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Proper management of soil organic matter (SOM) is needed for maintaining soil fertility and for mitigation of the global increase in atmospheric CO₂ concentrations and should be informed by knowledge about the sources, spatial organisation and stabilisation processes of SOM. Recently, microbial biomass residues (i.e. necromass) have been identified as a significant source of SOM. Here, we propose that cell wall envelopes of bacteria and fungi are stabilised in soil and contribute significantly to small-particulate SOM formation. This hypothesis is based on the mass balance of a soil incubation experiment with ¹³C-labelled bacterial cells and on the visualisation of the microbial residues by means of scanning electron microscopy (SEM). At the end of a 224-day incubation, 50% of the biomass-derived C remained in the soil, mainly in the non-living part of SOM (40% of the added biomass C). SEM micrographs only rarely showed intact cells. Instead, organic patchy fragments of 200–500 nm size were abundant and these fragments were associated with all stages of cell envelope decay and fragmentation. Similar fragments, developed on initially clean and sterile in situ microcosms during exposure to groundwater, provide clear evidence for their formation during microbial growth and surface colonisation. Microbial cell envelope fragments thus contribute significantly to SOM formation. This origin and the related macromolecular architecture of SOM are consistent with most observations on SOM, including the abundance of microbial-derived biomarkers, the low C/N ratio, the water repellency and the stabilisation of biomolecules, which in theory should be easily degradable.

Mineral-associated organic matter (MAOM) is a key component of the global carbon (C) and nitrogen (N) cycles, but the processes controlling its formation from plant litter are not well understood. Recent evidence suggests that more MAOM will form from higher quality litters (e.g., those with lower C/N ratios and lower lignocellulose indices), than lower quality litters. Shoots and roots of the same non-woody plant can provide good examples of high and low quality litters, respectively, yet previous work tends to show a majority of soil organic matter is root-derived. We investigated the effect of litter quality on MAOM formation from shoots versus roots using a litter-soil slurry incubation of isotopically labeled (¹³C and ¹⁵N) shoots or roots of Big Bluestem (Andropogon gerardii) with isolated silt or clay soil fractions. The slurry method minimized the influence of soil structure and maximized contact between plant material and soil. We tracked the contribution of shoot- and root-derived C and N to newly formed MAOM over 60 days. We found that shoots contributed more C and N to MAOM than roots. The formation of shoot-derived MAOM was also more efficient, meaning that less CO₂ was respired per unit MAOM formed. We suggest that these results are driven by initial differences in litter chemistry between the shoot and root material, while results of studies showing a majority of soil organic matter is root-derived may be driven by alternate mechanisms, such as proximity of roots to mineral surfaces, greater contribution of roots to aggregate formation, and root exudation. Across all treatments, newly formed MAOM had a low C/N ratio compared to the parent plant material, which supports the idea that microbial processing of litter is a key pathway of MAOM formation.

The halotolerant green alga Dunaliella salina is widely cultivated in the industrial-scale production of natural β-carotene (containing 9-cis and all-trans isomers) due to its high carotenoid content. The management of environmental factors such as nitrogen deficiency is an efficient way to induce the production of β-carotene. However, it is not clear how changes in nitrogen concentration and the resulting changes in the carbon–nitrogen (C/N) ratio affect the β-carotene isomer configuration ratio and content. In this study, D. salina HG01 was cultured under different C/N ratios, growth parameters, β-carotene content and the ratio of two isomers, intracellular substance content, and antioxidant enzyme activity were determined. The results showed that although a large amount of β-carotene was accumulated in algae cells under a high C/N ratio, the high C/N ratio was not conducive to the increase of β-carotene content in unit culture volume and the proportion of 9-cis-β-carotene. However, the β-carotene content in per unit of culture volume (14.70 μg mL−1) under a medium C/N ratio and the proportion of 9-cis-β-carotene (39.31%) under a low C/N ratio were relatively higher. In addition, transcriptome sequencing revealed changes in the expression of genes related to photosynthesis, carbon and nitrogen. This study integrates the physiological and transcriptomics data related with β-carotene configuration changes in D. salina under different C/N ratios, which has practical implications for the industrial production of β-carotene. By optimizing the C/N ratio during cultivation, it is possible to enhance β-carotene content and improve the desired 9-cis isomer proportion. These findings can contribute to the development of more efficient and cost-effective production methods for natural β-carotene, thereby benefiting D. salina-based β-carotene industry.

Wildfires burn approximately 1% of boreal forest yearly, being one of the most significant factors affecting soil organic matter (SOM) pools. Boreal forests are largely situated in the permafrost zone, which contains half of global soil carbon (C). Wildfires advance thawing of permafrost by burning the insulating organic layer and decreasing surface albedo, thus increasing soil temperatures. Fires also affect SOM quality through chemical and physical changes, such as the formation of resistant C compounds. The long-term post-fire effects on SOM quality, degradability and isotopic composition are not well known in permafrost forests. We studied the effect of forest fires on the proportional sizes of SOM pools with chemical fractionation (extracting with water, ethanol and acid) of soil samples (5, 30 and 50 cm depths) collected from a fire chronosequence in the upland mineral soils of the Canadian permafrost zone. We also determined the ¹³C and ¹⁵N isotopic composition of soil after fire. In the topsoil horizon (5 cm) recent fire areas contained a smaller fraction of labile SOM and were slightly more enriched with ¹⁵N and ¹³C than older fire areas. The SOM fraction ratios reverted towards pre-fire status with succession. Changes in SOM were less apparent deeper in the soil. Best predictors for the size of recalcitrant SOM fraction were active layer depth, vegetation biomass and soil C/N ratio, whereas microbial biomass was best predicted by the size of the recalcitrant SOM fraction. Results indicated that SOM in upland mineral soils at the permafrost surface could be mainly recalcitrant and its decomposition not particularly sensitive to changes resulting from fire.

Microbial oils are lipids produced by oleaginous microorganisms, which can be used as a potential feedstock for oleochemical production. The oleaginous yeast Rhodotorula toruloides can co-produce microbial oils and high-value compounds from low-cost substrates, such as xylose and acetic acid (from hemicellulosic hydrolysates) and raw glycerol (a byproduct of biodiesel production). One step towards economic viability is identifying the best conditions for lipid production, primarily the most suitable carbon-to-nitrogen ratio (C/N). Here, we aimed to identify the best conditions and cultivation mode for lipid production by R. toruloides using various low-cost substrates and a range of C/N ratios (60, 80, 100, and 120). Turbidostat mode was used to achieve a steady state at the maximal specific growth rate and to avoid continuously changing environmental conditions (i.e., C/N ratio) that inherently occur in batch mode. Regardless of the carbon source, higher C/N ratios increased lipid yields (up to 60% on xylose at a C/N of 120) but decreased the specific growth rate. Growth on glycerol resulted in the highest specific growth and lipid production (0.085 g lipids/gDW*h) rates at C/Ns between 60 and 100. We went on to study lipid production using glycerol in both batch and fed-batch modes, which resulted in lower specific lipid production rates compared with turbisdostat, however, fed batch is superior in terms of biomass production and lipid titers. By combining the data we obtained in these experiments with a genome-scale metabolic model of R. toruloides, we identified targets for improvements in lipid production that could be carried out either by metabolic engineering or process optimization.

PurposeThe control of the exogenous carbon-induced soil-priming effect (PE) by soil microbes, carbon, nitrogen, and carbon/nitrogen is still uncertain. To examine the relationship between diverse soil properties and the PE, the research was conducted using soils from forest, cropland, and grassland ecosystems.MethodsWe introduced a solution of 13C-labeled glucose (containing 6 atom% 13C) into soils collected from three distinct ecosystems. For the control group, we added an equal amount of water to the soils. Subsequently, all treatment and control samples were incubated at 60% of their water holding capacity and maintained at a temperature of 25 °C for a period of 28 days.ResultsThe magnitude of priming on native SOC was significantly higher in grassland ecosystems than in forest and cropland ecosystems. The results of structural equation modelling revealed a significant positive association of the PE with the soil carbon/nitrogen ratio, bacterial diversity, and community composition, as well as a negative association of the PE with SOC, dissolved organic carbon, and total nitrogen. Network analysis showed that the keystone taxa for each ecosystem were different. Sphingomonas, SBR1031, BD2-11-terrestrial-group, and Sebacina were the keystone taxa significantly positively associated with the PE, whereas Solirubrobacter, Bacillus, and Preussia were the keystone taxa significantly negatively associated with the PE.ConclusionOur findings are significant for studying carbon fluxes, improving soil carbon dynamics models, and understanding soil microbe, carbon, and nitrogen relationships with SOC mineralization. This understanding is crucial for mitigating climate change, promoting sustainable land management, and enhancing soil carbon stabilization.

Purpose The bacterial phylum Verrucomicrobia plays important roles in biogeochemical cycling processes, while the ecology of this phylum is still unclear. Previous elevational studies mainly focused on whole bacterial communities, while no study exclusively picked out Verrucomicrobia. Our objectives were to investigate the abundance, diversity and community composition of soil Verrucomicrobia across an elevation gradient on Changbai Mountain. Materials and methods In total, 24 soil samples representing six elevation gradients were collected. Primer set 515F/806R was used for PCR amplifications and sequencing was conducted on an Illumina HiSeq2000 platform. Data sets comprising of Verrucomicrobial phylum were culled from all quality sequences for the further analyses of Verrucomicrobial diversity and community composition. Results and discussion The relative abundance of Verrucomicrobia accounted for ~20% of the total bacterial communities, and Spartobacteria and DA101 were the most dominant class and genus, respectively. Verrucomicrobia community composition differed significantly among elevations, while the Verrucomicrobia diversity showed no apparent trend along elevation although the richness of some classes or genera significantly changed with elevation. The Verrucomicrobial community composition, diversity, and relative abundance of specific classes or genera were significantly correlated with soil pH and carbon/nitrogen ratio (C:N ratio). Conclusions These results indicated that Verrucomicrobia were abundant in Changbai Mountain soils, and Verrucomicrobial elevational distribution was strongly influenced by soil pH and C:N ratio. Our results also provide potential evidence that the dominant genus DA101 occupies different ecological niches and performs oligotrophic life history strategy in soil environment.

Soil carbon models typically scale decomposition linearly with soil carbon (C) concentration, but this linear relationship has not been experimentally verified. Here we investigated the underlying biogeochemical mechanisms controlling the relationships between soil C concentration and decomposition rates. We incubated a soil/sand mixture with increasing amounts of finely ground plant residue in the laboratory at constant temperature and moisture for 63 days. The plant residues were rye (Secale cereale, C/N ratio of 23) and wheat straw (Triticum spp., C/N ratio of 109) at seven soil C concentrations ranging from 0.38 to 2.99%. We measured soil respiration, dissolved organic carbon (DOC) concentrations, microbial biomass, and potential enzyme activities over the course of the incubation. Rye, which had higher N and DOC contents, lost 6 to 8 times more C as CO₂ compared to wheat residue. Under rye and wheat amendment, absolute C losses as CO₂ (calculated per g dry soil) increased linearly with C concentration while relative C losses as CO₂ (expressed as percent of initial C) increased with C concentration following a quadratic function. In low C concentration treatments (0.38–0.79% OC), DOC decreased gradually from day 3 to day 63, microbial C increased towards the end in the rye treatment or decreased only slightly with straw amendment, and microbes invested in general enzymes such as proteases and oxidative enzymes. At increasing C levels, enzyme activity shifted to degrading cellulose after 15 days and degrading microbial necromass (e. g. chitin) after 63 days. At the highest C concentrations (2.99% OC), microbial biomass peaked early in the incubation and remained high in the rye treatment and decreased only slightly in the wheat treatment. While wheat lost C as CO₂ constantly at all C concentrations, respiration dynamics in the rye treatment strongly depended on C concentration. Our results indicate that litter quality and C concentration regulate enzyme activities, DOC concentrations, and microbial respiration. The potential for non-linear relationships between soil C concentration and decomposition may need to be considered in soil C models and soil C sequestration management approaches.

Organic composts such as \"bokashi\", obtained from the fermentation of bran mixtures and inoculated with microorganisms, improve soil characteristics. In Brazil, the most widely used formulation for the production of this compost is obtained from a mixture of wheat and castor bean bran, but both have a high monetary cost. Replacing these components with regionally available sources represents the possibility of reducing costs and making more sustainable use of this waste. The aim of this study was to analyze the chemical characteristics and determine the availability of nitrogen for the plants. The study was divided into two stages, consisting of an incubation test in the laboratory and a bioassay in the greenhouse using forage sorghum as an indicator species. In the laboratory trial, the treatments consisted of two raw material sources with a low C/N ratio (castor bean bran—CAB and cottonseed bran—COB), corresponding to 40% of the mixture; three sources with a high C/N ratio (wheat bran—WHB or rice bran—RIB), gradually replaced by passion fruit peel bran—PFPB), corresponding to 60% of the mixture. The materials were mixed, moistened, inoculated with microorganisms (Embiotic ® ) and kept in sealed containers with a capacity of 620 cm 3 for 21 days. In the greenhouse, in addition to the aforementioned treatments, seven controls were included: no addition of organic and synthetic N sources; ammonium nitrate; CAB; COB; WHB; RIB and PFPB. In the second stage, dry mass production and N content in sorghum plant tissues were determined, and the rates of N availability were estimated. It was found that the pH of the standard compost was 4.75, and in the other formulations it ranged from 4.62 to 5.3, the highest values being observed when WHB was fully replaced by RIB There was a significant difference in the EC values, but all were well below the value considered adequate. Replacing CAB with COB and WHB with RIB and PFPB resulted in a reduction in N content and an increase in the C:N ratio. Replacing WHB with PFPB led to an increase in K content and a reduction in P and Mg content. In the bioassay, the highest biomass production was in the treatments with the fermented composts, and the highest biological recovery of N was obtained in the ammonium nitrate treatment, followed by the CAB, COB and WHB treatments.

PurposeThe main objective of this study was to understand the spatial characteristics of the ecological stoichiometry of carbon (C), nitrogen (N), and phosphorous (P) and their driving factors in farmland soils.Materials and methodsData were obtained from 7223 sampling sites of farmland topsoil (0–20 cm) collected by the soil testing and formulated fertilization project in 2012. Soil organic C was determined by potassium dichromate (K2Cr2O7) oxidation with heating in an oil bath. Total N was analyzed using an automatic Kjeldahl nitrogen analyzer. Total P was determined using HClO4–H2SO4 digestion followed by a Mo–Sb colorimetric assay.Results and discussionThe mean soil C/N, C/P, and N/P ratios were 11.76, 38.06, and 3.37, respectively and showed moderate variations. The spatial autocorrelation distances were larger for C/N and smaller for C/P and N/P. The nugget/sill ratios for soil C/N, C/P, and N/P were 79.43, 52.09, and 55.73%, respectively. Soil C/P and N/P ratios showed a similar spatial distribution with high heterogeneity, whereas the C/N ratio distribution was relatively smooth. Both C/N and N/P ratios were driven by N application rate, but the C/P ratio was more influenced by straw returning pattern.ConclusionsThe ecological stoichiometry of C, N, and P was affected by elevation, latitude, soil pH, soil type, parent material, straw returning pattern, and N application rate in farmland soils in Poyang Lake Plain. Soil C/N and N/P ratios were mainly limited by total N, whereas the soil C/P ratio was mainly limited by total P.