Wildfire effects on mercury fate in soils of North-Western Siberia.

Arctic soils store 49 Gg mercury (Hg) - an extremely toxic heavy metal, whereas soil Hg can be released to the atmosphere by wildfires. For the first time we investigated the effects of wildfires on the fate of soil Hg in North-Western (NW) Siberia based on GIS maps of areas burned during the last 38 years and a field paired comparison of unburned and burned areas in tundra (mosses, lichens, some grasses, and shrubs) and forest-tundra (multi-layered canopy of larch trees, shrubs, mosses, and lichens). These field surveys were deepened by soil controlled burning to assess the Hg losses from organic horizon and mineral soil. The soil Hg stocks in the organic horizon and in the top 10 cm of the mineral soil were 3.3 ± 0.6 and 16 ± 3 mg Hg m-2 for unburned tundra and forest-tundra, respectively. After the burning by wildfires, the soil Hg stocks decreased to 2.4 ± 0.1 and 6.6 ± 0.2 mg Hg m-2 for tundra and forest-tundra, respectively. By the averages annual burned areas in NW Siberia 527 km2, wildfires in tundra and forest-tundra released 0.19 and 2.9 Mg soil Hg per year, respectively, corresponding to 28 % and 59 % of the initial soil Hg stocks. These direct effects of wildfires on Hg volatilization are raised by indirect post-pyrogenic consequences on Hg fate triggered by the vegetation succession and adsorption of atmospheric Hg on the surface of charred biomass. Charred lichens and trees accumulated 4-16 times more Hg compared to the living biomass. Blackened burned vegetation and soil reduced surface albedo and slowly increased soil temperatures in Arctic after wildfires. This created favorable conditions for seeding grasses and shrubs after wildfire and transformed burned high-latitude ecosystems into greener areas, increasing their capacity to trap atmospheric Hg by vegetation, which partly compensate the burning losses of soil Hg.

Water limitation regulates positive feedback of increased ecosystem respiration.

Terrestrial ecosystem respiration increases exponentially with temperature, constituting a positive feedback loop accelerating global warming. However, the response of ecosystem respiration to temperature strongly depends on water availability, yet where and when the water effects are important, is presently poorly constrained, introducing uncertainties in climate-carbon cycle feedback projections. Here, we disentangle the effects of temperature and precipitation (a proxy for water availability) on ecosystem respiration by analysing eddy covariance CO2 flux measurements across 212 globally distributed sites. We reveal a threshold precipitation function, determined by the balance between precipitation and ecosystem water demand, which separates temperature-limited and water-limited respiration. Respiration is temperature limited for precipitation above that threshold function, whereas in drier areas water limitation reduces the temperature sensitivity of respiration and its positive feedback to global warming. If the trend of expansion of water-limited areas with warming climate over the last decades continues, the positive feedback of ecosystem respiration is likely to be weakened and counteracted by the increasing water limitation.

Limiting Resources Define the Global Pattern of Soil Microbial Carbon Use Efficiency.

Microbial carbon (C) use efficiency (CUE) delineates the proportion of organic C used by microorganisms for anabolism and ultimately influences the amount of C sequestered in soils. However, the key factors controlling CUE remain enigmatic, leading to considerable uncertainty in understanding soil C retention and predicting its responses to global change factors. Here, we investigate the global patterns of CUE estimate by stoichiometric modeling in surface soils of natural ecosystems, and examine its associations with temperature, precipitation, plant-derived C and soil nutrient availability. We found that CUE is determined by the most limiting resource among these four basic environmental resources within specific climate zones (i.e., tropical, temperate, arid, and cold zones). Higher CUE is common in arid and cold zones and corresponds to limitations in temperature, water, and plant-derived C input, while lower CUE is observed in tropical and temperate zones with widespread limitation of nutrients (e.g., nitrogen or phosphorus) in soil. The contrasting resource limitations among climate zones led to an apparent increase in CUE with increasing latitude. The resource-specific dependence of CUE implies that soils in high latitudes with arid and cold environments may retain less organic C in the future, as warming and increased precipitation can reduce CUE. In contrast, oligotrophic soils in low latitudes may increase organic C retention, as CUE could be increased with concurrent anthropogenic nutrient inputs. The findings underscore the importance of resource limitations for CUE and suggest asymmetric responses of organic C retention in soils across latitudes to global change factors.

Not all soil carbon is created equal: Labile and stable pools under nitrogen input.

Anthropogenic activities have raised nitrogen (N) input worldwide with profound implications for soil carbon (C) cycling in ecosystems. The specific impacts of N input on soil organic matter (SOM) pools differing in microbial availability remain debatable. For the first time, we used a much-improved approach by effectively combining the 13C natural abundance in SOM with 21 years of C3-C4 vegetation conversion and long-term incubation. This allows to distinguish the impact of N input on SOM pools with various turnover times. We found that N input reduced the mineralization of all SOM pools, with labile pools having greater sensitivity to N than stable ones. The suppression in SOM mineralization was notably higher in the very labile pool (18%-52%) than the labile and stable (11%-47%) and the very stable pool (3%-21%) compared to that in the unfertilized control soil. The very labile C pool made a strong contribution (up to 60%) to total CO2 release and also contributed to 74%-96% of suppressed CO2 with N input. This suppression of SOM mineralization by N was initially attributed to the decreased microbial biomass and soil functions. Over the long-term, the shift in bacterial community toward Proteobacteria and reduction in functional genes for labile C degradation were the primary drivers. In conclusion, the higher the availability of the SOM pools, the stronger the suppression of their mineralization by N input. Labile SOM pools are highly sensitive to N availability and may hold a greater potential for C sequestration under N input at global scale.

Assembly processes and co-occurrence of bacterial communities in tree rhizosphere under Pb-Zn contamination.

Rhizosphere bacteria are critical for supporting plant performance in stressful environments. Understanding the assembly and co-occurrence of rhizosphere bacterial communities contributes significantly to both plant growth and heavy metal accumulation. In this study, Ligustrum lucidum and Melia azedarach were planted in soils with simulated varying levels of Pb-Zn contamination. The Rhizosphere bacterial communities were investigated by using 16S rRNA gene sequencing. The impacts of Pb-Zn contamination on the diversity and structure of the rhizosphere bacterial community were found to be greater than those of both tree species. The variation in bacterial community structure in both trees was mainly driven by the combinations of Pb-Zn and soil properties. Deterministic processes (non-planted, 82 %; L. lucidum, 73 %; M. azedarach, 55 %) proved to be the most important assembly processes for soil bacterial communities, but both trees increased the importance of stochastic processes (18 %, 27 %, 45 %). The rhizosphere co-occurrence networks exhibited greater stability compared to the non-planted soil networks. Rare taxa played a dominant role in maintaining the stability of rhizosphere networks, as most of the keystone taxa within rhizosphere networks belonged to rare taxa. Dissimilarities in the structure and network complexity of rhizosphere bacterial communities were significantly associated with differences in tree biomass and metal accumulation. These variations in response varied between both trees, with L. lucidum exhibiting greater potential for phytoremediation in its rhizosphere compared to M. azedarach. Our results offer valuable insights for designing effective microbe-assisted phytoremediation systems.

Organic matter composition and stability in estuarine wetlands depending on soil salinity.

Coastal wetlands are key players in mitigating global climate change by sequestering soil organic matter. Soil organic matter consists of less stable particulate organic matter (POM) and more stable mineral-associated organic matter (MAOM). The distribution and drivers of MAOM and POM in coastal wetlands have received little attention, despite the processes and mechanisms differ from that in the upland soils. We explored the distribution of POM and MAOM, their contributions to SOM, and the controlling factors along a salinity gradient in an estuarine wetland. In the estuarine wetland, POM C and N were influenced by soil depth and vegetation type, whereas MAOM C and N were influenced only by vegetation type. In the estuarine wetland, SOM was predominantly in the form of MAOM (> 70 %) and increased with salinity (70 %-76 %), leading to long-term C sequestration. Both POM and MAOM increased with SOM, and the increase rate of POM was higher than that of MAOM. Aboveground plant biomass decreased with increasing salinity, resulted in a decrease in POM C (46 %-81 %) and N (52 %-82 %) pools. As the mineral amount and activity, and microbial biomass decreased, the MAOM C (2.5 %-64 %) and N pool (8.6 %-59 %) decreased with salinity. When evaluating POM, the most influential factors were microbial biomass carbon (MBC) and dissolved organic carbon (DOC). Key parameters, including MBC, DOC, soil salinity, soil water content, aboveground plant biomass, mineral content and activity, and bulk density, were identified as influencing factors for both MAOM abundance. Soil water content not only directly controlled MAOM, but together with salinity also indirectly regulated POM and MAOM by controlling microbial biomass and aboveground plant biomass. Our findings have important implications for improving the accumulation and increased stability of soil organic matter in coastal wetlands, considering the global sea level rise and increased frequency of inundation.

Increased straw return promoted soil organic carbon accumulation in China's croplands over the past 40 years.

Quantifying changes in soil organic carbon (SOC) stocks within croplands across a broad spatiotemporal scale in response to anthropogenic and environmental factors offers valuable insights for sustainable agriculture aimed to improve soil health. Using a validated and widely used soil carbon model RothC, we simulated the SOC dynamics across intensive croplands in China that support ∼22 % of the global population using only 7 % of the global cropland area. The modelling results demonstrate that the optimized RothC effectively captures SOC dynamics measured across 29 long-term field trials during 40 years. Between 1980 and 2020, the average SOC at the top 30 cm in croplands increased from 40 Mg C ha-1 to 49 Mg C ha-1, resulting in a national carbon sequestration of 1100 Tg C, with an average carbon sequestration rate of 27 Tg C yr-1. The annual increase rate of SOC (relative to the SOC stock of the previous year), starting at <0.2 % yr-1 in the 1980s, reached around 0.4 % yr-1 in the 1990s and further rose to about 0.8 % yr-1 in the 2000s and 2010s. Notably, the eastern and southern regions, comprising about 40 % of the croplands, contributed about two-thirds of the national SOC gain. In northeast China, SOC slightly decreased from 58 Mg C ha-1 in 1980 to 57 Mg C ha-1 in 2020, resulting in a total decline of 28 Tg C. Increased organic C inputs, particularly from the straw return, was the crucial factor in SOC increase. Future strategies should focus on region-specific optimization of straw management. Specifically, in northeast China, increasing the proportion of straw returned to fields can prevent further SOC decline. In regions with SOC increase, such as the eastern and southern regions, diversified straw utilization (e.g., bioenergy production), could further mitigate greenhouse gas emissions.

Soil organic matter priming: The pH effects.

Priming of soil organic matter (SOM) decomposition by microorganisms is a key phenomenon of global carbon (C) cycling. Soil pH is a main factor defining priming effects (PEs) because it (i) controls microbial community composition and activities, including enzyme activities, (ii) defines SOM stabilization and destabilization mechanisms, and (iii) regulates intensities of many biogeochemical processes. In this critical review, we focus on prerequisites and mechanisms of PE depending on pH and assess the global change consequences for PE. The highest PEs were common in soils with pH between 5.5 and 7.5, whereas low molecular weight organic compounds triggered PE mainly in slightly acidic soils. Positive PEs up to 20 times of SOM decomposition before C input were common at pH around 6.5. Negative PEs were common at soil pH below 4.5 or above 7 reflecting a suboptimal environment for microorganisms and specific SOM stabilization mechanisms at low and high pH. Short-term soil acidification (in rhizosphere, after fertilizer application) affects PE by: mineral-SOM complexation, SOM oxidation by iron reduction, enzymatic depolymerization, and pH-dependent changes in nutrient availability. Biological processes of microbial metabolism shift over the short-term, whereas long-term microbial community adaptations to slow acidification are common. The nitrogen fertilization induced soil acidification and land use intensification strongly decrease pH and thus boost the PE. Concluding, soil pH is one of the strongest but up to now disregarded factors of PE, defining SOM decomposition through short-term metabolic adaptation of microbial groups and long-term shift of microbial communities.

Mechanisms of Arbuscular Mycorrhizal Fungi Increasing Silicon Uptake by Rice.

Arbuscular mycorrhizal fungi (AMF) influence silicon (Si) uptake by plants, but the mechanisms remain unclear. This study investigated the mechanisms of AMF-mediated Si uptake by rice, a model Si-accumulating plant, and explored the tripartite interactions among AMF, Si, and phosphorus (P). AMF inoculation increased shoot Si content by 97% when supplied with silicic acid and by 29% with calcium silicate and upregulated expression of Si transporters Lsi1 and Lsi2 in roots. Supplying Si only to AMF hyphae increased the root Si content by 113%, indicating direct Si uptake by hyphae. Mechanisms of AMF-induced Si uptake were elucidated: 1) direct Si uptake by hyphae, 2) increased silicate dissolution, and 3) upregulation of Si transporters. Silicon application also increased AMF colonization by 28%, and the absence of interactions was observed on P uptake. Altogether, AMF support Si acquisition and Si fosters AMF colonization in rice, whereas the P uptake depends more on AMF than on Si.

Global pattern of organic carbon pools in forest soils.

Understanding the mechanisms of soil organic carbon (SOC) sequestration in forests is vital to ecosystem carbon budgeting and helps gain insight in the functioning and sustainable management of world forests. An explicit knowledge of the mechanisms driving global SOC sequestration in forests is still lacking because of the complex interplays between climate, soil, and forest type in influencing SOC pool size and stability. Based on a synthesis of 1179 observations from 292 studies across global forests, we quantified the relative importance of climate, soil property, and forest type on total SOC content and the specific contents of physical (particulate vs. mineral-associated SOC) and chemical (labile vs. recalcitrant SOC) pools in upper 10 cm mineral soils, as well as SOC stock in the O horizons. The variability in the total SOC content of the mineral soils was better explained by climate (47%-60%) and soil factors (26%-50%) than by NPP (10%-20%). The total SOC content and contents of particulate (POC) and recalcitrant SOC (ROC) of the mineral soils all decreased with increasing mean annual temperature because SOC decomposition overrides the C replenishment under warmer climate. The content of mineral-associated organic carbon (MAOC) was influenced by temperature, which directly affected microbial activity. Additionally, the presence of clay and iron oxides physically protected SOC by forming MAOC. The SOC stock in the O horizons was larger in the temperate zone and Mediterranean regions than in the boreal and sub/tropical zones. Mixed forests had 64% larger SOC pools than either broadleaf or coniferous forests, because of (i) higher productivity and (ii) litter input from different tree species resulting in diversification of molecular composition of SOC and microbial community. While climate, soil, and forest type jointly determine the formation and stability of SOC, climate predominantly controls the global patterns of SOC pools in forest ecosystems.

Enzyme stoichiometry reveals microbial nitrogen limitation in stony soils.

Resource limitation for soil microorganisms is the crucial factor in nutrient cycling and vegetation development, which are especially important in arid climate. Given that rock fragments strongly impact hydrologic and geochemical processes in arid areas, we hypothesized that microbial resource (C and N) limitation will increase along the rock fragment content (RFC) gradient. We conducted a field experiment in Minjiang river arid valleys with four RFC content (0 %, 25 %, 50 %, and 75 %, V V-1) and four vegetation types (Artemisia vestita, Bauhinia brachycarpa, Sophora davidii, and the soil without plants). Activities of C (ß-1,4-glucosidase, BG), N (ß-1,4-N-acetyl-glucosaminidase, NAG; L-leucine aminopeptidase, LAP), and P (acid phosphatase, ACP) acquiring enzymes were investigated to assess the limitations by C, N or P. In unplanted soil, the C acquiring enzyme activity decreased by 43 %, but N acquiring enzyme activity increased by 72 % in 75 % RFC than those in rock-free soils (0 % RFC). Increasing RFC reduced C:N and C:P enzymatic ratios, as well as vector length and vector angle (< 45°). Plants increased the activities of C and N acquiring enzymes in soils, as well as C:P and N:P enzyme activities, as well as vector length (by 5.6 %-25 %), but decreased vector angle (by 13 %-21 %). Enzyme stoichiometry was dependent on biotic and abiotic factors, such as soil water content, soil C:N, and total content of phospholipid fatty acids, reflecting microbial biomass content. Increased RFC shifted enzymatic stoichiometry toward lower C but stronger N limitation for microorganisms. Vegetation increased microbial C and N limitation, and impacted the enzymatic activities and stoichiometry depending on shrub functional groups. Consequently, the direct effects of vegetation, nutrient availability and microbial biomass content, as well as indirect effects of soil properties collectively increased microbial resource limitations along the RFC gradient.

Energy flows through nematode food webs depending on the soil carbon and nitrogen contents after forest conversion.

The swift proliferation of forests converted into monoculture plantations has profound impacts on soil nutrients, microbial communities, and many ecological processes and functions. Nematodes are soil microfauna that play a pivotal role in biogeochemical cycling and in soil food web, whereas the response of soil nematode communities and energy flows to forest conversion remains unknown. Here, we assessed the community composition and the energy flows of the nematode food webs as a function of soil chemistry after conversion from natural forests (Forest) to four plantations (8-year-old): Amygdalus persica (Peach), Myrica rubra (Berry), Camellia oleifera (Oil), and Cunninghamia lanceolata (Fir). After forest conversion, soil organic carbon (SOC) and total nitrogen (TN) contents decreased by 65 % and 55 %, respectively. Forest conversion strongly reduced the abundance (particularly large-bodied omnivorous-predatory nematodes), diversity, maturity, and stability of the soil nematode community. The shifts in composition and structure of nematode communities after forest conversion are reflected in changes in the abundance of predominant genera and trophic taxa, especially bacterivorous, fungivorous, and omnivorous-predatory nematodes. Acrobeloides notably increased, whereas Plectus, Prismatolaimus, Tylencholaimus, and Tripyla decreased. Accordingly, the abundances of r-strategy nematodes (cp value = 1-2) increased, but that of the K-strategists (cp value = 3-5) declined. Additionally, the energy flow across the soil nematode food web was reduced by 36 % and flow uniformity declined by 24 % after forest conversion. These changes in nematode diversity and abundance were triggered by diminishing soil C and N contents, thereby affecting the energy flows via the nematode food webs. Thus, forest conversion affects soil biotas and multi-functions from the perspective of nematode food web structure and energy flows, and underlines the interconnections between ecosystem and energy dynamics across multi-trophic levels, which is crucial for sustainable forest management.

Do Added Microplastics, Native Soil Properties, and Prevailing Climatic Conditions Have Consequences for Carbon and Nitrogen Contents in Soil? A Global Data Synthesis of Pot and Greenhouse Studies.

Microplastics threaten soil ecosystems, strongly influencing carbon (C) and nitrogen (N) contents. Interactions between microplastic properties and climatic and edaphic factors are poorly understood. We conducted a meta-analysis to assess the interactive effects of microplastic properties (type, shape, size, and content), native soil properties (texture, pH, and dissolved organic carbon (DOC)) and climatic factors (precipitation and temperature) on C and N contents in soil. We found that low-density polyethylene reduced total nitrogen (TN) content, whereas biodegradable polylactic acid led to a decrease in soil organic carbon (SOC). Microplastic fragments especially depleted TN, reducing aggregate stability, increasing N-mineralization and leaching, and consequently increasing the soil C/N ratio. Microplastic size affected outcomes; those <200 µm reduced both TN and SOC contents. Mineralization-induced nutrient losses were greatest at microplastic contents between 1 and 2.5% of soil weight. Sandy soils suffered the highest microplastic contamination-induced nutrient depletion. Alkaline soils showed the greatest SOC depletion, suggesting high SOC degradability. In low-DOC soils, microplastic contamination caused 2-fold greater TN depletion than in soils with high DOC. Sites with high precipitation and temperature had greatest decrease in TN and SOC contents. In conclusion, there are complex interactions determining microplastic impacts on soil health. Microplastic contamination always risks soil C and N depletion, but the severity depends on microplastic characteristics, native soil properties, and climatic conditions, with potential exacerbation by greenhouse emission-induced climate change.

Microbial life-history strategies mediate microbial carbon pump efficacy in response to N management depending on stoichiometry of microbial demand.

The soil microbial carbon pump (MCP) is increasingly acknowledged as being directly linked to soil organic carbon (SOC) accumulation and stability. Given the close coupling of carbon (C) and nitrogen (N) cycles and the constraints imposed by their stoichiometry on microbial growth, N addition might affect microbial growth strategies with potential consequences for necromass formation and carbon stability. However, this topic remains largely unexplored. Based on two multi-level N fertilizer experiments over 10 years in two soils with contrasting soil fertility located in the North (Cambisol, carbon-poor) and Southwest (Luvisol, carbon-rich), we hypothesized that different resource demands of microorganism elicit a trade-off in microbial growth potential (Y-strategy) and resource-acquisition (A-strategy) in response to N addition, and consequently on necromass formation and soil carbon stability. We combined measurements of necromass metrics (MCP efficacy) and soil carbon stability (chemical composition and mineral associated organic carbon) with potential changes in microbial life history strategies (assessed via soil metagenomes and enzymatic activity analyses). The contribution of microbial necromass to SOC decreased with N addition in the Cambisol, but increased in the Luvisol. Soil microbial life strategies displayed two distinct responses in two soils after N amendment: shift toward A-strategy (Cambisol) or Y-strategy (Luvisol). These divergent responses are owing to the stoichiometric imbalance between microbial demands and resource availability for C and N, which presented very distinct patterns in the two soils. The partial correlation analysis further confirmed that high N addition aggravated stoichiometric carbon demand, shifting the microbial community strategy toward resource-acquisition which reduced carbon stability in Cambisol. In contrast, the microbial Y-strategy had the positive direct effect on MCP efficacy in Luvisol, which greatly enhanced carbon stability. Such findings provide mechanistic insights into the stoichiometric regulation of MCP efficacy, and how this is mediated by site-specific trade-offs in microbial life strategies, which contribute to improving our comprehension of soil microbial C sequestration and potential optimization of agricultural N management.

Mechanisms and implications of bacterial-fungal competition for soil resources.

Elucidating complex interactions between bacteria and fungi that determine microbial community structure, composition, and functions in soil, as well as regulate carbon (C) and nutrient fluxes, is crucial to understand biogeochemical cycles. Among the various interactions, competition for resources is the main factor determining the adaptation and niche differentiation between these two big microbial groups in soil. This is because C and energy limitations for microbial growth are a rule rather than an exception. Here, we review the C and energy demands of bacteria and fungi-the two major kingdoms in soil-the mechanisms of their competition for these and other resources, leading to niche differentiation, and the global change impacts on this competition. The normalized microbial utilization preference showed that bacteria are 1.4-5 times more efficient in the uptake of simple organic compounds as substrates, whereas fungi are 1.1-4.1 times more effective in utilizing complex compounds. Accordingly, bacteria strongly outcompete fungi for simple substrates, while fungi take advantage of complex compounds. Bacteria also compete with fungi for the products released during the degradation of complex substrates. Based on these specifics, we differentiated spatial, temporal, and chemical niches for these two groups in soil. The competition will increase under the main five global changes including elevated CO2, N deposition, soil acidification, global warming, and drought. Elevated CO2, N deposition, and warming increase bacterial dominance, whereas soil acidification and drought increase fungal competitiveness.

Microbial phosphorus-cycling genes in soil under global change.

The ongoing climate change on the Tibetan Plateau, leading to warming and precipitation anomalies, modifies phosphorus (P) cycling in alpine meadow soils. However, the interactions and cascading effects of warming and precipitation changes on the key "extracellular" and "intracellular" P cycling genes (PCGs) of bacteria are largely unknown for these P-limited ecosystems. We used metagenomics to analyze the individual and combined effects of warming and altered precipitation on soil PCGs and P transformation in a manipulation experiment. Warming and increased precipitation raised Olsen-P (bioavailable P, AP) by 13% and 20%, respectively, mainly caused by augmented hydrolysis of organic P compounds (NaOH-Po). The decreased precipitation reduced soil AP by 5.3%. The richness and abundance of the PCGs' community in soils on the cold Tibetan plateau were more sensitive to warming than altered precipitation. The abundance of PCGs and P cycling processes decreased under the influence of individual climate change factors (i.e., warming and altered precipitation alone), except for the warming combined with increased precipitation. Pyruvate metabolism, phosphotransferase system, oxidative phosphorylation, and purine metabolism (all "intracellular" PCG) were closely correlated with P pools under climate change conditions. Specifically, warming recruited bacteria with the phoD and phoX genes, which encode enzymes responsible for phosphoester hydrolysis (extracellular P cycling), strongly accelerated organic P mineralization and so, directly impacted P bioavailability in alpine soil. The interactions between warming and altered precipitation profoundly influenced the PCGs' community and facilitated microbial adaptation to these environmental changes. Warming combined with increased precipitation compensated for the detrimental impacts of the individual climate change factors on PCGs. In conclusion, warming combined with rising precipitation has boosting effect on most P-related functions, leading to the acceleration of P cycling within microbial cells and extracellularly, including mineralization and more available P release for microorganisms and plants in alpine soils.

Carbon starvation raises capacities in bacterial antibiotic resistance and viral auxiliary carbon metabolism in soils.

Organic carbon availability in soil is crucial for shaping microbial communities, yet, uncertainties persist concerning microbial adaptations to carbon levels and the ensuing ecological and evolutionary consequences. We investigated organic carbon metabolism, antibiotic resistance, and virus-host interactions in soils subjected to 40 y of chemical and organic fertilization that led to contrasting carbon availability: carbon-poor and carbon-rich soils, respectively. Carbon-poor soils drove the enrichment of putative genes involved in organic matter decomposition and exhibited specialization in utilizing complex organic compounds, reflecting scramble competition. This specialization confers a competitive advantage of microbial communities in carbon-poor soils but reduces their buffering capacity in terms of organic carbon metabolisms, making them more vulnerable to environmental fluctuations. Additionally, in carbon-poor soils, viral auxiliary metabolic genes linked to organic carbon metabolism increased host competitiveness and environmental adaptability through a strategy akin to "piggyback the winner." Furthermore, putative antibiotic resistance genes, particularly in low-abundance drug categories, were enriched in carbon-poor soils as an evolutionary consequence of chemical warfare (i.e., interference competition). This raises concerns about the potential dissemination of antibiotic resistance from conventional agriculture that relies on chemical-only fertilization. Consequently, carbon starvation resulting from long-term chemical-only fertilization increases microbial adaptations to competition, underscoring the importance of implementing sustainable agricultural practices to mitigate the emergence and spread of antimicrobial resistance and to increase soil carbon storage.

Carbon footprint of synthetic nitrogen under staple crops: A first cradle-to-grave analysis.

More than half of the world's population is nourished by crops fertilized with synthetic nitrogen (N) fertilizers. However, N fertilization is a major source of anthropogenic emissions, augmenting the carbon footprint (CF). To date, no global quantification of the CF induced by N fertilization of the main grain crops has been performed, and quantifications at the national scale have neglected the CO2 assimilated by plants. A first cradle-to-grave life cycle assessment was performed to quantify the CF of the N fertilizers' production, transportation, and application to the field and the uses of the produced biomass in livestock feed and human food, as well as biofuel production. We quantified the direct and indirect inventories emitted or sequestered by N fertilization of main grain crops: wheat, maize, and rice. Grain food produced with N fertilization had a net CF of 7.4 Gt CO2eq. in 2019 after excluding the assimilated C in plant biomass, which accounted for a quarter of the total CF. The cradle (fertilizer production and transportation), gate (fertilizer application, and soil and plant systems), and grave (feed, food, biofuel, and losses) stages contributed to the CF by 2%, 11%, and 87%, respectively. Although Asia was the top grain producer, North America contributed 38% of the CF due to the greatest CF of the grave stage (2.5 Gt CO2eq.). The CF of grain crops will increase to 21.2 Gt CO2eq. in 2100, driven by the rise in N fertilization to meet the growing food demand without actions to stop the decline in N use efficiency. To meet the targets of climate change, we introduced an ambitious mitigation strategy, including the improvement of N agronomic efficiency (6% average target for the three crops) and manufacturing technology, reducing food losses, and global conversion to healthy diets, whereby the CF can be reduced to 5.6 Gt CO2eq. in 2100.

Composition and metabolism of microbial communities in soil pores.

Delineation of microbial habitats within the soil matrix and characterization of their environments and metabolic processes are crucial to understand soil functioning, yet their experimental identification remains persistently limited. We combined single- and triple-energy X-ray computed microtomography with pore specific allocation of 13C labeled glucose and subsequent stable isotope probing to demonstrate how long-term disparities in vegetation history modify spatial distribution patterns of soil pore and particulate organic matter drivers of microbial habitats, and to probe bacterial communities populating such habitats. Here we show striking differences between large (30-150 µm Ø) and small (4-10 µm Ø) soil pores in (i) microbial diversity, composition, and life-strategies, (ii) responses to added substrate, (iii) metabolic pathways, and (iv) the processing and fate of labile C. We propose a microbial habitat classification concept based on biogeochemical mechanisms and localization of soil processes and also suggests interventions to mitigate the environmental consequences of agricultural management.