Ecotoxicological impact of butisanstar and clopyralid herbicides on soil microbial respiration and the enzymatic activities

The application of herbicides in soil has been noted for its detrimental effect on the soil microbial community, crucial for various biochemical processes. This study provides a comprehensive assessment of the impact of butisanstar and clopyralid herbicides, both individually and in combination at different dosage (recommended field dose (RFD), ½, 2 and 5-times RFD). The assessment focuses on soil basal respiration (SBR), cumulative microbial respiration (CMR), and the activities dehydrogenase (DH), catalase (CAT), urease, acid and alkaline phosphatases (Ac-P and Alk-P) enzymes, along with their variations on days 10, 30, 60, and 90 post-herbicide application. Results indicate that, although herbicides, even at lower doses of RFD, demonstrate inhibitory effects on DH, CAT, and microbial respiration, they paradoxically lead to a significant enhancement in urease and phosphatase activities, even at higher doses. The inhibitory/enhancing intensity varies based on herbicide type, incubation period, and dosage. Co-application of herbicides manifests synergistic effects compared to individual applications. The most notable inhibitory effects on DH, CAT, and SBR are observed on the 30th day, coinciding with the highest activities of urease and phosphatases on the same day. The persistent inability to restore respiration and enzyme activities to initial soil (control) levels emphasizes the lasting adverse and inhibitory effects of herbicides, especially clopyralid, over the long term. It becomes apparent that soil microorganisms require an extended duration to decompose and acclimate to the presence of herbicides. Consequently, these agrochemical compounds pose a potential risk to crucial biochemical processes, such as nutrient cycling, ultimately impacting crop production.

The role of edaphic variables and management practices in regulating soil microbial resilience to drought - A meta-analysis

Environmental disturbances such as drought can impact soil health and the resistance (ability to withstand environmental stress) and resilience (ability to recover functional and structural integrity after stress) of soil microbial functional activities. A paucity of information exists on the impact of drought on soil microbiome and how soil biological systems respond to and demonstrate resilience to drought stress. To address this, we conducted a systematic review and meta-analysis (using only laboratory studies) to assess the response of soil microbial biomass and respiration to drought stress across agriculture, forest, and grassland ecosystems. The meta-analysis revealed an overall negative response of microbial biomass in resistance (-31.6 %) and resilience (-0.3 %) to drought, suggesting a decrease in soil microbial biomass content. Soil microbial respiration also showed a negative response in resistance to drought stress indicating a decrease in soil microbial respiration in agriculture (-17.5 %), forest (-64.0 %), and grassland (-65.5 %) ecosystems. However, it showed a positive response in resilience to drought, suggesting an effective recovery in microbial respiration post-drought. Soil organic carbon (SOC), clay content, and pH were the main regulating factors of the responses of soil microbial biomass and respiration to drought. In agriculture ecosystem, soil pH was primarily correlated with soil microbial respiration resistance and resilience to drought, potentially influenced by frequent land preparation and fertilizer applications, while in forest ecosystem SOC, clay content, and pH significantly impacted microbial biomass and respiration resistance and resilience. In grassland ecosystem, SOC was strongly associated with biomass resilience to drought. The impact of drought stress on soil microbiome showed different patterns in natural and agriculture ecosystems, and the magnitude of microbial functional responses regulated by soil intrinsic properties. This study highlighted the importance of understanding the role of soil properties in shaping microbial responses to drought stress for better ecosystem management.

Microbial-driven mechanisms for the effects of heavy metals on soil organic carbon storage: A global analysis

Heavy metal (HM) enrichment is closely related to soil organic carbon (SOC) pools in terrestrial ecosystems, which are deeply intertwined with soil microbial processes. However, the influence of HMs on SOC remains contentious in terms of magnitude and direction. A global analysis of 155 publications was conducted to integrate the synergistic responses of SOC and microorganisms to HM enrichment. A significant increase of 13.6 % in SOC content was observed in soils exposed to HMs. The response of SOC to HMs primarily depends on soil properties and habitat conditions, particularly the initial SOC content, mean annual precipitation (MAP), initial soil pH, and mean annual temperature (MAT). The presence of HMs resulted in significant decreases in the activities of key soil enzymes, including 31.9 % for soil dehydrogenase, 24.8 % for β-glucosidase, 35.8 % for invertase, and 24.3 % for cellulose. HMs also exerted inhibitory effects on microbial biomass carbon (MBC) (26.6 %), microbial respiration (MR) (19.7 %), and the bacterial Shannon index (3.13 %) but elevated the microbial metabolic quotient (qCO2) (20.6 %). The HM enrichment-induced changes in SOC exhibited positive correlations with the response of MBC (r = 0.70, p < 0.01) and qCO2 (r = 0.50, p < 0.01), while it was negatively associated with β-glucosidase activity (r = 0.72, p < 0.01) and MR (r = 0.39, p < 0.01). These findings suggest that the increase in SOC storage is mainly attributable to the inhibition of soil enzymes and microorganisms under HM enrichment. Overall, this meta-analysis highlights the habitat-dependent responses of SOC to HM enrichment and provides a comprehensive evaluation of soil carbon dynamics in an HM-rich environment.

Impact of redox fluctuations on microbe-mediated elemental sulfur disproportionation and coupled redox cycling of iron

Elemental sulfur (S0) plays a vital role in the coupled cycling of sulfur and iron, which in turn affects the transformation of carbon and various pollutants. These processes have been well characterized under static anoxic or oxic conditions, however, how the natural redox fluctuations affect the bio-mediated sulfur cycling and coupled iron cycling remain enigmatic. The present work examined S0 disproportionation as driven by natural microbial communities under fluctuating redox conditions and the contribution of S0 disproportionation to ferrihydrite transformation. Samples were incubated at either neutral or alkaline pH values, applying sequential anaerobic, aerobic and anaerobic conditions over 60 days. Under anaerobic conditions, S0 was found to undergo disproportionation to sulfate and sulfide, which subsequently reduced ferrihydrite at both pH 7.4 and 9.5. Ferrihydrite promoted S0 disproportionation by scavenging biogenic sulfide and maintaining a suitable degree of sulfate formation. After an oxic period, during the subsequent anoxic incubation, bioreduction of sulfate occurred and the biogenic sulfide reduced iron (hydr)oxides at a rate approximately 25 % lower than that observed during the former anoxic period. A 16S rDNA-based microbial community analysis revealed changes in the microbial community in response to the redox fluctuations, implying an intimate association with the coupled cycling of sulfur and iron. Microscopic and spectroscopic analyses confirmed the S0-mediated transformation of ferrihydrite to crystalline iron (hydr)oxide minerals such as lepidocrocite and magnetite and the formation of iron sulfides precipitated under fluctuating redox conditions. Finally, a reaction mechanism based on mass balance was proposed, demonstrating that bio-mediated sulfur transformation maintained a sustainable redox reaction with iron (hydr)oxides under fluctuating anaerobic-aerobic-anaerobic conditions tested in this study. Altogether, the finding of our study is critical for obtaining a more complete understanding of the dynamics of iron redox reactions and pollutant transformation in sulfur-rich aquatic environments.

Electromotive force induced by dynamic magnetic field electrically polarized sediment to aggravate methane emission

As a primary driving force of global methane production, methanogens like other living organisms are exposed to an environment filled with dynamic electromagnetic waves, which might induce electromotive force (EMF) to potentially influence the metabolism of methanogens. However, no reports have been found on the effects of the induced electromotive force on methane production. In this study, we found that exposure to a dynamic magnetic field enhanced bio-methanogenesis via the induced electromotive force. When exposed to a dynamic magnetic field with 0.20 to 0.40 mT of intensity, the methane emission of the sediments increased by 41.71%. The respiration of methanogens and bacteria was accelerated by the EMF, as the ratios of F420H2/F420 and NAD+/NADH of the sediment increased by 44.12% and 55.56%, respectively. The respiratory enzymes in respiration chains might be polarized with the EMF to accelerate the proton-coupled electron transfer to enhance microbial metabolism. Together with the enriched exoelectrogens and electrotrophic methanogens, as well as the increased sediment electro-activities, this study indicated that the EMF could enhance the electron exchange among extracellular respiratory microorganisms to increase the methane emission from sediments.

Environmental stress stimulates microbial activities as indicated by cyclopropane fatty acid enhancement

Soil microbial responses to environmental stress remain a critical question in microbial ecology. The content of cyclopropane fatty acid (CFA) in cytomembrane has been widely used to evaluate environmental stress on microorganisms. Here, we used CFA to investigate the ecological suitability of microbial communities and found a stimulating impact of CFA on microbial activities during wetland reclamation in Sanjiang Plain, Northeastern China. The seasonality of environmental stress resulted in the fluctuation of CFA content in the soil, which suppressed microbial activities due to nutrient loss upon wetland reclamation. After land conversion, the aggravation of temperature stress to microbes increased the CFA content by 5 % (autumn) to 163 % (winter), which led to the suppression of microbial activities by 7 %-47 %. By contrast, the warmer soil temperature and permeability decreased the CFA content by 3 % to 41 % and consequently aggravated the microbial reduction by 15 %-72 % in spring and summer. Complex microbial communities of 1300 CFA-produced species were identified using a sequencing approach, suggesting that soil nutrients dominated the differentiation in these microbial community structures. Further analysis with structural equation modeling highlighted the important function of CFA content to environmental stress and the stimulating influence of CFA induced by environmental stress on microbial activities. Our study shows the biological mechanisms of seasonal CFA content for microbial adaption to environmental stress under wetland reclamation. It advances our knowledge of microbial physiology affecting soil element cycling caused by anthropogenic activities.

Soil contamination by diesel fuel destabilizes the soil microbial pools: Insights from permafrost soil incubations

Arctic contamination by diesel fuel (DF) is of great concern because of the uncertain feedback of permafrost carbon (C) and soil microbiota to DF in the context of climate change in high latitudes. We conducted a laboratory incubation experiment with a gradient of DF addition ratios to examine the responses of the soil microbiota of the typical permafrost soils in the tundra ecosystems of the Norilsk region (Siberia). The study revealed initial heterogeneity in the microbial activity of the studied soils (Histic Gleyic Cryosols (CR-hi,gl), Turbic Cryosols (CR-tu), Turbic Spodic Folic Cryosols (CR-tu,sd,fo), Gleyic Fluvisols (FL-gl)). We applied the two-pool model for evaluation of the effect of DF on the proportions of C pools and revealed significant differences between soil types in the fast and slow C pools in response to DF addition. The results showed that DF addition treatments had varying effects on the fast and slow C pools, microbial activity, and microbial community structure in the studied soils. For minor exceptions, DF dramatically accelerated C loss from the slow C pool in all soil types. We assume that differences in C pool and microbiota responses to DF addition were caused by soil texture and changes in microbial community structure. We isolated Serratia proteamaculans, S. liquefaciens, S. plymuthica, Rhodococcus erythropolis, Pseudomonas antarctica, P. libanensis, P. brassicacearum, and P. chlororaphis from the DF-polluted soils. These species are recommended for bioremediation to mitigate the DF contamination of permafrost soils, especially regarding climate change and the sustainable well-being of Arctic ecosystems.

Land use intensification in a dry-hot valley reduced the constraints of water content on soil microbial diversity and multifunctionality but increased CO2 production

Conversion of abandoned land (mainly savanna) into cropland generally occurs in fragile ecosystems such as dry-hot valleys (DHVs) in southwest China, with the intent of increasing land productivity and conducting ecological restoration. However, the effects of conversion on soil microbial communities and carbon turnover of savanna ecosystems remain unclear, since savannas could be a vital but overlooked carbon sink. To illustrate the ecological consequences of land-use change (LUC) for savanna ecosystems, a 1-year field experiment was conducted in DHVs of southwest China. The soil properties, microbial respiration, and metagenomics from two different land-use types (grassland and mango plantation) were examined to reveal the effects of regional LUC on soil C turnover and microbial traits. Conversion from degraded grassland into cropland increased the contribution of soil microclimate to the microbial community composition, reduced the constraints of soil water content (SWC), and further decreased nutrient availability. LUC reshaped the composition and structure of soil bacterial communities. Specifically, soil dominant microbes that belonged to Actinobacteria and Proteobacteria were significantly enriched by conversion, while rare microbes that belonged to a wider range of phyla were generally depleted, leading to an overall decrease in community diversity. In addition, LUC-induced changes in soil characteristics and microbial communities further decreased soil multifunctionality as well as the carbon use efficiency of microbes. Intensified microbial respiration and a significant increase in the soil CO₂ efflux were observed following LUC, which could drive changes in soil microbial community composition and functions (such as growth and regeneration). In summary, through simultaneously reducing constraints on SWC and decreasing nutrient availability, conversion from degraded grassland to cropland in a DHV decreased soil microbial diversity and multifunctionality, and increased microbial respiration and soil CO₂ efflux. Our study provides new insights for understanding the role and mechanisms of LUC in soil carbon turnover in ecologically fragile areas such as DHVs.

Spatial responses of soil carbon stocks, total nitrogen, and microbial indices to post-wildfire in the Mediterranean red pine forest

Wildfire is a key ecological event that alters vegetation and soil quality attributes including biochemical attributes at spatial scale. This knowledge can provide insights into the development of better rehabilitation or restoration strategies that depend on the ecological dynamics of vegetation, fungi, and animals. The present study aimed to understand the causes and consequences of spatial variability of soil organic carbon, microbial biomass C concentrations, and soil quality indices as impacted by wildfire in a red pine forest. This study was conducted using kriging and inverse distance neighborhood similarity (IDW) interpolations methods. The carbon stocks were significantly (P = 0.002) higher in burned areas compared to those of unburned areas by 255% whereas microbial biomass carbon and microbial respiration were significantly (P < 0.0001 and P = 0.02) lower in burned areas by 66% and 90%, The Pearson's correlation analysis showed that carbon stocks were positively correlated with pH (0.61), total nitrogen (0.60) and ash quantity (0.41), but negatively correlated with microbial biomass carbon (-0.46) and nitrogen (-0.61), and microbial respiration (-0.48). The IDW interpolation method better-predicted pH, bulk density, and microbial biomass carbon and nitrogen compared to kriging interpolation, whereas the kriging interpolation method was better than IDW interpolation for the other studied soil properties. We concluded that pH, EC, SOC, C/N, MR, MBC/SOC, and MBC/MBN can be reliable indicators to monitor the effect of wildfire on forest soils. The wildfire event increased soil carbon stocks, TN, pH, and _qCO₂, but decreased MBC and MBN.

Combined maize straw-biochar and oxalic acids induced a relay activity of abundant specific degraders for efficient phenanthrene degradation: Evidence based on the DNA-SIP technology

Biochar-oxalic acid composite application (BCOA) have shown to be efficient in the remediation of polycyclic aromatic hydrocarbon (PAH)-contaminated soil, but the functional degraders and the mechanism of improving biodegradation remains unclear. In this study, with the help of stable isotope probing technology of phenanthrene (Phe), we determined that BCOA significantly improved Phe mineralization by 2.1 times, which was ascribed to the increased numbers and abundances of functional degraders. The BCOA increased contents of dissolved organic carbon and available nutrients and decreased pH values in soil, thus promoting the activity, diversity and close cooperation of the functional Phe-degraders, and stimulating their functions associated with Phe degradation. In addition, there is a relay activity among more and diverse functional Phe-degraders in the soil with BCOA. Specifically, Pullulanibacillus persistently participated in Phe-degradation in the soil with BCOA throughout the incubation period. Moreover, Pullulanibacillus, Blastococcus, Alsobacter, Ramlibacter, and Mizugakiibacter were proved to be potential Phe-degraders in soil for the first time. The specific Phe degraders and their relay and cooperation activity in soils as impacted by BCOA were first identified with DNA-stable isotope probing technology. Our findings provided a novel perspective to understand the efficient degradation of PAH in the BCOA treatments, revealed the potential of soil native microbes in the efficient bioremediation of PAH-contaminated natural soil, and provided a basis for the development of in-situ phytoremediation technologies to remediate PAH pollution in future.

Spatial heterogeneity of soil carbon exchanges and their drivers in a boreal forest

Boreal forests have a large impact on the global greenhouse gas balance and their soils constitute an important carbon (C) reservoir. Mature boreal forests are typically a net CO₂ sink, but there are also examples of boreal forests that are persistent CO₂ sources. The reasons remain often unknown, presumably due to a lack of understanding of how biotic and abiotic drivers interact to determine the microbial respiration of soil organic matter (SOM). This study aimed at identifying the main drivers of microbial SOM respiration and CO₂ and CH₄ soil chamber-fluxes within dry and wet sampling areas at the mature boreal forest of Norunda, Sweden, a persistent net CO₂ source. The spatial heterogeneity of the drivers was assessed with a geostatistical approach combined with stepwise multiple regression. We found that heterotrophic soil respiration increased with SOM content and nitrogen (N) availability, while the SOM reactivity, i.e., SOM specific respiration, was determined by soil moisture and N availability. The latter suggests that microbial activity was N rather than C limited and that microbial N mining might be driving old-SOM decomposition, which was observed through a positive correlation between soil respiration and its δ¹³C values. SOM specific heterotrophic respiration was lower in wet than in dry areas, while no such dependencies were found for chamber-based soil CO₂ fluxes, implying that oxygen depletion resulted in lower SOM reactivity. The chamber-based soil CH₄ flux differed significantly between the wet and dry areas. In the wet area, we observed net CH₄ emission that was positively related to soil moisture and NH₄⁺-N content. Taken together, our findings suggest that N availability has a strong regulatory effect on soil CO₂ and CH₄ emissions at Norunda, and that microbial decomposition of old-SOM to release bioavailable N might be partly responsible for the net CO₂ emission at the site.

Seasonal Variations in Litter Layers' Characteristics Control Microbial Respiration and Microbial Carbon Utilization Under Mature Pine, Cedar, and Beech Forest Stands in the Eastern Mediterranean Karstic Ecosystems

The forest floor is hotspot of several functions integral to the stability of forest ecosystems. However, seasonal variations in litter decomposition rate contribute to biochemical and structural heterogeneity in the forest floor carbon (C) and nutrient cycling. We investigated the influence of seasonal variations in litter layers' micro-climate (temperature and moisture content) and chemical characteristics such as pH, electrical conductivity (EC), total organic C (TOC), total nitrogen (TN), and C/N ratio on microbial respiration, biomass, and C use efficiency under mature (> 80 years stage age) pine, beech, and cedar forests in eastern Mediterranean Karstic ecosystems. In contrast to significantly higher microbial respiration in fall, winter, and spring under pine, beech, and cedar forests, the significantly lowest microbial biomass C (MBC) and microbial biomass N (MBN) were observed in winter under each forest. Microbial C use efficiency, measured as the metabolic quotient (qCO₂ = CO₂/MBC), varied strongly between forest stands and seasons but was generally higher in winter. The significant positive correlations between litter layer and microbial biomass C/N ratios, under beech and cedar forests, suggested strong CN stoichiometric coupling and microbial adaptation to substrate resource stoichiometry. qCO₂ correlated significantly negatively with litter layers' temperature, positively with moisture content and EC. However, qCO₂ had significant negative relationships with pH in pine and beech forests but significant positive under cedar forest. qCO₂ showed significant positive relationships with C/N ratios under all forests but much stronger in beech and cedar forests suggesting higher C respired per unit MBC with an increase in C/N ratio. Despite variations between forest species, the highest MBC/TOC and MBN/TN ratios in fall indicated greater C and N incorporation into microbial biomass. Changes in MBC/MBN ratios under pine (9.62-10.6), beech (8.63-15.6), and cedar (7.32-16.2) forests indicated the shift in microbial communities as fungi have a higher C/N ratio than bacteria. Stepwise regression analysis further revealed that microbial respiration and biomass were controlled differently by litter layer characteristics in each forest. This study suggested that qCO₂ independently or with other microbial indices can show litter layers' controls on organic matter turnover in Karst ecosystems and, taking into account the strong seasonal variations, can enhance the predictive potential of decomposition models.

Biogeochemical Fe(II) generators as a new strategy for limiting Cd uptake by rice and its implication for agricultural sustainability

This work has developed a new strategy of biogeochemical Fe(II) generators for activating microbial Fe(II) generation to immobilize Cd in soils through protons scavenging and coprecipitation. A new biochar modified magnetite (FeBC15) has been fabricated through a top-down method, with which microbial respiration can be stimulated in paddy soil. The FeBC15 exhibits a higher adsorption capacity for Cd than pristine magnetite (1.7 times). The results show that the available Cd can be reduced by 14.4% after adding FeBC15 compared to the control. More importantly, FeBC15 particles promote the conversion of MgCl₂ - Cd to stable crystalline Fe/Al bound Cd under the incubation period. The enhanced pH and Fe(II) leads to a comparably lower Cd availability in soils than in pristine soils, which are supported by the enhanced relative abundance of Geobacter and Clostridium with the FeBC15 treatment (i.e. up to 7.44-7.68 × 10⁹ copies/g soil). The Diffusive Gradients in Thin-films (DGT) study indicates that FeBC15 can lower the replenish capacity of soils (i.e. K_dL values of 0.2-3.6 mL/g) to soil pore waters and limit root absorption. Pot experiments demonstrate that this strategy can alleviate the rice Cd content by 38.4% (< 0.2 mg/kg). This work paves a new pathway for reducing Cd uptake in rice, enabling sustainable remediation of paddy soil.

Mixing effects of three Eurasian plants on root decomposition in the existence of living plant community in a meadow steppe

In grasslands, roots of different plant species decay in combination in the presence of living plants, besides, most root decomposition studies are conducted on how roots of plants decomposed alone or in artificial compositions in the absence of living plants. Therefore, we evaluated how roots of different perennial plants induced effects on decomposition process under living plants and their associated mechanisms. By using litter bag technique, we determined the root decomposition process of three perennial plants, Leymus chinensis, Phragmites australis, and Kalimeris integrifolia grown in monocultures, bi- and tri-species mixtures, after 12 months of incubation under living plants and bare soil communities. We found both additive and non-additive effects on decomposition dynamics indicating that root mass losses of compositions cannot be calculated from decaying rates of individual species. The rich-nutrient roots of K. integrifolia in monocultures and in mixtures with other plant species decayed faster. Compared with bare soil, microbial activities were enhanced under living plant communities and hence stimulated decomposition rates. Our results indicated that microbial activities are important but secondary factors to root physico-chemical properties impacting root decomposition rates. In conclusion, the empirical relationships developed here are helpful to better understand the effects of root properties and microbial activities on decay rates.

Algae, shrimp grazing, and fecal pellets synergistically increase microbial activity and enhance N immobilization during Typha angustifolia leaf litter decomposition

Algae play an important role in ecological processes of aquatic ecosystems. Understanding the interactive effects of algae with invertebrates in litter decomposition is important for predicting the effects of global change on aquatic ecosystems. We manipulated Typha angustifolia litter to control exposure to shrimp fecal pellets and/or grazing, and the green alga Chlorella vulgaris were added to test their interactive effects on T. angustifolia litter decomposition. Our results showed that algae largely shortened microbial conditioning time and improved litter palatability (increasing litter quality), resulting in greater decomposition and higher fecal pellet production. Fecal pellets enhanced grazing effects on decomposition by increasing litter ash content. The effects of algae and especially fecal pellets on decomposition were dependent on or mediated by grazing. Without grazing, algae slightly promoted decomposition and marginally offset the negative effect of fecal pellets on litter decomposition. Shrimp grazing dramatically decreased microbial activity (extracellular enzyme activity and microbial respiration) at microbial conditioning stage while enhanced microbial activity after 84 days especially with both algae and fecal pellets present. Algae significantly upregulated N- and P-acquiring and slightly downregulated C-acquiring enzyme activity. Fecal pellets significantly depressed recalcitrant C-decomposition enzyme activity. Nevertheless, the three factors synergistically and significantly increased C loss and most enzyme activities, microbial respiration, and N immobilization, resulting in the decrease of litter C:N. Our results reveal the synergistic action of different trophic levels (autotrophs, heterotrophs, and primary consumers) in the complicated nutrient pathways of litter decomposition and provide support for predicting the effects of global changes (e.g., N deposition and CO₂ enrichment), which have dramatically effects on alga dynamics and on ecological processes in aquatic ecosystems.

Demonstrational gardens with EDTA-washed soil. Part II: Soil quality assessment using biological indicators

In this study, we evaluated the impact of washing of Pb, Zn and Cd contaminated soil using EDTA-based technology (ReSoil®) on soil biological properties by measuring some of the most commonly used/sensitive biological indicators of soil perturbation. We estimated the temporal dynamics of the soil respiration, the activities of soil enzymes (dehydrogenase, β-glucosidase, urease, acid and alkaline phosphatase), and the effect of the remediation process on arbuscular mycorrhizal (AM) fungi in original (Orig), remediated (Rem) and remediated vitalized (Rem+V) soils during a more than one-year garden experiment. ReSoil® technology initially affected the activity level of soil microbial respiration and all enzyme activities except urease and reduced AM fungal potential in the soil. However, after one year of vegetable cultivation and standard gardening practices, soil microbial respiration, acid and alkaline phosphatase in the Rem and Rem+V reached similar activities as in the Orig. Only the activities of dehydrogenase and β-glucosidase remained lower in the remediated soil compared to the Orig. The frequency of arbuscular mycorrhiza in the root system, arbuscular density in the colonized root fragment, and the intensity of mycorrhizal colonization in the colonized root fragments in the remediated treatments increased with time; at the end of the experiment, no consistent differences in these parameters of mycorrhizal colonization were found among the treatments. Our results suggest a restored biological functioning of the remediated soil after one year of vegetable cultivation. In general, no differences were found between the Rem and Rem+V treatments, indicating that simple common garden practices are sufficient to restore soil functioning after remediation.

Effects of plant species on soil quality in natural and planted areas of a forest park in northern Iran

Reforestation may help protect the health of endangered forest ecosystems. To implement this action, it is important to evaluate the effects of the planted species on soil quality. Previous studies have demonstrated that soil properties are closely driven by the effects of plant roots and plant remains (quantity and quality) reaching the soil surface. However, little research is available about the effects of plant species on soil quality of reforested sites compared to natural forest ecosystems. This study evaluates the changes in the main soil properties between two 30-40 year-old stand types in forest areas of northern Iran: i) two stands, each one comprising a natural species (Parrotia persica or Pinus taeda); and ii) two stands, each one with planted trees (Quercus castaneifolia or Alnus glutinosa). Compared to reforested sites, the soils with natural trees showed higher root weight density (+43%), pH (+17%), and organic carbon (+64%). These differences led to higher nutrient contents, microbial respiration, aggregate stability, and water retention in soils with natural trees, as confirmed by the correlation analysis. A principal component analysis provided a meaningful combined factor (the first principal component) that showed a clear discrimination in soil quality and fertility among natural and reforested species. The calculation of a soil quality index confirms that planted species may lead to an overall lower quality of soils with planted species compared to natural forest. Since the lower soil quality of planted forests can be also the result of unsuitable management practices, this study suggest that forest operations in reforested areas should be avoided, since this could lead to negative effects on soil quality and contribute to an increase in the risk of soil degradation.

Land use intensification destabilizes stream microbial biodiversity and decreases metabolic efficiency

Urbanization and agricultural intensification can transform landscapes. Changes in land-use can lead to increases in storm runoff and nutrient loadings which can impair the health and function of stream ecosystems. Microorganisms are an integral component of stream ecosystems. Due to the sensitivity of microorganisms to perturbations, changes in hydrology and water chemistry may alter microbial activity and structure. These shifts in microbial community dynamics may alter stream metabolism and water quality, potentially impacting higher trophic levels. Here we examine the effects of land-use and associated changes in water chemistry on sediment microbial communities by studying the West Run Watershed (WRW) a mixed-land-use system in West Virginia, USA. Streams were sampled throughout the growing season at six sites within the WRW spanning different levels of land use intensification. The proportion of land impacted by agricultural and urban development was positively correlated with temporal variation in stream sediment microbial community composition (adj R² = 0.65), suggesting development can destabilize microbial communities. Moreover, streams in developed watersheds had an increased metabolic quotient (20-50% higher), this indicates that microorganisms have greater respiration per unit biomass and signifies reduced metabolic efficiency. Further, our results suggest that land use associated changes in water chemistry alter microbial function both directly and indirectly via changes in microbial community composition and biomass. Taken together our results suggest that highly developed watersheds with elevated conductivity, metal ion concentration, and pH impose stress on microbial communities resulting in reduced microbial efficiency and elevated respiration.

Microbial activity and metamitron degrading microbial communities differ between soil and water-sediment systems

The herbicide metamitron is frequently detected in the environment, and its degradation in soil differs from that in aquatic sediments. In this study, we applied ¹³C₆-metamitron to investigate the differences in microbial activity, metamitron mineralization and metamitron degrading microbial communities between soil and water-sediment systems. Metamitron increased soil respiration, whereas it suppressed respiration in the water-sediment system as compared to controls. Metamitron was mineralized two-fold faster in soil than in the water-sediment. Incorporation of ¹³C from ¹³C₆-metamitron into Phospholipid fatty acids (PLFAs) was higher in soil than in sediment, suggesting higher activity of metamitron-degrading microorganisms in soil. During the accelerated mineralization of metamitron, biomarkers for Gram-negative, Gram-positive bacteria and actinobacteria dominated within the ¹³C-PLFAs in soil. Gram-negative bacteria dominated among the metamitron degraders in sediment throughout the incubation period. Actinobacteria, and actinobacteria and fungi were the main consumers of necromass of primary degraders in soil and water-sediment, respectively. This study clearly showed that microbial groups involved in metamitron degradation depend on the system (soil vs. water-sediment) and on time. It also indicated that the turnover of organic chemicals in complex environments is driven by different groups of synthropic degraders (primary degraders and necromass degraders) rather than by a single degrader.

Plant material and its biochar differ in their effects on nitrogen mineralization and nitrification in a subtropical forest soil

Natural wildfires have a great effect on soil N transformation in subtropical forest. The pyrogenic organic matter (PyOM) in forest soils is mainly derived from the plant material burnt during forest fires, which affects soil N composition, N mineralization and nitrification. This study examined the effects of typical fresh plant material (leaves and twigs of Castanopsis sclerophylla, representing litter) and its biochar (representing PyOM) on N mineralization and nitrification in a subtropical forest soil. The soils were incubated with the plant material (PM), its biochar (BC) and their combinations for 84 days. Both PM and BC considerably increased soil pH and dissolved organic C, whereas PM decreased NO₃^--N and dissolved organic N. The additions of PM alone, and its combinations with BC resulted in net N immobilization. The rates of net N mineralization rapidly increased in first 14 days and then became stable following the addition of PM to soil. Moreover, the additions of PM and BC increased the abundances of archaeal amoA and bacterial amoA, especially with PM. The abundance of bacterial amoA correlated positively with soil pH and dissolved organic C, while archaeal amoA showed the opposite. Biochar affected soil properties and N transformation more significantly in the presence of PM, highlighting the need for further research on the interactions of plant litter and its biochar.

Timing of oviposition influences the effects of a non-native grass on amphibian development

Land-use change can alter the energy dynamics in aquatic systems by changing the subsidies that form the nutrient base within them. However, experimental evaluations of subsidy change often fail to consider how effects, such as differences in individual growth and survival, may differ under varying ecological contexts experienced in the field. We used a mesocosm approach to investigate how litter (Native Prairie or Non-Native Tall-Fescue Grass) surrounding wetlands and timing of oviposition affected larval amphibian development. We found that survival differed between litter types in the Early-Oviposition treatment, with nearly 100% mortality in Fescue treatments. Conversely, survival  was similar across litter types in the Late Oviposition treatment (~ 43%), and larvae in Late-Fescue treatments metamorphosed more quickly and were larger post-metamorphosis than larvae in Prairie treatments. Follow-up experiments confirmed that low dissolved oxygen (DO) was responsible for high mortality in Early-Fescue treatments; high quantities of Fescue resulted in a microbial bloom that reduced DO to < 2 mg/L for several days, resulting in low hatching success. This effect was eliminated in treatments with supplemental aeration. Finally, we confirmed that experimentally observed DO patterns also occurred in the field. Context (i.e., timing of inundation relative to amphibian breeding) is critical to understanding the effects of subsidies on amphibian populations; early and explosively breeding species may experience catastrophic mortality due to DO depletion; whereas, species that breed later may experience enhanced fitness of recruits. Considering the effects of non-native species across different ecological contexts is necessary for elucidating the extent of their impacts.

The Legacy Effects of Winter Climate on Microbial Functioning After Snowmelt in a Subarctic Tundra

Warming-induced increases in microbial CO₂ release in northern tundra may positively feedback to climate change. However, shifts in microbial extracellular enzyme activities (EEAs) may alter the impacts of warming over the longer term. We investigated the in situ effects of 3 years of winter warming in combination with the in vitro effects of a rapid warming (6 days) on microbial CO₂ release and EEAs in a subarctic tundra heath after snowmelt in spring. Winter warming did not change microbial CO₂ release at ambient (10 °C) or at rapidly increased temperatures, i.e., a warm spell (18 °C) but induced changes (P < 0.1) in the Q₁₀ of microbial respiration and an oxidative EEA. Thus, although warmer winters may induce legacy effects in microbial temperature acclimation, we found no evidence for changes in potential carbon mineralization after spring thaw.

Contribution of root respiration to total soil respiration in a semi-arid grassland on the Loess Plateau, China

Using the trenching method, a study was conducted in a grassland on the Loess Plateau of northern China in 2008 and 2009 to partition total soil respiration (Rt) into microbial respiration (Rm) and root respiration (Rr). Using the measurements of soil CO₂ diffusivity and soil CO₂ production, an analytical model was applied to correct the data, aiming to quantify the method-induced error. The results showed that Rm and Rr responded differently to biotic and abiotic factors and exhibited different diurnal and seasonal variations. The diurnal variation of Rm was strongly controlled by soil temperature, while Rr might be mainly controlled by photosynthesis. The combination of soil temperature and moisture could better explain the seasonal variation in Rm (r²=0.76, P<0.001). The seasonal variation of Rr was influenced mainly by the plant activity. The contribution of root respiration to total soil respiration (Rr/Rt ratio) also exhibited substantial diurnal and seasonal variations, being higher at nighttime and lower at daytime. In the different growing stages, the Rr/Rt ratios ranged from 15.0% to 62.0% in 2008 and 14.5% to 63.6% in 2009. The mean values of the Rr/Rt ratio in the growing season and the annual mean Rr/Rt ratio were 41.7% and 41.9%, respectively, during the experiment period. Different precipitation distributions in the two years did not change the yearly Rr/Rt ratio. Corrected with the analytical model, the trenching method in small root-free plots led to an underestimation of Rr and Rr/Rt ratio by 4.2% and 1.8%.

Effect of long-term fertilization on humic redox mediators in multiple microbial redox reactions

This study investigated the effects of different long-term fertilizations on humic substances (HSs), humic acids (HAs) and humins, functioning as redox mediators for various microbial redox biotransformations, including 2,2',4,4',5,5'- hexachlorobiphenyl (PCB₁₅₃) dechlorination, dissimilatory iron reduction, and nitrate reduction, and their electron-mediating natures. The redox activity of HSs for various microbial redox metabolisms was substantially enhanced by long-term application of organic fertilizer (pig manure). As a redox mediator, only humin extracted from soils with organic fertilizer amendment (OF-HM) maintained microbial PCB₁₅₃ dechlorination activity (1.03 μM PCB₁₅₃ removal), and corresponding HA (OF-HA) most effectively enhanced iron reduction and nitrate reduction by Shewanella putrefaciens. Electrochemical analysis confirmed the enhancement of their electron transfer capacity and redox properties. Fourier transform infrared analysis showed that C=C and C=O bonds, and carboxylic or phenolic groups in HSs might be the redox functional groups affected by fertilization. This research enhances our understanding of the influence of anthropogenic fertility on the biogeochemical cycling of elements and in situ remediation ability in agroecosystems through microorganisms' metabolisms.

Optimizing vermifilter depth by process performance collaborated with the evolutions of microbial characteristics during sewage sludge treatment

The present study focuses on optimizing filter depth on sludge reduction in a four-stage vermifiltration during the course of treating excess sludge continuously. The results indicated that when the filter depth exceeded 75 cm, though the fourth stage can further advance the sludge reduction, its contribution for the total sludge reduction was lower than 10%, while the aerobic bacteria, especially the dominant bacteria (Proteobacteria and Bacteroidetes), kept a high similarity as the filter depth varied. Furthermore, earthworm activities attributed to aerobic bacteria being preferentially selected in the system, positively supporting the organic decomposition. As far as economic cost and process performance are concerned, a 75-cm vermifilter was recommended to efficiently and economically achieve the required standard for sewage sludge reduction and stabilization.

Use of sugarcane filter cake and nitrogen, phosphorus and potassium fertilization in the process of bioremediation of soil contaminated with diesel

This study evaluated the use of sugarcane filter cake and nitrogen, phosphorus and potassium (NPK) fertilization in the bioremediation of a soil contaminated with diesel fuel using a completely randomized design. Five treatments (uncontaminated soil, T1; soil contaminated with diesel, T2; soil contaminated with diesel and treated with 15 % (wt) filter cake, T3; soil contaminated with diesel and treated with NPK fertilizer, T4; and soil contaminated with diesel and treated with 15 % (wt) filter cake and NPK fertilizer, T5) and four evaluation periods (1, 60, 120, and 180 days after the beginning of the experiment) were used according to a 4 × 5 factorial design to analyze CO2 release. The variables total organic carbon (TOC) and total petroleum hydrocarbons (TPH) remaining in the soil were analyzed using a 5 × 2 factorial design, with the same treatments described above and two evaluation periods (1 and 180 days after the beginning of the experiment). In T3 and T5, CO2 release was significantly higher, compared with the other treatments. Significant TPH removal was observed on day 180, when percent removal values were 61.9, 70.1, 68.2, and 75.9 in treatments T2, T3, T4, and T5, respectively, compared with the initial value (T1).

Dynamics of dissolved organic carbon (DOC) through stormwater basins designed for groundwater recharge in urban area: Assessment of retention efficiency

Managed aquifer recharge (MAR) has been developed in many countries to limit the risk of urban flooding and compensate for reduced groundwater recharge in urban areas. The environmental performances of MAR systems like infiltration basins depend on the efficiency of soil and vadose zone to retain stormwater-derived contaminants. However, these performances need to be finely evaluated for stormwater-derived dissolved organic matter (DOM) that can affect groundwater quality. Therefore, this study examined the performance of MAR systems to process DOM during its transfer from infiltration basins to an urban aquifer. DOM characteristics (fluorescent spectroscopic properties, biodegradable and refractory fractions of dissolved organic carbon -DOC-, consumption by micro-organisms during incubation in slow filtration sediment columns) were measured in stormwater during its transfer through three infiltration basins during a stormwater event. DOC concentrations sharply decreased from surface to the aquifer for the three MAR sites. This pattern was largely due to the retention of biodegradable DOC which was more than 75% for the three MAR sites, whereas the retention of refractory DOC was more variable and globally less important (from 18% to 61% depending on MAR site). Slow filtration column experiments also showed that DOC retention during stormwater infiltration through soil and vadose zone was mainly due to aerobic microbial consumption of the biodegradable fraction of DOC. In parallel, measurements of DOM characteristics from groundwaters influenced or not by MAR demonstrated that stormwater infiltration increased DOC quantity without affecting its quality (% of biodegradable DOC and relative aromatic carbon content -estimated by SUVA254-). The present study demonstrated that processes occurring in soil and vadose zone of MAR sites were enough efficient to limit DOC fluxes to the aquifer. Nevertheless, the enrichments of DOC concentrations measured in groundwater below infiltration basins need to be considered in future studies to especially assess their impact on groundwater quality.

The use of concentrated monosodium glutamate wastewater as a conditioning agent for adjusting acidity and minimizing ammonia volatilization in livestock manure composting

In this study, concentrated monosodium glutamate waste (CMGW) was proposed as a conditioning agent to adjust acidity and decrease ammonia (NH3) volatilization in thermophilic aerobic composting based on two incubation experiments. The results showed that with the addition of CMGW, NH3 volatilization of compost mixture under high temperature phase decreased significantly and pH met the current national standard within 5.5-8.5. When CMGW dosage increased to 2% (v/w), the decrease in NH3 volatilization was as high as 78.9%. This effect was enhanced by repeated application of CMGW. Furthermore, although the electrical conductivity increased with the application of CMGW, both the germination index and the microbial respiration of compost mixture implied that CMGW had no negative effects on the maturity of compost, instead, a comprehensive maturity might be accelerated. It was concluded that CMGW was an optional conditioning agent for thermophilic aerobic composting of livestock manure in regards to adjusting acidity and preventing nitrogen loss from NH3 volatilization.

Effects of nitrogen addition on soil microbes and their implications for soil C emission in the Gurbantunggut Desert, center of the Eurasian Continent

Nitrogen (N) deposition can influence carbon cycling of terrestrial ecosystems. However, a general recognition of how soil microorganisms respond to increasing N deposition is not yet reached. We explored soil microbial responses to two levels of N addition (2.5 and 5 gN m(-2) yr(-1)) in interplant soil and beneath shrubs of Haloxylon ammodendron and their consequences to soil respiration in the Gurbantunggut Desert, northwestern China from 2011 to 2013. Microbial biomass and respiration were significantly higher beneath H. ammodendron than in interplant soil. The responses of microbial biomass carbon (MBC) and microbial respiration (MR) showed opposite responses to N addition in interplant and beneath H. ammodendron. N addition slightly increased MBC and MR in interplant soil and decreased them beneath H. ammodendron, with a significant inhibition only in 2012. N addition had no impacts on the total microbial physiological activity, but N addition decreased the labile carbon substrate utilization beneath H. ammodendron when N addition level was high. Phospholipid fatty acid (PLFA) analysis showed that N addition did not alter the soil microbial community structure as evidenced by the similar ratios of fungal to bacterial PLFAs and gram-negative to gram-positive bacterial PLFAs. Microbial biomass and respiration showed close correlations with soil water content and dissolved carbon, and they were independent of soil inorganic nitrogen across three years. Our study suggests that N addition effects on soil microorganisms and carbon emission are dependent on the respiratory substrates and water availability in the desert ecosystem.

Column studies to assess the effects of climate variables on redox processes during riverbank filtration

Riverbank filtration is an established technique used world-wide to produce clean drinking water in a reliable and cost-efficient way. This practice is, however, facing new challenges posed by climate change, as already observed during past heat waves with the local occurrence of anoxic conditions. In this study we investigated the effect of direct (temperature) and indirect (dissolved organic matter (DOM) concentration and composition, flow rate) climate change variables on redox processes (aerobic respiration, denitrification and Mn(III/IV)/Fe(III) reduction) by means of column experiments. Natural river water, modified river water and river water mixed with treated wastewater effluent were used as feed waters for the columns filled with natural sand from a river-infiltration system in Switzerland. Biodegradable dissolved organic matter was mainly removed immediately at the column inlet and particulate organic matter (POM) associated with the natural sand was the main electron donor for aerobic respiration throughout the column. Low infiltration rates (≤0.01 m/h) enhanced the oxygen consumption leading to anoxic conditions. DOM consumption did not seem to be sensitive to temperature, although oxygen consumption (i.e., associated with POM degradation) showed a strong temperature dependence with an activation energy of ∼70 kJmol(-1). Anoxic conditions developed at 30 °C with partial denitrification and formation of nitrite and ammonium. In absence of oxygen and nitrate, Mn(II) was mobilized at 20 °C, highlighting the importance of nitrate acting as a redox buffer under anoxic conditions preventing the reductive dissolution of Mn(III/IV)(hydr)oxides. Reductive dissolution of Fe(III)(hydr)oxides was not observed under these conditions.

Mobility and distribution of arsenic in contaminated mine soils and its effects on the microbial pool

Three soils, coming from a former mining site and characterized by a different degree of pollution, were analysed in terms of Arsenic (As) content, using three different analytical approaches, and its distribution in various soil fractions. The effect of As on soil microbial biomass (size, respiration and microbial quotients) was also analysed. Total arsenic concentration between soil fractions was significantly different and ranged from 189 to 4357mgkg(-1), indicating a high level of pollution. Soil sequential fractioning showed that more than 60 percent of total As was bound to Fe-Al oxides, suggesting a minor availability and environmental risk regardless the total concentration of As in the sample. On the contrary, water soluble As fraction showed a significant difference among the three samples. The largest water soluble As concentration was found in the sample with intermediate total As amount. As far as microbial biomass is concerned, it was found that bioavailable As negatively impacted microbial metabolism in terms of basal and cumulative respiration, and microbial quotients, suggesting a strong selection within microbial pool.

A closeup study of early beech litter decomposition: potential drivers and microbial interactions on a changing substrate

AIMS

Litter decomposition and subsequent nutrient release play a major role in forest carbon and nutrient cycling. To elucidate how soluble or bulk nutrient ratios affect the decomposition process of beech (Fagus sylvatica L.) litter, we conducted a microcosm experiment over an 8 week period. Specifically, we investigated leaf-litter from four Austrian forested sites, which varied in elemental composition (C:N:P ratio). Our aim was to gain a mechanistic understanding of early decomposition processes and to determine microbial community changes.

METHODS

We measured initial litter chemistry, microbial activity in terms of respiration (CO2), litter mass loss, microbial biomass C and N (Cmic and Nmic), non purgeable organic carbon (NPOC), total dissolved nitrogen (TDN), NH4+, NO3- and microbial community composition (phospholipid fatty acids - PLFAs).

RESULTS

At the beginning of the experiment microbial biomass increased and pools of inorganic nitrogen (N) decreased, followed by an increase in fungal PLFAs. Sites higher in NPOC:TDN (C:N of non purgeable organic C and total dissolved N), K and Mn showed higher respiration.

CONCLUSIONS

The C:N ratio of the dissolved pool, rather than the quantity of N, was the major driver of decomposition rates. We saw dynamic changes in the microbial community from the beginning through the termination of the experiment.