Methane turnover and temperature response of methane-oxidizing bacteria in permafrost-affected soils of northeast Siberia

The abundance, activity, and temperature response of aerobic methane-oxidizing bacteria were studied in permafrost-affected tundra soils of northeast Siberia. The soils were characterized by both a high accumulation of organic matter at the surface and high methane concentrations in the water-saturated soils. The methane oxidation rates of up to 835 nmol CH₄ h⁻¹ g⁻¹ in the surface soils were similar to the highest values reported so far for natural wetland soils worldwide. The temperature response of methane oxidation was measured during short incubations and revealed maximum rates between 22 °C and 28 °C. The active methanotrophic community was characterized by its phospholipid fatty acid (PLFA) concentrations and with stable isotope probing (SIP). Concentrations of 16:1ω8 and 18:1ω8 PLFAs, specific to methanotrophic bacteria, correlated significantly with the potential methane oxidation rates. In all soils, distinct 16:1 PLFAs were dominant, indicating a predominance of type I methanotrophs. However, long-term incubation of soil samples at 0 °C and 22 °C demonstrated a shift in the composition of the active community with rising temperatures. At 0 °C, only the concentrations of 16:1 PLFAs increased and those of 18:1 PLFAs decreased, whereas the opposite was true at 22 °C. Similarly, SIP with ¹³CH₄ showed a temperature-dependent pattern. When the soils were incubated at 0 °C, most of the incorporated label (83%) was found in 16:1 PLFAs and only 2% in 18:1 PLFAs. In soils incubated at 22 °C, almost equal amounts of ¹³C label were incorporated into 16:1 PLFAs and 18:1 PLFAs (33% and 36%, respectively). We concluded that the highly active methane-oxidizing community in cold permafrost-affected soils was dominated by type I methanotrophs under in situ conditions. However, rising temperatures, as predicted for the future, seem to increase the importance of type II methanotrophs, which may affect methane cycling in northern wetlands.     

Long term repeated burning in a wet sclerophyll forest reduces fungal and bacterial biomass and responses to carbon substrates

Soils from a long term experiment, established in 1972, incorporating replicated treatments of burning every 2 and 4 years with control plots were sampled in 2005 to determine the changes in microbial community structure, measured using phospholipid fatty acids (PLFAs) and functional diversity measured using multiple substrate induced respiration (SIR) tests (MicroResp™). Microbial biomass (total PLFA) in the 2 year burn treatments was 50% less than both the control and 4-year burn treatments. There was also concomitantly less respiratory activity which mirrored the known changes in soil C and substrate quality. Contrary to other studies soil bacterial PLFAs were reduced as much as fungal PLFAs in the 2-year burn and the short term (6 h) SIR of arginine, lysine, galactose and trehalose were significantly inhibited in the 2-year burn soils. The data suggest that a 4-year burn is a more sustainable practice for maintaining the original structure and function of the forest belowground ecosystem.     

Changes in soil microbial community structure following the abandonment of agricultural terraces in mountainous areas of Eastern Spain

In Eastern Spain, almond trees have been cultivated in terraced orchards for centuries, forming an integral part of the Mediterranean forest scene. In the last decades, orchards have been abandoned due to changes in society. This study investigates effects of changes in land use from forest to agricultural land and the posterior land abandonment on soil microbial community, and the influence of soil physico-chemical properties on the microbial community composition (assessed as abundances of phospholipids fatty acids, PLFA). For this purpose, three land uses (forest, agricultural and abandoned agricultural) at four locations in SE Spain were selected. Multivariate analysis showed a substantial level of differentiation in microbial community structure according to land use. The microbial communities of forest soils were highly associated with soil organic matter content. However, we have not found any physical or chemical soil property capable of explaining the differences between agricultural and abandoned agricultural soils. Thus, it was suggested that the cessation of the perturbation caused by agriculture and shifts in vegetation may have led to changes in the microbial community structure. PLFAs indicative of fungi and ratio of fungal to bacterial PLFAs were higher in abandoned agricultural soils, whereas the relative abundance of bacteria was higher in agricultural soils. Actinomycetes were generally lower in abandoned agricultural soils, while the proportions of vesicular–arbuscular mycorrhyzal fungi were, as a general trend, higher in agricultural and abandoned agricultural soils than in forests. Total microbial biomass and richness increased as agricultural < abandoned agricultural < forest soils.     

Distinct microbial and faunal communities and translocated carbon in Lumbricus terrestris drilospheres

Lumbricus terrestris is a deep-burrowing anecic earthworm that builds permanent, vertical burrows with linings (e.g., drilosphere) that are stable and long-lived microhabitats for bacteria, fungi, micro- and mesofauna. We conducted the first non-culture based field study to assess simultaneously the drilosphere (here sampled as 0–2 mm burrow lining) composition of microbial and micro/mesofaunal communities relative to bulk soil. Our study also included a treatment of surface-applied ¹³C- and ¹⁵N-labeled plant residue to trace the short-term (40 d) translocation of residue C and N into the drilosphere, and potentially the assimilation of residue C into drilosphere microbial phospholipid fatty acids (PLFAs). Total C concentration was 23%, microbial PLFA biomass was 58%, and PLFAs associated with protozoa, nematodes, Collembola and other fauna were 200-to-300% greater in the drilosphere than in nearby bulk soil. Principal components analysis of community PLFAs revealed that distributions of Gram-negative bacteria and actinomycetes and other Gram-positive bacteria were highly variable among drilosphere samples, and that drilosphere communities were distinct from bulk soil communities due to the atypical distribution of PLFA biomarkers for micro- and mesofauna. The degree of microbial PLFA ¹³C enrichment in drilosphere soils receiving ¹³C-labeled residue was highly variable, and only one PLFA, 18:1ω9c, was significantly enriched. In contrast, 11 PLFAs from diverse microbial groups where enriched in response to residue amendment in bulk soil 0–5 cm deep. Among control soils, however, a significant δ¹³C shift between drilosphere and bulk soil at the same depth (5–15 cm) revealed the importance of L. terrestris for translocating perennial ryegrass-derived C into the soil at depth, where we estimated the contribution of the recent grass C (8 years) to be at least 26% of the drilosphere soil C. We conclude that L. terrestris facilitates the translocation of plant C into soil at depth and promotes the maintenance of distinct soil microbial and faunal communities that are unlike those found in the bulk soil.     

The microbial PLFA composition as affected by pH in an arable soil

The influence of soil pH on the phospholipid fatty acid (PLFA) composition of the microbial community was investigated along the Hoosfield acid strip, Rothamsted Research, UK - a uniform pH gradient between pH 8.3 and 4.5. The influence of soil pH on the total concentration of PLFAs was not significant, while biomass estimated using substrate induced respiration decreased by about 25%. However, the PLFA composition clearly changed along the soil pH gradient. About 40% of the variation in PLFA composition along the gradient was explained by a first principal component, and the sample scores were highly correlated to pH (R² = 0.97). Many PLFAs responded to pH similarly in the Hoosfield arable soil compared with previous assessments in forest soils, including, e.g. monounsaturated PLFAs 16:1ω5, 16:1ω7c and 18:1ω7, which increased in relative concentrations with pH, and i16:0 and cy19:0, both of which decreased with pH. Some PLFAs responded differently to pH between the soil types, e.g. br18:0. We conclude that soil pH has a profound influence on the microbial PLFA composition, which must be considered in all applications of this method to detect changes in the microbial community.     

Comparison of soil fungal/bacterial ratios in a pH gradient using physiological and PLFA-based techniques

We have compared the total microbial biomass and the fungal/bacterial ratio estimated using substrate-induced respiration (SIR) in combination with the selective inhibition technique and using the phospholipid fatty acid (PLFA) technique in a pH gradient (3.0-7.2) consisting of 53 mature broad-leaved forest soils. A fungal/bacterial biomass index using the PLFA technique was calculated using the PLFA 18:2ω6,9 as an indicator of fungal biomass and the sum of 13 bacterial specific PLFAs as indicator of the bacterial biomass. Good linear correlation (p<0.001) was found between the total microbial biomass estimated with SIR and total PLFAs (totPLFA), indicating that 1 mg biomass-C was equivalent to 130 nmol totPLFA. Both biomass estimates were positively correlated to soil pH. The fungal/bacterial ratio measured using the selective inhibition technique decreased significantly with increasing pH from about 9 at pH 3 to approximately 2 at pH 7, while the fungal/bacterial biomass index using PLFA measurements tended to increase slightly with increasing soil pH. Good correlation between the soil content of ergosterol and of the PLFA 18:2ω6,9 indicated that the lack of congruency between the two methods in estimating fungal/bacterial ratios was not due to PLFA 18:2ω6,9-related non-fungal structures to any significant degree. Several PLFAs were strongly correlated to soil pH (R² values >0.8); for example the PLFAs 16:1ω5 and 16:1ω7c increased with increasing soil pH, while i16:0 and cy19:0 decreased. A principal component analysis of the total PLFA pattern gave a first component that was strongly correlated to soil pH (R²=0.85, p<0.001) indicating that the microbial community composition in these beech/beech-oak forest soils was to a large extent determined by soil pH.     

不同肥力水平和利用历史的红壤磷脂脂肪酸图谱

Analysis of phospholipid fatty acids(PLFAs) was used to estimate the microbial community structures of eight Chinese red soils with different fertility levels and land use histories.The total amounts of PLFAs in the soils were significantly correltaed with soil organic carbon, total nitrogen,microbial biomass C and basal respiration,indicating that total PLFA was closely related to fertility and sustainbility in these highly weathered soils.Soils of the eroded wastelan were rich in Gram-positive species .When the eroded soils were planted with citrus trees,the soil microbial population had changed little in 4 years but took up to 8-12 yearss before it reached a significantly different population,Multivariate analysis of PLFAs demonstrated that land use history and plant cover type had a significant impact on microbial community structure.Howver,the difference of soil microbial community structure in the paddy field compared to other land uses was not larger than expected in this experiment.     

Does history matter? Temperature effects on soil microbial biomass and community structure based on the phospholipid fatty acid (PLFA) analysis

Background, aim, and scope

Temperature is an important environmental factor regulating soil microbial biomass, activity, and community. Soils from different climatic regions may have very different dominant soil microbes, which are acclimated to the local conditions like temperature. Changing soil temperature especially warming has been shown to increase the mortality rate of soil microbes. However, little is known about the responses of soil microbes coming from different climatic regions to different incubation temperatures. The objective of this study was to examine the temperature effects on microbial biomass and community of soils collected from cold, intermediate, and hot natural sites.

Materials and methods

Soils were collected from northern (Heilongjiang province), central (Jiangsu province), and southern (Guangxi province) China, these soils having very different temperature histories. The Heilongjiang soil was from the coldest region with a mean annual temperature of 1.2°C, the Jiangsu soil was intermediate with a mean annual temperature of 15.7°C, and Guangxi soil was from the hottest area, with a mean annual temperature of 21.2°C. These three soils were incubated at 4°C, 15°C, 25°C, and 35°C for up to 56 days. Phospholipid fatty acid (PLFA) analyses were conducted on days 0, 3, 7, 14, 28, and 56 to track the dynamics of soil microbes.

Results

Soil microbial biomass indexed by total phospholipid fatty acid concentration decreased with increasing incubation temperature, with that of the Heilongjiang soil decreasing most. At the end of incubation, the biomass at 35°C in the Heilongjiang, Jiangsu, and Guangxi soils had declined to 65%, 72%, and 96% of the initial biomass, respectively. The PLFA patterns shifted with increasing temperatures in all the soils, especially at 35°C; the change was biggest in the Heilongjiang soil.

Discussion

History does have effects on soil microbes responding to environmental stress. Soil microbial biomass and PLFA profiles shifted least in the Guangxi soil with the hottest temperature history and most in the Heilongjiang soil with the coldest temperature, indicating that the distribution of free-living microorganisms is influenced by climatic factors. The majority of soil microorganisms coming from the hot regions are more adapted to high temperature (35°C) compared to those from the cold area. There are some regular changes of PLFA profiles when increasing incubation temperature to 35°C. However, the effect of incubation temperature on soil microbial community structure was inconclusive. As PLFA profile community structure is the phenotypic community structure. Genotype experiments are required to be done in future studies for the better understanding of soil microbes in different climate regions with concerning temperature variation.

Conclusions

With the increasing incubation temperature, soil microbial biomass and PLFA profiles shifted most in the soil with the coldest temperature history and least in the soil with the hottest temperature. History does matter in determining soil microbial dynamics when facing thermal stress.     

Soil microbial communities response to herbicide 2,4-dichlorophenoxyacetic acid butyl ester

2,4-Dichlorophenoxyacetic acid butyl ester (2,4-D butyl ester) is extensively applied for weed control in cultivation fields in China, but its effect on soil microbial community remains obscure. This study investigated the microbial response to 2,4-D butyl ester application at different concentrations (CK, 10, 100 and 1000 μg g^?1) in the soils with two fertility levels, using soil dilution plate method and phospholipid fatty acid (PLFA) analysis. Culturable microorganisms were affected by the herbicide in both soils, particularly at the higher concentration. After treating soil with 100 μg g^?1 herbicide, culturable bacteria and actinomycetes were significantly higher, compared to other treatments. Treatment of soil with 1000 μg g^?1 2,4-D butyl ester caused a decline in culturable microbial counts, with the exception of fungal numbers, which increased over the incubation time. PLFA profiles showed that fatty acids for Gram-negative (GN) bacteria, Gram-positive (GP) bacteria, total bacteria and total fungi, as well as total PLFAs, varied with herbicide concentration for both soil samples. As herbicide concentration increased, the GN/GP ratio decreased dramatically in the two soils. The higher stress level was in the treatments with high concentrations of herbicide (1000 μg g^?1) for both soils. Principal component analysis of PLFAs showed that the addition of 2,4-D butyl ester significantly shifted the microbial community structure in the two soils. These results showed that the herbicide 2,4-D butyl ester might have substantial effects on microbial population and microbial community structure in agricultural soils. In particular, the effects of 2,4-D butyl ester were greater in soil with low organic matter and fertility level than in soil with high organic matter and fertility level.     

The influence of freeze–thaw cycles and soil moisture on aggregate stability of three soils in Norway

Winter conditions with seasonally frozen soils may have profound effects on soil structure and erodibility, and consequently for runoff and erosion. Such effects on aggregate stability are poorly documented for Nordic winter conditions. The purpose of this study was to quantify the effect of variable freeze–thaw cycles and soil moisture conditions on aggregate stability of three soils (silt, structured clay loam—clay A and levelled silty clay loam—clay B), which are representative of two erosion prone areas in southeastern Norway. A second purpose was to compare aggregate stabilities measured by the Norwegian standard procedure (rainfall simulator) and the more widely used wet-sieving procedure. Surface soil was sampled in autumn. Field moist soil was sieved into the fraction 1–4 mm and packed into cylinders. The water content of the soil was adjusted, corresponding to matric potentials of − 0.75, − 2 and − 10 kPa. The soil cores were insulated and covered, and subjected to 0, 1, 3 or 6 freeze–thaw cycles: freezing at − 15 °C for 24 h and thawing at 9 °C for 48 h. Aggregate stability was measured in a rainfall simulator (all soils) and a wet-sieving apparatus (silt and clay B). The rainfall stability of silt was found to be significantly lower than of clay A and clay B. Clay A and clay B had similar rainfall stabilities, even though it was expected that the artificially levelled clay B would have lower stability. Freezing and thawing decreased the rainfall stability of all soils, but the effect was more severe on the silt soil. There was no evident effect of water content on the stability, probably due to experimental limitations. The same effects were observed for wet-sieved soil, but the wet-sieving resulted in less aggregate breakdown than the rainfall simulator. Rainfall impact seemed to be more detrimental than wet-sieving on more unstable soil, that is, on silt soil and soil subjected to many freeze–thaw cycles. Such conditions are expected to occur frequently during field conditions in unstable winters.     

Solvent-based washing removes lipophilic contaminant interference with phospholipid fatty acid analysis of soil communities

Phospholipid fatty acid (PLFA) analysis is an informative method for characterising and quantifying changes in the phenotypic profile of the soil microbial community when soils are exposed to chemical toxicants and other xenobiotics. However, where such materials are lipophilic, a range of non-polar compounds can be co-extracted with PLFAs and can consequently mask PLFA chromatograms. We found this to be the case with the lipophilic anti-microbial compound triclosan, which can enter the soil via the addition of sewage sludge. A simple method of washing soil in solvent prior to extraction was developed in order to remove triclosan without altering the relative abundance of PLFAs. Three contrasting soils were spiked with 500 mg kg⁻¹ of triclosan before being washed with methanol (MeOH), dichloromethane (DCM), hexane or aqueous solutions of these solvents. PLFAs were then extracted and analysed. All treatments were found to remove triclosan effectively, allowing all peaks to be identifiable. Whilst the polar solvents MeOH and DCM significantly altered the relative abundance of extracted fatty acids in most of the soils tested, soil washing with a small quantity of hexane was able to remove triclosan whilst best preserving the fidelity of the PLFA profiles.     

Structural diversity and enzyme activity of volcanic soils at different stages of development and response to experimental disturbance

We investigated the phospholipid fatty acid (PLFA) diversity and enzyme activities in soils from the volcano, Mt. Etna (Sicily). The soils were at sites which have been developing for different periods of time and have formed in volcanic lava of differing ages that have been supplemented with volcanic ejecta from subsequent eruptions. However, the plant communities indicated a marked successional difference between the sites and we have used this as a proxy for developmental stage. We have compared the structural and functional properties of the microbial communities in soils from the two sites and tested experimentally the hypothesis that the more diverse community was more resistant and resilient to disturbance. The experimental disturbance imposed was heating (60 °C for 48 h) and the recovery of enzyme activities (β-glucosidase, acid phosphatase and arylsulfatase) and structural properties (PLFA profiles) were then followed over six months. The microbial community in the soil from the older site was more structurally diverse and had a larger total PLFA concentration before disturbance than that of the soil from the younger site. The older soil community was not more resistant and resilient following an environmental disturbance as the younger soil community was equally or more resistant and resilient for all parameters. Changes in enzyme activities following disturbance were almost entirely attributable to changes in biomass (total PLFA).     

Exploring soil microbial communities and soil organic matter: Variability and interactions in arable soils under minimum tillage practice

This study describes an integrated approach (1) to monitor the quantity and quality of water extractable organic matter (WEOM) and size, structure and function of microbial communities in space (depth) and time, and (2) to explore the relationships among the measured properties. The study site was an arable field in Southern Germany under integrated farming management including reduced tillage. Samples of this Eutric Cambisol soil were taken in July 2001, October 2001, April 2002 and July 2002 and separated into three depths according to the soil profile (0–10 cm, 10–28 cm and 28–40 cm). For each sample, the quantity and quality (humification index, HIX) of water extractable organic matter (WEOM) were measured concomitantly with soil enzyme activities (alkaline phosphatase, β-glucosidase, protease) and microbial community size (C_mic). Furthermore, microbial community structure was characterised based on the fingerprints of nucleic acids (DNA) as well as phospholipid fatty acids (PLFA). We observed strong influences of sampling date and depth on the measured parameters, with depth accounting for more of the observed variability than date. Increasing depth resulted in decreases in all parameters, while seasonal effects differed among variants. Principal component (PC) analysis revealed that both DNA and PLFA fingerprints differentiated among microbial communities from different depths, and to a smaller extent, sampling dates. The majority of the 10 PLFAs contributing most to PC 1 were specific for anaerobes. Enzyme activities were strongly related to C_mic, which was depending on water extractable organic carbon and nitrogen (WEOC and WEON) but not to HIX. HIX and WEOM interact with the microbial community, illustrated by (1) the correlation with the number of PLFA peaks (community richness), and (2) the correlations with community PC analysis scores.     

Experimentally induced effects of heavy metal on microbial activity and community structure of forest mor layers

This study compared the toxic effects of adding chromium (Cr), zinc (Zn), lead (Pb), molybdenum (Mo), nickel (Ni), and cadmium (Cd) at three dose levels to mor layer samples in laboratory experiments. Microbial activity in the form of soil respiration was monitored for 64 days. At the end of the experimental period, the composition of the soil microbial community structure was analysed by phospholipid fatty acid (PLFA) analysis. The metals added induced changes in the microbial community structure and affected respiration negatively, indicating toxicity. The microbial community structure (principal component analysis of the PLFA pattern) for all metals was significantly related to microbial activity (cumulative respiration), indicating intimate links between microbial community structure and activity. The most striking result in this study was that the shift in the microbial community because of metal stress was similar for all metals. Thus, the PLFA i16:0 increased most in relative abundance in metal-polluted soils, followed by other PLFAs indicative of Gram-positive bacteria (10Me16:0, 10Me17:0, 10Me18:0, a17:0 and br18:0). The PLFA 16:1ω5 was consistently negatively affected by metal stress, as were the PLFAs 18:1, 18:1ω7 and 19:1a. However, a significant separation between Cr- and Cd-polluted soils was observed in the response of the PLFA cy19:0, which decreased in abundance with Cr stress, and increased with Cd stress. Furthermore, the PLFA 18:2w6, indicating fungi, only increased with Cr and Zn stress. The effective doses of the metals, ranked with regard to background metal concentrations, decreased in the order: Zn > Cr > Pb > Mo > Ni > Cd. We concluded that interpretation of results of microbial activity from experiments of metal toxicity should include microbial structural patterns and background metal concentrations.     

Dynamics of Microbial Community in Japanese Andisol of Apple Orchard Production Systems

Abstract

In this study, two fields of temperate Andisols from high‐input and low‐input (zero‐input) management practices of an apple orchard were selected to assess microbial community dynamics based on environmental variables. Soils from an Ap horizon were sampled in five consecutive months from May to September and assessed for phospholipid fatty acids as a biomarker of soil microbial community, soil hardness, bulk density, porosity, pH, electrical conductivity (EC), organic carbon (C), available nitrogen (N) and phosphorus (P), and exchangeable cations as soil environmental variables. For all sample dates, total phospholipid fatty acids (PLFAs), total bacterial PLFAs, fungal PLFA, mycorrhizal PLFA, PLFA for actinomycetes, and earthworm were higher in low‐input management than high‐input management. Total PLFAs showed a high degree of seasonality, having August maxima and May minima. Significant effect on the relationships among soil environmental variables and microbiological attributes were observed. Soil management practices also showed a remarkable effect on the relationships among microbiological traits, indicating that some mechanism regulated soil microbial dynamics under two soil management practices. Comparatively higher correlations among the microbiological attributes were observed in low‐input management than high‐input management. Irrespective of soil management practices, bacterial and fungal lipid biomarkers were negatively correlated, suggesting that these subsets of fatty acids are contrasting components of the microbial biomass. Bulk density has negative influence on all soil microbial communities except fungi. On the other hand, linoleic acid and organic C were positively correlated, referring to the distribution of soil organic C implying an upper layer of soils. Microbial community composition and structure were greatly affected by sampling date and to a lesser extent by long‐term management practice. In this study, both ecosystems were characterized by a very diverse microbial community.     

Substrate inputs and pH as factors controlling microbial biomass, activity and community structure in an arable soil

Our aim was to determine whether the smaller biomasses generally found in low pH compared to high pH arable soils under similar management are due principally to the decreased inputs of substrate or whether some factor(s) associated with pH are also important. This was tested in a soil incubation experiment using wheat straw as substrate and soils of different pHs (8.09, 6.61, 4.65 and 4.17). Microbial biomass ninhydrin-N, and microbial community structure evaluated by phospholipid fatty acids (PLFAs), were measured at 0 (control soil only), 5, 25 and 50 days and CO₂ evolution up to 100 days. Straw addition increased biomass ninhydrin-N, CO₂ evolution and total PLFA concentrations at all soil pH values. The positive effect of straw addition on biomass ninhydrin-N was less in soils of pH 4.17 and 4.65. Similarly total PLFA concentrations were smallest at the lowest pH. This indicated that there is a direct pH effect as well as effects related to different substrate availabilities on microbial biomass and community structure. In the control soils, the fatty acids 16:1ω5, 16:1ω7c, 18:1ω7c&9t and i17:0 had significant and positive linear relationships with soil pH. In contrast, the fatty acids i15:0, a15:0, i16:0 and br17:0, 16:02OH, 18:2ω6,9, 17:0, 19:0, 17:0c9,10 and 19:0c9,10 were greatest in control soils at the lowest pHs. In soils given straw, the fatty acids 16:1ω5, 16:1ω7c, 15:0 and 18:0 had significant and positive linear relationships with pH, but the concentration of the monounsaturated 18:1ω9 PLFA decreased at the highest pHs. The PLFA profiles indicative of Gram-positive bacteria were more abundant than Gram-negative ones at the lowest pH in control soils, but in soils given straw these trends were reversed. In contrast, straw addition changed the microbial community structures least at pH 6.61. The ratio: [fungal PLFA 18:2w6,9]/[total PLFAs indicative of bacteria] indicated that fungal PLFAs were more dominant in the microbial communities of the lowest pH soil. In summary, this work shows that soil pH has marked effects on microbial biomass, community structure, and response to substrate addition.     

Soil texture affects soil microbial and structural recovery during grassland restoration

Many biotic and abiotic factors influence recovery of soil communities following prolonged disturbance. We investigated the role of soil texture in the recovery of soil microbial community structure and changes in microbial stress, as indexed by phospholipid fatty acid (PLFA) profiles, using two chronosequences of grasslands restored from 0 to 19 years on silty clay loam and loamy fine sand soils in Nebraska, USA. All restorations were formerly cultivated fields seeded to native warm-season grasses through the USDA’s Conservation Reserve Program. Increases in many PLFA concentrations occurred across the silty clay loam chronosequence including total PLFA biomass, richness, fungi, arbuscular mycorrhizal fungi, Gram-positive bacteria, Gram-negative bacteria, and actinomycetes. Ratios of saturated:monounsaturated and iso:anteiso PLFAs decreased across the silty clay loam chronosequence indicating reduction in nutrient stress of the microbial community as grassland established. Multivariate analysis of entire PLFA profiles across the silty clay loam chronosequence showed recovery of microbial community structure on the trajectory toward native prairie. Conversely, no microbial groups exhibited a directional change across the loamy fine sand chronosequence. Changes in soil structure were also only observed across the silty clay loam chronosequence. Aggregate mean weighted diameter (MWD) exhibited an exponential rise to maximum resulting from an exponential rise to maximum in the proportion of large macroaggregates (>2000 μm) and exponential decay in microaggregates (<250 μm and >53 μm) and the silt and clay fraction (<53 μm). Across both chronosequences, MWD was highly correlated with total PLFA biomass and the biomass of many microbial groups. Strong correlations between many PLFA groups and the MWD of aggregates underscore the interdependence between the recovery of soil microbial communities and soil structure that may explain more variation than time for some soils (i.e., loamy fine sand). This study demonstrates that soil microbial responses to grassland restoration are modulated by soil texture with implications for estimating the true capacity of restoration efforts to rehabilitate ecosystem functions.     

Soil microbial community dynamics and phenanthrene degradation as affected by rape oil application

The effects of rape oil application on soil microbial communities and phenanthrene degradation were characterized by examining phenanthrene concentrations, changes in microbial composition and incorporation of [¹³C] phenanthrene-derived carbon into phospholipid fatty acids (PLFAs). A Haplic Chernozem was incubated with and without rape oil in combination with and without phenanthrene over 60 days. High-performance liquid chromatography (HPLC) analysis showed a net reduction in extractable phenanthrene in the soils treated with rape oil but no net reduction in the soils without rape oil. Rape oil application increased the total PLFA content and changed microbial community composition predominantly due to growth of fungal groups and Gram-positive bacterial groups. Under rape oil and phenanthrene amendment all detected microbial groups grew until day 24 of incubation. The ¹³C PLFA profiles showed ¹³C enrichment for the PLFAs i14:0, 15:0, 18:0, 18:1ω5 and the fungal biomarker 18:2ω6,9 under rape oil application. Fungal PLFA growth was highest among detected all PLFAs, but its ¹³C incorporation was lower compared to the Gram-positive and Gram-negative bacteria PLFAs. Our results demonstrate the effect of rape oil application on the abundance of microbial groups in soil treated with phenanthrene and its impact on phenanthrene degradation.     

Responses of soil microbial biomass and community composition to biological soil crusts in the revegetated areas of the Tengger Desert

As a key component of desert ecosystems, biological soil crusts (BSCs) play an important role in dune fixation and maintaining soil biota. Soil microbial properties associated with the colonization and development of BSCs may indicate soil quality changes, particularly following dune stabilization. However, very little is known about the influence of BSCs on soil microbes in sand dunes. We examined the influence of BSCs on soil microbial biomass and community composition in revegetated areas of the Tengger Desert. BSCs increased soil microbial biomass (biomass C and N), microbial phospholipid fatty acid (PLFA) concentrations and the ratio of fungal to bacterial PLFAs. The effects varied with crust type and crust age. Moss crusts had higher microbial biomass and microbial PLFA concentrations than cyanobacteria-lichen crusts. Crust age was positively correlated with microbial biomass C and N, microbial PLFA concentrations, bacterial PLFA concentrations, fungal PLFA concentrations and the ratio of fungal to bacterial PLFAs. BSCs significantly affected microbial biomass C and N in the 0–20 cm soil layers, showing a significant negative correlation with soil depth. The study demonstrated that the colonization and development of BSCs was beneficial for soil microbial properties and soil quality in the revegetated areas. This can be attributed to BSCs increasing topsoil thickness after dunes have been stabilized, creating suitable habitats and providing an essential food source for soil microbes.