parameters (clay content, cation exchange capacity (CEC) and metal oxides) in the subsoil soil (B and C horizons) originating from three contrasting parent materials (sandshale, granite, limestone). The results showed that MAOC accounts for an average of 88% of total SOC, of which organo‐metal complexes (extracted with sodium pyrophosphate) are the dominant form at all three sites. MAOC and its three fractions were significantly correlated with soil clay, CEC, sodium pyrophosphate extractable Fe and oxalate extractable Fe concentrations (P &lt; 0.05), which varied significantly with soil parent material. Since the sodium pyrophosphate plus oxalate‐ extracted mineral‐associated carbon accounted for more than 70% of MAOC, and the molar ratio between sodium pyrophosphate or oxalate‐ extracted mineral‐associated carbon and Fe (C:Fe) ranged from 2.7 to 17.1. The formation of Fe‐associated organic complex is dominated by co‐precipitation when C:Fe is &gt;1. Therefore we conclude that non‐sorptive interactions with the mineral phase through co‐precipitation are the dominant mechanism for SOC stabilization in subtropical forest soils.

The reducing effect of aglime onN2OandCO2emissions balance from an acidic soil: A study on intact soil cores

AbstractThe functioning of the nitrous oxide (N2O) reductase enzyme involved in the last step of denitrification is pH sensitive, with an optimum of 6.8. A solution to mitigate N2O emissions would be to bring soil pH close to neutrality by adding agricultural liming products (aglime). Nevertheless, the influence of aglime on the soil greenhouse gas (GHG) balance (CO2–N2O) is a subject of debate, particularly when considering the fate of the carbon (C) derived from carbonates. Our objective was to investigate the results of the effect of calcium carbonate (CaCO3) aglime on the CO2–N2O balance. Sixteen cylinders of undisturbed acidic soil were taken from a sandy loam profile and incubated at 20°C for 107 days in anaerobic conditions (water‐filled pore space &gt;60%). Eight limed treatment cylinders received 1.45 g of aglime on the soil surface (2 t NV ha−1) and 0.08 g of N (100 kg of N ha−1). Eight control treatment cylinders received only 0.08 g of N. N2O and CO2fluxes were measured and converted into CO2equivalents to perform a GHG balance calculation. Furthermore, soil and leachate properties were measured. Aglime application triggered a reduction of N2O emissions, probably due to an increase in soil pH at the beginning of the experiment, which would have led to the N2O reductase activation. High ‐N content in the soil may inhibit the high N2O reduction potential in the limed treatment. CO2emissions were unexpectedly lower in the limed treatment. Aglime addition did not enhance C mineralisation, which may be explained by the possible stabilisation of soil organic carbon. A significant 11.3% reduction of GHG emissions was observed in the limed treatment. Overall, our results show that a strategy of liming acidic agricultural soil could be implemented for its potential in GHG mitigation. Nevertheless, future in‐depth research is necessary to better understand the fate of the C brought about by aglime.

Long-term tillage and irrigation effects on aggregation and soil organic carbon stabilization mechanisms

Sustainable soil management practices are required in agriculture to enhance carbon sequestration and restore soil functions. Here, the aim was to investigate the effect of different tillage practices combined with or without irrigation on (i) soil organic carbon (SOC) content, (ii) fungal biomass and their relationships with aggregate size classes in the soil surface layer; further, (iii) the concept of soil particle saturation with SOC was tested to evaluate if a threshold was reached in a 14 year-experiment. Our hypothesis was that long-term irrigation, intensive tillage and their combination, would negatively affect soil aggregation and SOC stabilization. The experiment has started in 2003 on a research farm in Canterbury, New Zealand. The present work focused on two contrasting tillage practices –intensive tillage with 20–25 cm ploughing (IT) and direct drill (DD)– combined with sprinkler-irrigated and non-irrigated (hereafter called Rainfed) conditions in a split-plot experimental design. Soil samples (0–5 cm layer) were analyzed for pore size distribution, specific surface area and microbial biomass. Further, wet sieving was used to isolate large macroaggregates (LM, > 2000 μm), small macroaggregates (SM, 250–2000 μm), microaggregates (m, 53–250 μm), particle sized silt + clay fractions (s+c, < 53 μm) and Fine20 particles (<20 μm), followed by the analysis of aggregate morphology and SOC quantification in them. Results showed that both DD and Rainfed management increased total SOC content of the bulk soil. Only the LM fraction and the SOC therein (OC-LM) increased significantly in DD compared to IT, while m and s+c fractions and OC-m and OC-s+c did not differ between treatments. Macroaggregate breakdown processes and measured SOC therein had likely not reached steady-state conditions, as suggested by the lack of any SOC differences in the aggregate size classes < 250 µm. In contrast, the Fines20:SOC ratio differentiated between soils that had reached (i.e., DD) or not reached (i.e., IT) the saturation threshold. Finally, it was observed that a higher fungal:bacteria (F:B) ratio was generally accompanied by a greater LM fraction and mean weight aggregate diameter, highlighting the importance of fungi in the formation of LM. These results suggested that our hypothesis of detrimental effects on soil aggregation and SOC accumulation of both tillage and irrigation was not fully demonstrated yet. A longer study period would be required to better understand the effects of such practices of SOC storage.

Soil organic carbon stability mediate soil phosphorus in greenhouse vegetable soil by shifting phoD-harboring bacterial communities and keystone taxa

Addition of organic amendments, such as manure and straw, to arable fields as a partial substitute for mineral phosphorus (P), are a sustainable practice in high-efficiency agricultural production. Different organic inputs may induce varied soil organic carbon (OC) stability and phoD harboring microbes, subsequently regulate P behavior, but the underlying mechanisms are poorly understood. A 11-year field experiment examined P forms by 31P-nuclear magnetic resonance (NMR), OC chemical composition by 13C NMR, and biologically-based P availability methods, phoD bacterial communities, and their co-occurrence in soils amended with chemical P fertilizer (CF), chemical P partly substituted by organic amendments including pig manure (CM), a mixture of pig manure and corn straw (CMS), and corn straw (CS), with equal P input in all treatments. Organic amendments significantly increased soil labile Pi (CaCl2-P, citrate-P, 2.91–3.26 and 1.16–1.32 times higher than CF) and Po (enzyme-P, diesters, 4.08–7.47 and 1.71–2.14 times higher than CF) contents and phosphatase activities, while significantly decreased aromaticity (AI) and recalcitrance indexes (RI) of soil C, compared with CF. The keystone genera in manured soils (Alienimomas and Streptomyces) and straw-applied soils (Janthinobacterium and Caulobacter) were significantly correlated with soil enzyme-P, microbial biomass P (MBP), diesters, and citrate-P. Soil AI and RI were significantly correlated with the phoD keystone and soil P species. It suggested that the keystone was impacted by soil OC stability and play a role in regulating P redistribution in amended soils. This study highlights how manure and straw incorporation altered soil OC stability, shaped the phoD harboring community, and enhanced soil P biological processes promoted by the keystone taxa. The partial substitution of mineral P by mixture of manure and straw is effectively promote soil P availability and beneficial for environmental sustainability.

Soil organic carbon pool distribution and stability with grazing and topography in a Mongolian grassland

Livestock grazing and topography have long been recognized as essential factors that control soil organic carbon (SOC) dynamics. However, the interplay effects of grazing and topography on SOC pool distribution and stability are not well understood, hindering the precise prediction of the organic carbon (OC) fate over the hillslope grassland. Here, we examine the relationship between grazing and topography on SOC pool distribution and stability in a Mongolian grassland. We determined a suite of OC metrics (e.g., particulate OC (POC), mineral-associated OC (MAOC), dissolved OC (DOC), and microbial biomass carbon (MBC) along a topographic gradient (e.g., upper slope, middle slope, and downslope) with grazing or not, and incubation was conducted to quantify OC stability. The results showed that POC and SOC content was 106–138% and 24–63% greater in the grazed than the ungrazed site at low topographic positions, respectively. Across the landscape, more variations in POC and SOC across the topographic gradient were observed at the grazed sites than at ungrazed sites. The MBC content of the downslope was greater in the grazing than in ungrazed sites. Furthermore, the humic-like substances in the grazing area were lower than those in the enclosed area and showed a significant and positive linear relationship with the aromaticity and humification of DOC. The aggregated boosted tree (ABT) analysis showed that the POC and MBC were the most important OC components to affect OC pool stability (accounting for 40.87%). The structural equation model (SEM) supported that the varies of POC regulated the downstream soil biochemical processes and changed SOC pool stability. Collectively, the results suggest the POC could be primarily responsible for the changes in SOC pools and stability in a hillslope grassland. These findings enhanced our understanding of the ecological mechanisms in SOC sequestration.