Deterministic assembly of grassland soil microbial communities driven by climate warming amplifies soil carbon loss

Perturbations in soil microbial communities caused by climate warming are expected to have a strong impact on biodiversity and future climate-carbon (C) feedback, especially in vulnerable habitats that are highly sensitive to environmental change. Here, we investigate the impact of four-year experimental warming on soil microbes and C cycling in the Loess Hilly Region of China. The results showed that warming led to soil C loss, mainly from labile C, and this C loss is associated with microbial response. Warming significantly decreased soil bacterial diversity and altered its community structure, especially increasing the abundance of heat-tolerant microorganisms, but had no effect on fungi. Warming also significantly increased the relative importance of homogeneous selection and decreased "drift" of bacterial and fungal communities. Moreover, warming decreased bacterial network stability but increased fungal network stability. Notably, the magnitude of soil C loss was significantly and positively correlated with differences in bacterial community characteristics under ambient and warming conditions, including diversity, composition, network stability, and community assembly. This result suggests that microbial responses to warming may amplify soil C loss. Combined, these results provide insights into soil microbial responses and C feedback in vulnerable ecosystems under climate warming scenarios.

Improved carbon sequestration by utilization of ferrous ions during different organic wastes composting

The humic acid (HA) possesses a more recalcitrant structure, making it crucial carbon components that improve carbon sequestration. Moreover, ferrous ions could improve microbial activity and enhance compost humification, and their oxidation into iron oxides could adsorb carbon components for sequestration. Based on the advantages of low cost and easy availability of ferrous sulfate (FeSO4), this study investigated the effect of FeSO4 on carbon sequestration during composting. Chicken manure (CM) and food waste (FW) composting were carried out in four treatments, namely control (CM, FW) and 5% (w/w) FeSO4 treated groups (CM+, FW+). Results indicated that FeSO4 increased HA content, improved organic carbon stability. Carbon loss for CM, CM+, FW and FW + treatments were 48.5%, 46.2%, 45.0%, and 40.3%, respectively. Meanwhile, FeSO4 enhanced the function of bacterial taxa involved in HA synthesis in CM + treatment, and improved the number of core bacteria significantly associated with formation of HA and iron oxide. SEM analysis verified that role of FeSO4 was significant in promoting HA synthesis during CM + composting, while it was remarkably in enhancing HA sequestration during FW + composting. This article provided fundamental theoretical backing for enhancing HA production and improving carbon sequestration during different materials composting.

High-level and -yield production of L-leucine in engineered Escherichia coli by multistep metabolic engineering

L-leucine is an essential amino acid widely used in food and pharmaceutical industries. However, the relatively low production efficiency limits its large-scale application. In this study, we rationally developed an efficient L-leucine-producing Escherichia coli strain. Initially, the L-leucine synthesis pathway was enhanced by overexpressing feedback-resistant 2-isopropylmalate synthase and acetohydroxy acid synthase both derived from Corynebacterium glutamicum, along with two other native enzymes. Next, the pyruvate and acetyl-CoA pools were enriched by deleting competitive pathways, employing the nonoxidative glycolysis pathway, and dynamically modulating the citrate synthase activity, which significantly promoted the L-leucine production and yield to 40.69 g/L and 0.30 g/g glucose, respectively. Then, the redox flux was improved by substituting the native NADPH-dependent acetohydroxy acid isomeroreductase, branched chain amino acid transaminase, and glutamate dehydrogenase with their NADH-dependent equivalents. Finally, L-leucine efflux was accelerated by precise overexpression of the exporter and deletion of the transporter. Under fed-batch conditions, the final strain LXH-21 produced 63.29 g/L of L-leucine, with a yield and productivity of 0.37 g/g glucose and 2.64 g/(L h), respectively. To our knowledge, this study achieved the highest production efficiency of L-leucine to date. The strategies presented here will be useful for engineering E. coli strains for producing L-leucine and related products on an industrial scale.

Impacts of microplastics and heavy metals on the earthworm Eisenia fetida and on soil organic carbon, nitrogen, and phosphorus

Microplastics (MPs) are increasingly being studied because they have become ubiquitous in aquatic and terrestrial environments. However, little is known about the negative effects of co-contamination by polypropylene microplastic (PP MPs) and heavy metal mixtures on terrestrial environment and biota. This study assessed the adverse effects of co-exposure to PP MPs and heavy metal mixture (Cu²⁺, Cr⁶⁺, and Zn²⁺) on soil quality and the earthworm Eisenia fetida. Soil samples were collected in the Dong Cao catchment, near Hanoi, Vietnam, and analyzed for changes in extracellular enzyme activity and carbon, nitrogen, and phosphorus availability in the soil. We determined the survival rate of earthworms Eisenia fetida that had ingested MPs and two doses of heavy metals (the environmental level - 1 × - and its double - 2 ×). Earthworm ingestion rates were not significantly impacted by the exposure conditions, but the mortality rate for the 2 × exposure conditions was 100%. Metal-associated PP MPs stimulated the activities of β-glucosidase, β-N-acetyl glucosaminidase, and phosphatase enzymes in soil. Principle component analysis showed that these enzymes were positively correlated with Cu²⁺ and Cr⁶⁺ concentrations, but negatively correlated with microbial activity. Zn²⁺ showed no correlation with soil extracellular enzyme activity or soil microbial activity. Our results showed that co-exposure of earthworms to MPs and heavy metals had no impact on soil nitrogen and phosphorus but caused a decrease in total soil carbon content, with a possible associated risk of increased CO₂ emissions.

Tree mortality and carbon emission as a function of wildfire severity in south-eastern Australian temperate forests

Disturbance trends over recent decades indicate that climate change is resulting in increased fire severity and extent in Australia's temperate Eucalyptus forests. As disturbance cycles become shorter and more severe, empirical measurements are required to identify potential change in forest carbon (C) stock and emissions. However, such estimates are rare in the literature. The 2019-2020 wildfires burnt through 6 to 7 million ha of mainly temperate open Eucalyptus forest in south-east Australia, with top down emission estimates ranging from 97 to 130 tonnes CO₂ ha^-1. Study sites that had been assessed for all aboveground C pools prior to the wildfires, were burnt in January 2020 by wildfire that varied in severity. Here we quantify the impact of high and low/moderate fire severities on tree mortality, C loss and C redistribution and assess implications for future C storage in these temperate Eucalyptus forests. Higher fire severity resulted in greater overstorey tree mortality but not understorey or loss of dead standing trees than in low/moderate severity fires. High severity fires combusted almost twice as much C from live trees (42 Mg C ha^-1) as low/moderate severity fires (25 Mg C ha^-1), while C loss from dead standing trees was similar among fire severity classes (average 17 Mg C ha^-1). Total aboveground C lost across study sites was 42 Mg C ha^-1 for high and 47 Mg C ha^-1 for low/moderate severity, with an average of 45 Mg C ha^-1 equivalent to 15 % (high severity) and 14 % (low/moderate severity) of AGC. Extrapolating our findings to other tall to medium open Eucalyptus forests across Victoria revealed that 37.33 ± 12.25 Tg C (mean ± s.e.) or 152 ± 50 Mg CO₂ ha^-1 was lost to the atmosphere from the 0.9 million ha of these productive forests, equating to about 20 % of Australia's total net annual emissions.

Sustainable production of 4-hydroxyisoleucine with minimised carbon loss by simultaneously utilising glucose and xylose in engineered Escherichia coli

4-Hydroxyisoleucine is a promising drug for diabetes therapy; however, microbial production of 4-hydroxyisoleucine is not economically efficient because of the carbon loss in the form of CO₂. This study aims to achieve de novo synthesis of 4-hydroxyisoleucine with minimised carbon loss in engineered Escherichia coli. Initially, an L-isoleucine-producing strain, ILE-5, was established, and the 4-hydroxyisoleucine synthesis pathway was introduced. The flux toward α-ketoglutarate was enhanced by reinforcing the anaplerotic pathway and disrupting competitive pathways. Subsequently, the metabolic flux for 4-hydroxyisoleucine synthesis was redistributed by dynamically modulating the α-ketoglutarate dehydrogenase complex activity, achieving a 4-hydroxyisoleucine production of 16.53 g/L. Finally, carbon loss was minimised by employing the Weimberg pathway, resulting in a 24.5% decrease in sugar consumption and a 31.6% yield increase. The 4-hydroxyisoleucine production by strain IEOH-11 reached 29.16 g/L in a 5-L fermenter. The 4-hydroxyisoleucine yield (0.29 mol/mol sugar) and productivity (0.91 g/(L⋅h)) were higher than those previously reported.

A data-driven estimate of litterfall and forest carbon turnover and the drivers of their inter-annual variabilities in forest ecosystems across China

Strong influences of climate and land-cover changes on terrestrial ecosystems urgently need to re-estimate forest carbon turnover time (τ_forest), i.e., the residence time of carbon (C) in the living forest carbon reservoir in China, to reduce uncertainties in ecosystem carbon sinks under ongoing climate change. However, in absence of accurate carbon loss (e.g., forest litterfall), τ_forest estimate based on the non-steady-state assumption (NSSA) in forest ecosystems across China is still unclear. In this study, thus, we first compiled a litterfall dataset with 1025 field observations, and applied a Random Forest (RF) algorithm with the linkage of gridded environmental variables to predict litterfall from 2000 to 2019 with a fine spatial resolution of 1 km and a temporal resolution of one year. Finally, τ_forest was also estimated with the data-driven litterfall product. Results showed that RF algorithm could well predict the spatial and temporal patterns of forest litterfall with a model efficiency of 0.58 and root mean square error of 78.7 g C m^-2 year^-1. Mean litterfall was 205.4 ± 1.1 Tg C year^-1 (mean ± standard error) with a significant increasing trend of 0.65 ± 0.14 Tg C year^-2 from 2000 to 2019 (p < 0.01), indicating an increasing carbon loss from litterfall. Mean τ_forest was 26.2 ± 0.1 years with a significant decreasing trend of -0.11 ± 0.02 years (p < 0.01) from 2000 to 2019. Climate change dominated the inter-annual variability of τ_forest in high latitude areas, and land-cover change dominated the regions with intensive human activities. These findings suggested that carbon loss from vegetation to the atmosphere becomes more quickly in recent decades, with significant implication for vegetation carbon cycling-climate feedbacks. Meanwhile, the developed litterfall and τ_forest datasets can serve as a benchmark for biogeochemical models to accurately estimate global carbon cycling.

Canopy structure and phenology modulate the impacts of solar radiation on C and N dynamics during litter decomposition in a temperate forest

Decomposition of plant organic matter plays a key role in the terrestrial biogeochemical cycles. Sunlight has recently been identified as an important contributor to carbon [C] turnover through photodegradation, accelerating decomposition even in forest ecosystems where understorey solar irradiance remains relatively low. However, it is uncertain how C and nutrients dynamics respond to fluctuations in solar spectral irradiance caused by canopy structure (understorey vs. gaps) and season (open vs. closed canopy phenology). Spectral-attenuation treatments were used to compare litter decomposition over eight months, covering canopy phenology, in a temperate deciduous forest and an adjacent gap. Exposure to the full spectrum of sunlight increased the loss of litter C and lignin by 75% and 64% in the forest gap, and blue light was responsible for respectively 27% and 42% of that loss. Whereas in the understorey, C and lignin loss were similar among spectral-attenuation treatments over the experimental period, except prior to and during spring canopy flush when exposure to the full spectrum of sunlight promoted C loss by 15% overall, 80% of which was attributable to ultraviolet-B (UV-B) radiation. Nitrogen [N] was immobilized in the understorey during canopy flush before the canopy completely closed but N was swiftly released during canopy leaf-fall. Our study suggests that blue-driven photodegradation plays an important role in lignin decomposition and N dynamics in canopy gaps, whereas seasonal canopy phenology affecting sunlight reaching the forest floor drastically changes patterns of C and N in litter during decomposition. Hence, including sunlight dynamics driven by canopy structure and phenology would improve estimates of biogeochemical cycling in forests responding to changes in climate and land-use.

Soil carbon loss from drained agricultural peatland after coverage with mineral soil

Drainage for agriculture has turned peatlands from a net sink to a net source of carbon (C). In order to reduce the environmental footprint of agricultural peatland drainage, and to counteract soil subsidence, mineral soil coverage is becoming an increasingly used practice in Switzerland. To explore the effect of mineral soil coverage on soil C loss and the source of CO₂ from peatland drained for agriculture, we utilized the radiocarbon signature (F¹⁴C) of soil C and emitted CO₂ in the field. The experiment, located in the Swiss Rhine Valley, was carried out on two adjacent drained organic soils, either without mineral soil cover (reference 'Ref'), or covered with mineral soil (thickness ~ 40 cm) (coverage 'Cov') 13 years ago. Drainage already commenced 130 years ago and the site was managed as meadow since the 1970ies. Drainage induced 41-75 kg C m^-2 loss, which is equivalent to annual C loss rates of 0.49-0.58 kg C m^-2 yr^-1 and 0.31-0.63 kg C m^-2 yr^-1 for Cov and Ref, respectively. Mineral soil coverage had no significant effect on the amount of heterotrophic respiration, however, at Cov, the radiocarbon signature of heterotrophic CO₂ was significantly (p<0.01) younger than at Ref, indicating that mineral soil coverage moved the source of decomposition of soil organic carbon (SOC) from a higher share of old peat towards a higher share of relatively younger material. In summary, our study lends support to the hypothesis that mineral soil coverage might reduce the decomposition of old peat underneath, and may therefore be a promising peatland management technique for the future use of drained peatland for agriculture.

Reducing ammonia and greenhouse gas emission with adding high levels of superphosphate fertilizer during composting

Previous studies revealed that superphosphate fertilizer (SSP) as an additive in compost can reduce the nitrogen loss and improve the effectiveness of phosphorus during composting. However, few studies have explored the influence of adding SSP with high levels on ammonia and greenhouse gas emission and the suitable amount for SSP addition according to a combined assessment of the composting process and product. The present study aimed to evaluate the impact of SSP with high additive amounts on NH₃, CO₂, CH₄, and N₂O emission and organic carbon loss. All piles were mixtures of pig manure and cornstalks with different levels of SSP addition including 10%, 14%, 18%, 22%, 26%, and 30% dry weight basis of raw materials. Compared with the control without SSP, the amount of NH₃ cumulative emissions was decreased by 23.8-48.1% for the treatments with 10-30% SSP addition, and the emission of greenhouse gas including CO₂, CH₄, and N₂O by 20.9-35.5% (CO₂ equivalent) was reduced by 20.9-35.5%. Adding SSP with the amount exceeding 14% to compost could reduce CO₂ emissions by 32.0-38.4% and more than 30% carbon loss at the end of composting but exceeding 26% had an adverse impact on maturity of the composts. Therefore, considering the maturity and safety of compost and gas emission reduction, 14-26% SSP was the optimum amount for composting addition, which is an effective and economical way to increase the nutrient level of carbon, nitrogen, and phosphorus in compost and reduce environmental risks.

Effects of different biochar amendments on carbon loss and leachate characterization from an agricultural soil

Selection of an appropriate biochar as a soil amendment requires a thorough investigation of the effects on soil ecosystems and adjacent water systems via leaching. Different biochar characteristics influence retention or leaching of different soil and biochar components. A lab lysimeter study was conducted to investigate carbon (C) balance and leachate quality with biochar additions. Biochar made from wood pellets (WP) and sewage sludge (SS) produced at 400 °C (WP400 and SS400) and 700 °C (WP700 and SS700), respectively, were applied to silt loam soil at an application rate of 4%. Fluorescence excitation-emission spectrophotometry (EEMs) was utilized to understand the compositional changes in leachate dissolved organic carbon (DOC). Our results show that DOC contributed the largest portion of C leaching loss. The WP treatments increased DOC mass loss, but did not significantly change leachate DOC quality. SS400, in comparison, increased mass loss of DOC and SS700 decreased it probably due to its higher adsorptive capacity to DOC. Unlike WP treatments, SS treatments significantly changed leachate DOC quality. Chemical oxygen demand (COD) was reduced with SS400 and SS700 biochar additions, which is assumed to be related to SS biochar's high oxygen-containing surface functional groups. Reduction in total nitrogen (TN) leaching by WP700 and SS700 treatments might be related to the higher micropore surface area. Over all, our findings imply that changes in the different components of the leachate from biochar-amended soil are related to different biochar properties, such as labile matter content, total surface area, micropore volume and cation exchange capacity.

Impact of fertiliser, water table, and warming on celery yield and CO2 and CH4 emissions from fenland agricultural peat

Peatlands are globally important areas for carbon preservation; although covering only 3% of global land area, they store 30% of total soil carbon. Lowland peat soils can also be very productive for agriculture, but their cultivation requires drainage as most crops are intolerant of root-zone anoxia. This leads to the creation of oxic conditions in which organic matter becomes vulnerable to mineralisation. Given the demand for high quality agricultural land, 40% of the UK's peatlands have been drained for agricultural use. In this study we present the outcomes of a controlled environment experiment conducted on agricultural fen peat to examine possible trade-offs between celery growth (an economically important crop on the agricultural peatlands of eastern England) and emissions of greenhouse gases (carbon dioxide (CO₂) and methane (CH₄)) at different temperatures (ambient and ambient +5 °C), water table levels (-30 cm, and -50 cm below the surface), and fertiliser use. Raising the water table from -50 cm to -30 cm depressed yields of celery, and at the same time decreased the entire ecosystem CO₂ loss by 31%. A 5 °C temperature increase enhanced ecosystem emissions of CO₂ by 25% and increased celery dry shoot weight by 23% while not affecting the shoot fresh weight. Fertiliser addition increased both celery yields and soil respiration by 22%. Methane emissions were generally very low and not significantly different from zero. Our results suggest that increasing the water table can lower emissions of greenhouse gases and reduce the rate of peat wastage, but reduces the productivity of celery. If possible, the water table should be raised to -30 cm before and after cultivation, and only decreased during the growing season, as this would reduce the overall greenhouse gas emissions and peat loss, potentially not affecting the production of vegetable crops.

Soil organic carbon depletion and degradation in surface soil after long-term non-growing season warming in High Arctic Svalbard

Arctic tundra active-layer soils are at risk of soil organic carbon (SOC) depletion and degradation upon global climate warming because they are in a stage of relatively early decomposition. Non-growing season (NGS) warming is particularly pronounced, and observed increases of CO₂ emissions during experimentally warmed NGSs give concern for great SOC losses to the atmosphere. Here, we used snow fences in Arctic Spitsbergen dwarf shrub tundra to simulate 1.86 °C NGS warming for 9 consecutive years, while growing season temperatures remained unchanged. In the snow fence treatment, the 4-11 cm thick A-horizon had a 2% lower SOC concentration and a 0.48 kg C m^-2 smaller pool size than the controls, indicating SOC pool depletion. The snow fence treatment's A-horizon's alkyl/O-alkyl ratio was also significantly increased, indicating an advance of SOC degradation. The underlying 5 cm of B/C-horizon did not show these effects. Our results support the hypothesis that SOC depletion and degradation are connected to the long-term transience of observed ecosystem respiration (ER) increases upon soil warming. We suggest that the bulk of warming induced ER increases may originate from surface and not deep active layer or permafrost horizons. The observed losses of SOC might be significant for the ecosystem in question, but are in magnitude comparatively small relative to anthropogenic greenhouse gas enrichment of the atmosphere. We conclude that a positive feedback of carbon losses from surface soils of Arctic dwarf shrub tundra to anthropogenic forcing will be minor, but not negligible.

Effect of water table management and elevated CO2 on radish productivity and on CH4 and CO2 fluxes from peatlands converted to agriculture

Anthropogenic activity is affecting the global climate through the release of greenhouse gases (GHGs) e.g. CO₂ and CH₄. About a third of anthropogenic GHGs are produced from agriculture, including livestock farming and horticulture. A large proportion of the UK's horticultural farming takes place on drained lowland peatlands, which are a source of significant amounts of CO₂ into the atmosphere. This study set out to establish whether raising the water table from the currently used -50cm to -30cm could reduce GHGs emissions from agricultural peatlands, while simultaneously maintaining the current levels of horticultural productivity. A factorial design experiment used agricultural peat soil collected from the Norfolk Fens (among the largest of the UK's lowland peatlands under intensive cultivation) to assess the effects of water table levels, elevated CO₂, and agricultural production on GHG fluxes and crop productivity of radish, one of the most economically important fenland crops. The results of this study show that a water table of -30cm can increase the productivity of the radish crop while also reducing soil CO₂ emissions but without a resultant loss of CH₄ to the atmosphere, under both ambient and elevated CO₂ concentrations. Elevated CO₂ increased dry shoot biomass, but not bulb biomass nor root biomass, suggesting no immediate advantage of future CO₂ levels to horticultural farming on peat soils. Overall, increasing the water table could make an important contribution to global warming mitigation while not having a detrimental impact on crop yield.

Comparison of solid-state anaerobic digestion and composting of yard trimmings with effluent from liquid anaerobic digestion

Solid-state anaerobic digestion (SS-AD) and composting of yard trimmings with effluent from liquid AD were compared under thermophilic condition. Total solids (TS) contents of 22%, 25%, and 30% were studied for SS-AD, and 35%, 45%, and 55% for composting. Feedstock/effluent (F/E) ratios of 2, 3, 4, 5, and 6 were tested. In composting, the greatest carbon loss was obtained at 35% TS, which was 2-3 times of that at 55% TS and was up to 50% higher than that in SS-AD. In SS-AD, over half of the degraded carbon was converted to methane with the greatest methane yield of 121 L/kg VS(feedstock). Methane production from SS-AD was low at F/E ratios of 2 and 3, likely due to the inhibitory effect of high concentrations of ammonia nitrogen (up to 5.6g/kg). The N-P-K values were similar for SS-AD digestate and compost with different dominant nitrogen forms.

Soil CO2 efflux in a bioenergy plantation with fast-growing Populus trees - influence of former land use, inter-row spacing and genotype

AIMS

In this study we quantified the annual soil CO2 efflux (annual SCE) of a short rotation coppice plantation in its establishment phase. We aimed to examine the effect of former (agricultural) land use type, inter-row spacing and genotype.

METHODS

Annual SCE was quantified during the second growth year of the establishment rotation in a large scale poplar plantation in Flanders. Automated chambers were distributed over the two former land use types, the two different inter-row spacings and under two poplar genotypes. Additional measurements of C, N, P, K, Mg, Ca and Na concentrations of the soil, pH, bulk density, fine root biomass, microbial biomass C, soil mineralization rate, distance to trees and tree diameters were performed at the end of the second growth year.

RESULTS

Total carbon loss from soil CO2 efflux was valued at 589 g m-2 yr-1. Annual SCE was higher in former pasture as compared to cropland, higher in the narrow than in the wider inter-row spacings, but no effect of genotype was found.

CONCLUSIONS

Spatial differences in site characteristics are of great importance for understanding the effect of ecosystem management and land use change on soil respiration processes and need to be taken into account in modeling efforts of the carbon balance.