The accumulation of heavy metals in the soil not only causes serious damage to the soil ecosystem, but also threatens human health through the food chain. Exopolysaccharides have the functions of adsorbing and chelating heavy metals and reducing their bioavailability in the soil. In our study, exopolysaccharide-producing bacteria with a high efficiency in adsorbing cadmium (Cd) and lead (Pb) were screened from heavy metal-contaminated farmland. Through pot experiments, the influence of functional strains on the size distribution, heavy metal content, and bacterial community structure of soil aggregates in lettuce was studied using high-throughput sequencing technology. The results show that 11 strains secreting exopolysaccharides were initially screened from heavy metal-contaminated soil. Among them, strain Z23 had a removal rate of 88.6% for Cd and 93.2% for Pb. The rate at which Cd was removed by strain Z39 was 92.3%, and the rate at which Pb was removed was 94.4%. Both strains belong to Bacillus sp. Strains Z23 and Z39 induced the formation of Fe2Pb(PO4)(2), Cd-2(PO4)(2), and Pb2O3 in the solution. The pot experiments showed that strains Z23 and Z39 increased (19.1 similar to 23.9%) the dry weight and antioxidant enzyme activity of lettuce roots and leaves, while reducing (40.1 similar to 61.7%) the content of Cd and Pb. Strains Z23 and Z39 increased the proportion of microaggregates (<0.25 mm) and the content of exopolysaccharides in rhizosphere soil and reduced (38.4-59.7%) the contents of available Cd and Pb in microaggregates, thus inhibiting the absorption of heavy metals by lettuce. In addition, the exopolysaccharide content and the bacterial community associated with heavy metal resistance and nitrogen (N) cycling (Patescibacteria, Saccharimonadales, Microvirga, and Pseudomonas) in microaggregates were key factors affecting the available heavy metal content in soil. These results show that the exopolysaccharide-producing bacteria Z23 and Z39 reduced the absorption of Cd and Pb by lettuce tissues, thus providing strain resources for the safe utilization of soils that exceed heavy metal standards for farmland and for reducing the heavy metal content in vegetables.

Microbes in peatlands provide key ecosystem services and are essential for their role in biogeochemical cycling. Prescribed burning is a common aspect of peatland management but the practice has been criticized for being ecologically damaging due to its effect on the biological, chemical and physical properties of the soil. It is poorly understood how burning affects soil N cycling and previous studies have focused predominantly on the topsoil whilst giving less attention to changes with soil depth. This study investigated the changes of microbial abundance (bacterial 16S rRNA and fungal 18S rRNA) and the abundance of N-cycle functional genes involved in archaeal and bacterial ammonia oxidation (amoA-AOA and amoA-AOB), denitrification (nirK and nirS), N fixation (nifH) and organic N decomposition (chiA) in soil profiles across three burn treatments on a managed peatland landscape (a 'non-burn' since 1954 control, 20 years burn interval, and 10 years burn interval). Our results indicate the abundance of bacterial 16S rRNA and fungal 18 s rRNA was affected by burn treatment, soil depth and their interaction and were greater in the non-burn control plots. The abundances of amoA-AOA, amoA-AOB, and nifH were significantly higher in the topsoil of the non-burn control plots while the abundance of nirK was higher in plots subject to short rotation and long rotation burn regimes but also decreased significantly with soil depth. The abundance of nirS was not affected by burn treatment or soil depth. ChiA abundance was affected by burn treatment, soil depth and their interaction. N-cycle functional gene abundance responded differently to environmental factors associated with prescribed burning and varied with soil depth. These findings suggest that the practice of burning affects microbial N turnover potential and provides an important insight into the soil Ncycling potential of peatlands under different burning regimes.

Arctic extreme winter warming events (WW events) have increased in frequency with climate change. WW events have been linked to damaged tundra vegetation (Arctic browning), but the mechanisms that link episodic winter thaw to plant damage in summer are not fully understood. We suggest that one mechanism is microbial N immobilization during the WW event, which leads to a smaller release of winter-mineralized N in spring and therefore more N limitation for vegetation in summer. We tested this hypothesis in a Western Greenlandic Low arctic tundra, where we experimentally simulated a 6 day field-scale extreme WW event and 1) used stable isotopes to trace the movement of N as a consequence of the WW event, 2) measured the effect of a WW event on spring N release in top soils in the laboratory, and 3) measured the carry-over effect on summer aboveground vegetation C/N ratio in tundra subject to a WW event. Our results show that soil mineral N released by a WW event followed by soil thaw is taken up by microbes and stored in the soil, whereas vascular plants acquired almost none, and significant amounts were lost to leaching and gaseous emissions. As soils thawed in spring, we saw weak but not significant evidence (P = 0.067) for a larger N release over the first month of spring thaw in Control soils compared to WW event soils, although not significantly. A weak signal (P = 0.07) linked WW event treatment to higher summer C/N ratios in evergreen shrubs, whereas deciduous shrubs were not affected. We conclude that our results did not show significant evidence for WW events causing Arctic browning via N immobilization and summer N limitation, but that we had indications (P < 0.1) which merits further testing of the theory in various tundra types and with repeated WW events. Evergreen shrubs could be especially sensitive to winter N immobilization, with implications for future vegetation community composition and tundra C storage.

Litter decomposition represents a major path for atmospheric carbon influx into Arctic soils, thereby controlling below-ground carbon accumulation. Yet, little is known about how tundra litter decomposition varies with microenvironmental conditions, hindering accurate projections of tundra soil carbon dynamics with future climate change. Over 14 months, we measured landscape-scale decomposition of two contrasting standard litter types (Green tea and Rooibos tea) in 90 plots covering gradients of micro-climate and -topography, vegetation cover and traits, and soil characteristics in Western Greenland. We used the tea bag index (TBI) protocol to estimate relative variation in litter mass loss, decomposition rate (k) and stabilisation factor (S) across space, and structural equation modelling (SEM) to identify relationships among environmental factors and decomposition. Contrasting our expectations, microenvironmental factors explained little of the observed variation in both litter mass loss, as well as k and S, suggesting that the variables included in our study were not the major controls of decomposer activity in the soil across the studied tundra landscape. We use these unexpected findings of our study combined with findings from the current literature to discuss future avenues for improving our understanding of the drivers of tundra decomposition and, ultimately, carbon cycling across the warming Arctic.

Forest degradation, driven by human and natural factors, diminishes ecological functions and carbon storage. Understanding the complex dynamics of soil carbon pools is crucial for the global carbon cycle, although these dynamics are poorly understood. This study examines how different thinning intensities influence seasonal soil carbon cycling in degraded forests. ANOVA revealed significant differences in soil properties across treatments (p < 0.05). Redundancy analysis and random forest analyses were used to explore relationships among thinning intensities, soil properties, and carbon sequestration. Thinning significantly altered soil attributes, as revealed by field experiments and data analysis. Moderate thinning (20% intensity) significantly enhanced litter retention and soil nutrient levels year-round (p < 0.05). Seasonal variations affected soil carbon dynamics and lower thinning intensities improved carbon sequestration in spring and summer. Conversely, higher thinning intensities led to carbon loss in autumn and winter. Litter carbon, fine root carbon, and correction factor significantly respond to thinning intensities year-round as examined through redundancy analysis and random forest analyses. Findings indicate moderate thinning effectively enhances soil carbon sequestration in degraded forests. Strategically planned thinning could aid climate change mitigation by boosting forest soil carbon storage, influencing forest management and conservation.

Permafrost degradation in peatlands is altering vegetation and soil properties and impacting net carbon storage. We studied four adjacent sites in Alaska with varied permafrost regimes, including a black spruce forest on a peat plateau with permafrost, two collapse scar bogs of different ages formed following thermokarst, and a rich fen without permafrost. Measurements included year-round eddy covariance estimates of net carbon dioxide (CO2), mid-April to October methane (CH4) emissions, and environmental variables. From 2011 to 2022, annual rainfall was above the historical average, snow water equivalent increased, and snow-season duration shortened due to later snow return. Seasonally thawed active layer depths also increased. During this period, all ecosystems acted as slight annual sources of CO2 (13-59 g C m(-2) year(-1)) and stronger sources of CH4 (11-14 g CH4 m(-2) from similar to April to October). The interannual variability of net ecosystem exchange was high, approximately +/- 100 g C m(-2) year(-1), or twice what has been previously reported across other boreal sites. Net CO2 release was positively related to increased summer rainfall and winter snow water equivalent and later snow return. Controls over CH4 emissions were related to increased soil moisture and inundation status. The dominant emitter of carbon was the rich fen, which, in addition to being a source of CO2, was also the largest CH4 emitter. These results suggest that the future carbon-source strength of boreal lowlands in Interior Alaska may be determined by the area occupied by minerotrophic fens, which are expected to become more abundant as permafrost thaw increases hydrologic connectivity. Since our measurements occur within close proximity of each other (<= 1 km(2)), this study also has implications for the spatial scale and data used in benchmarking carbon cycle models and emphasizes the necessity of long-term measurements to identify carbon cycle process changes in a warming climate.

Tundra soils are one of the world's largest organic carbon stores, yet this carbon is vulnerable to accelerated decomposition as climate warming progresses. The landscape-scale controls of litter decomposition are poorly understood in tundra ecosystems, which hinders our understanding of the global carbon cycle. We examined the extent to which the thermal sum of surface air temperature, soil moisture and permafrost thaw depth influenced litter mass loss and decomposition rates (k), and at which spatial thresholds an environmental variable becomes a reliable predictor of decomposition, using the Tea Bag Index protocol across a heterogeneous tundra landscape on Qikiqtaruk-Herschel Island, Yukon, Canada. We found greater green tea litter mass loss and faster decomposition rates (k) in wetter areas within the landscape, and to a lesser extent in areas with deeper permafrost active layer thickness and higher surface thermal sums. We also found higher decomposition rates (k) on north-facing relative to south-facing aspects at microsites that were wetter rather than warmer. Spatially heterogeneous belowground conditions (soil moisture and active layer depth) explained variation in decomposition metrics at local scales (< 50 m(2)) better than thermal sum. Surprisingly, there was no strong control of elevation or slope on litter decomposition. Our results reveal that there is considerable scale dependency in the environmental controls of tundra litter decomposition, with moisture playing a greater role than the thermal sum at < 50 m(2) scales. Our findings highlight the importance and complexity of microenvironmental controls on litter decomposition in estimates of carbon cycling in a rapidly warming tundra biome.

In cold regions, climate change is expected to result in warmer winter temperatures and increased temperature variability. Coupled with changing precipitation regimes, these changes can decrease soil insulation by reducing snow cover, exposing soils to colder temperatures and more frequent and extensive soil freezing and thawing. Freeze-thaw events can exert an important control over winter soil processes and the cycling of nitrogen (N), with consequences for soil health, nitrous oxide (N2O) emissions, and nearby water quality. These impacts are especially important for agricultural soils and practices in cold regions. We conducted a lysimeter experiment to assess the effects of winter pulsed warming, soil texture, and snow cover on N cycling in agricultural soils. We monitored the subsurface soil temperature, moisture, and porewater geochemistry together with air temperature, precipitation, and N2O fluxes in four agricultural field-controlled lysimeter systems (surface area of 1 m(2) and depth of 1.5 m) at the University of Guelph's Elora Research Station over one winter (December 2020 to April 2021). The lysimeters featured two soil types (loamy sand and silt loam) which were managed under a corn-soybean-wheat rotation with cover crops. Additionally, ceramic infrared heaters located above two of the lysimeters were turned on after each snowfall event to melt the snow and then turned off to mimic snow-free winter conditions with increased soil freezing. Porewater samples collected from five depths in the lysimeters were analyzed for total dissolved nitrogen (TDN), nitrate (NO3 (-)), nitrite (NO2 (-)), and ammonium (NH4 (+)). N2O fluxes were measured using automated soil gas chambers installed on each lysimeter. The results from the snow removed lysimeters were compared to those of lysimeters without heaters (with snow). As expected, the removal of the insulating snow cover resulted in more intense soil freeze-thaw events, causing increased dissolved N loss from the lysimeter systems as N2O (from the silt loam system) and via NO3 (-) leaching (from the loamy sand system). In the silt loam lysimeter, we attribute the freeze thaw-enhanced N2O fluxes to de novo processes rather than gas build up and release. In the loamy sand lysimeter, we attribute the increased NO3 (-) leaching to the larger pore size and therefore lower water retention capacity of this soil type. Overall, our study illustrates the important role of winter snow cover dynamics and soil freezing in modulating the coupled responses of soil moisture, temperature, and N cycling.

Microorganisms are major constituents of the total biomass in permafrost regions, whose underlain soils are frozen for at least two consecutive years. To understand potential microbial responses to climate change, here we examined microbial community compositions and functional capacities across four soil depths in an Alaska tundra site. We showed that a 5-year warming treatment increased soil thaw depth by 25.7% (p = .011) within the deep organic layer (15-25 cm). Concurrently, warming reduced 37% of bacterial abundance and 64% of fungal abundances in the deep organic layer, while it did not affect microbial abundance in other soil layers (i.e., 0-5, 5-15, and 45-55 cm). Warming treatment altered fungal community composition and microbial functional structure (p < .050), but not bacterial community composition. Using a functional gene array, we found that the relative abundances of a variety of carbon (C)-decomposing, iron-reducing, and sulphate-reducing genes in the deep organic layer were decreased, which was not observed by the shotgun sequencing-based metagenomics analysis of those samples. To explain the reduced metabolic capacities, we found that warming treatment elicited higher deterministic environmental filtering, which could be linked to water-saturated time, soil moisture, and soil thaw duration. In contrast, plant factors showed little influence on microbial communities in subsurface soils below 15 cm, despite a 25.2% higher (p < .05) aboveground plant biomass by warming treatment. Collectively, we demonstrate that microbial metabolic capacities in subsurface soils are reduced, probably arising from enhanced thaw by warming.

Permafrost is experiencing climate warming at a rate that is two times faster than the rest of the Earth's surface. However, it is still lack of a quantitative basis for predicting the functional stability of permafrost ecosystems in carbon (C) and nutrient cycling. We compiled the data of 708 observations from 89 air-warming experiments in the Northern Hemisphere and characterized the general effects of temperature increase on permafrost C exchange and balance, biomass production, microbial biomass, soil nutrients, and vegetation N dynamics through a meta-analysis. Also, an investigation was made on how responses might change with habitat-specific (e.g., plant functional groups and soil moisture status) conditions and warming variables (e.g., warming phases, levels, and timing). The net ecosystem C exchange (NEE) was found to be downregulated by warming as a result of a stronger sensitivity to warming in respiration (15.6%) than in photosynthesis (6.2%). Vegetation usually responded to warming by investing more C to the belowground, as belowground biomass increased much more (30.1%) than aboveground biomass (2.9%). Warming had a minor effect on microbial biomass. Warming increased soil ammonium and nitrate concentrations. What's more, a synthesis of 70 observations from 11 herbs and 9 shrubs revealed a 2.5% decline of N in green leaves. Compared with herbs, shrubs had a stronger response to respiration and had a decline in green leaf N to a greater extent. Not only in dry condition did green leaf N decline with warming but also in wet conditions. Warming in nongrowing seasons would negatively affect soil water, C uptake, and biomass production during growing seasons. Permafrost C loss and vegetation N decline may increase with warming levels and timing. Overall, these findings suggest that besides a positive C cycling-climate feedback, there will be a negative feedback between permafrost nutrient cycling and climate warming.