Phospholipid Fatty Acid Pattern Research Articles

Exploring Biomarker Signatures in Glaciovolcanic Environments: Implications for the Search for Life on Mars

Glaciovolcanic systems, where hydrothermal heat interacts with ice, offer favorable conditions for life by providing liquid water, nutrients, and physicochemical gradients (Cousins and Crawford 2011). Despite significant climate change, evidence for glaciovolcanism is widespread through Mars’ history. Such sites may have offered refugia for life after Mars lost much of its surface water, representing some of the most recent habitable areas and promising sites to recover biomarkers. We examined a terrestrial glaciovolcanic site to study the indigenous biological community structure, the supporting physicochemical parameters, and the distribution of biomarkers within the geologic context. The insights will help refine Mars exploration of analogous sites. Method: We studied a partially subglacial hydrothermal area at the summit of the active Kverkfjöll volcano, Iceland. The heated ground has created a large ice-damned meltwater lake with shoreline hot springs, thermal streams, and mud pots of variable activity. We collected water and sediment samples aseptically across the breadth of features, including the lake (surface & depth). Samples were kept at -4°C. Environmental parameters were measured at each sample site. Sediment samples were split for mineralogical and organic analysis. Mineralogy was measured by X-ray Diffraction. Organic samples were freeze-dried and extracted with a Bligh & Dyer method (Bligh and Dyer 1959). Extracts were divided into hydrocarbons, neutral lipids, glycolipids, and phospholipid fatty acids (PLFA) with hexane, dichloromethane, acetone, and methanol. Gas Chromatography/Mass Spectrometry was used to analyze hydrocarbons and PLFA as fatty acid methyl esters. Results: Environmental: Fluids ranged from acidic to alkaline (pH 3-9), low to high temperature (8-87°C), and severely dysoxic to oxic (0.5-5 mg/L dissolved O). Mineralogy comprised a dioctahedral swelling clay, heulandite, and minor quartz, anatase, and pyrite. The assemblage suggests argillic-grade alteration at 100-140°C (Fulignati 2020), confirming that the glaciovolcanic conditions were within theoretical boundaries for life (<150°C (Merino et al. 2019)), and formed minerals capable of protecting organic matter (e.g., swelling clay). Lipids: PLFA are essential components of cell membranes and degrade rapidly upon cell death. PLFA profiles thus provide insight into the composition and distribution of the viable community. Results revealed a diversity of PLFA with low molecular weights and several bacterial-diagnostic structures, indicating an active prokaryote-dominated biosphere. Molecular patterns correlated (p<0.05) with pH, temperature, and oxygen, suggesting homeoviscous adaptations or community composition variations. In either case, microbes demonstrate adaptability to extreme conditions in glaciovolcanic settings. Hydrocarbons are inert and used as molecular fossils. We detected multiple patterns attributable to microbial biosynthesis, including abundant C17 (straight and branched) diagnostic of cyanobacteria; high concentrations of short-chain n-alkane/alkene doublets (<C22) with an even carbon preference (CPI<1) indicating diverse microbial input; and a narrow unresolved complex mixture with elution parameters characteristic of microbial heterotrophy (Finkel et al. 2023). The pattern of even-numbered alkane/alkene pairs has been scarcely reported and offers insight into rare hydrocarbon distributions. Interestingly, the hydrocarbon and PLFA patterns do not align. Thus, PLFA are not a significant source of hydrocarbons here and we suggest direct microbial synthesis dominates. Implications of this hypothesis will be discussed in the context of palaeobiological reconstruction. Conclusion: Our site provides an array of physicochemical niches for diverse life modes and a mineral assemblage amenable to organic preservation. Analogous sites on Mars should be explored, but more work is needed to understand preservation biases of hydrocarbon sources and the implications for interpretation.

Microbial Communities as Affected by Clarithromycin Addition in Four Acid Soils (NW Iberian Peninsula)

A laboratory experiment was carried out to investigate the response of the microbial communities in acid agricultural soils located in the NW Iberian Peninsula to the presence of clarithromycin. Four soils, with different organic C content and similar pH, and seven different concentrations of clarithromycin (0.49, 1.95, 7.81, 31.25, 125, 500 and 2,000 mg kg−1 of soil) were used, and microbial estimates were made after 8 and 42 incubation days. The phospholipid fatty acids (PLFA) technique was used to estimate the total microbial biomass and biomass of specific microbial groups as well as the microbial community structure (PLFA pattern). The microbial biomass (total and specific groups) was different in the four studied soils, the lowest values being exhibited by soils with the lowest organic C. The antibiotic addition showed a positive effect on microbial biomass (total and specific groups), especially at the highest dose; the effect being similar or even more accentuated with time passed after the addition (42 days ≥8 days). Principal component analysis (PCA) of the PLFA data carried out with the whole data set showed that the main determining factors of the microbial structure followed the order: soil > time incubation ≥ antibiotic dose. When the PCA was performed individually for each incubation time, the results indicated that microbial communities of the four soils were different. Likewise, for each soil, different microbial communities were observed depending on antibiotic concentration. The microbial biomass and PLFA pattern data were coincidentally showing that the clarithromycin addition favored fungi and G− bacteria more that bacteria and G+ bacteria; the effect being dose-dependent. Our data (microbial biomass, PLFA pattern) also demonstrated that the effect of clarithromycin addition on microbial communities in these four acid agricultural soils persisted even after 42 incubation days.

Changes in vineyard soil parameters after repeated application of organic-inorganic amendments based on spent mushroom substrate

The changes of physicochemical and biochemical parameters of a silty loam (S1) and sandy loam (S2) vineyard soils added with spent mushroom substrate (SMS) or SMS composted with ophite (OF) as rock dust (SMS + OF) were studied. Two doses of SMS or SMS + OF (25 and 100 Mg ha−1) were applied for two consecutive years (2020–2021) and changes of soil physicochemical parameters, and dehydrogenase activity (DHA), respiration (RES), microbial biomass (BIO), and the phospholipid fatty acids (PLFAs) profile were assayed on a temporal basis. The results showed an increase in soil organic carbon (OC) content, total and mineralised N, P, and K, especially when the highest SMS dose was applied to soils. Repeated application caused OC content over time up to 2.3 times higher than initial content in the silty loam soil. This increase was not observed in sandy soil, possibly due to a higher bioavailability of OC, as indicated by the evolution of extractable humic acid/fulvic acid pools. In both soils, all biochemical parameters increased after amendment, being favoured both by the OC and by the presence of OF. Significant positive correlations were found between DHA, RES and BIO, and OC content especially in the first part and then levelled off after the second dose application. Total bacterial or fungal PLFAs patterns reflected the variation of BIO by SMS application. The higher growth of fungi vs. bacterial community in amended soils was recorded after the first SMS application, although the opposite effect occurred after the second application, with similar results in both soils. The findings indicate that the application of SMS or SMS + OF in vineyard soils could be an appropriate agronomic management practice for maintaining soil sustainability, although doses and application times of these amendments should first be evaluated depending on soil texture.

Biomonitoring and Assessment of Dumpsites Soil Using Phospholipid Fatty Acid Analysis (PLFA) Method—Evaluation of Possibilities and Limitations

Dumped waste is not only a problem from an aesthetic point of view, but also has an environmental polluting effect, or can even pose a direct danger if the waste is dumped in illegal landfills in an uncontrolled manner with unknown composition. In the case of soil pollution, the assessment of the changing microbial state can be used as an indicator of initial changes, since waste as a pollutant impacts the diversity of the landfill’s microbial community. The degree of change depends on the qualitative and quantitative composition of the pollutants, which can be measured through the microbial phospholipid fatty acid (PLFA) pattern. The aim was a comprehensive assessment of the soil microbiological and toxicological hazards of various illegal landfill. Cluster-analysis of the average principal component revealed significant differences between the experimental sites. In comparison with the control site, the percentage of fatty acid biomarkers of Gram-positive bacteria was significantly higher in the contaminated areas, as well as the ratio of trans/cis isomerization in the case of 16:1ω7 and 18:1ω7 fatty acids. The inverse tendency was observed in the relative quantities of fatty acid biomarkers of Gram-negative bacteria compared to Actinomycetes, and in the fungal-bacterial ratio.

Fairy ring‐induced soil potassium depletion gradients reshape microbial community composition in a montane grassland

AbstractFairy rings promoting circular greening belts in the vegetation can shape soil microbial communities by altering soil conditions. Knowledge about soil variables involved in this process is incomplete. We characterised the soil microbial communities of six fairy rings in a montane grassland using phospholipid fatty acid (PLFA) profiling, and studied if changes in soil properties corresponded to changes in soil microbial PLFA patterns. Exchangeable potassium (K) decreased inside the current rings, while soil moisture increased in the zones where the greening belts were two years before sampling (R2015). Fairy ring associated changes in PLFA composition were highly related to soil K. Gram‐negative bacteria were associated with the zones outside the ring with the highest K content, whereas Gram‐positive bacteria proportions increased inside the ring‐affected zones. An environmental stress indicator, the iso to anteiso ratio of PLFA 17:0, decreased in the R2015 zones, coinciding with the highest soil moisture contents. Our findings highlight the unreported importance of soil K in fairy ring dynamics affecting microbial communities. This common omission could lead to incorrect conclusions. Hence, the effects of fairy rings on soil should be further tested.Highlights Exchangeable potassium (K) decreased inside the current fairy rings. Fairy ring associated changes in PLFA microbial composition were present related to soil K. Soil moisture increased in the zones where the rings were 2 years before sampling. An environmental stress indicator decreased in those areas.

Soil Enzymatic Activities and Microbial Community Structure in Soils Polluted with Tetracycline Antibiotics

A laboratory experiment was performed to examine the medium-term influence of three tetracycline antibiotics (chlortetracycline, CTC; tetracycline, TC and oxytetracycline, OTC) at different concentrations in four agricultural soils with similar pH and different soil organic content. After a 42-days incubation period, three different soil enzymes (β-glucosidase, urease, and phosphomonoesterase) were estimated, as well as the phospholipid fatty acids (PLFAs). A residual effect was observed on all microbial parameters measured in the four soils affecting to the soil enzymes activity and soil microbial communities structure (PLFA pattern). A different microbial sensitivity to antibiotics was detected depending on both, soil type and the microbial property considered. Specifically, in general, no antibiotic effect or even a slight positive effect was observed for phosphomonoesterase and β-glucosidase enzyme activities, respectively, while a negative effect was detected for urease activity values, particularly at higher doses of the antibiotics in a soil with a low organic matter content. The principal component analysis performed with the PLFAs data obtained for all soil samples showed different microbial communities depending mainly on soil type, followed by the antibiotic added to the soil (CTC, TC or OTC) and, in a lesser extent, by its concentration. In general, the PLFA patterns showed similar microbial communities structure due to OTC and TC addition in comparison to the microbial communities structure of soil treated with CTC. These results could be environmentally relevant, especially as regards potential effects of antibiotics on the soil microbiome and hence on health risk assessment of these antibiotics in soils.

Turnover of gram-negative bacterial biomass-derived carbon through the microbial food web of an agricultural soil

Soil organic matter (SOM) represents an important terrestrial carbon reservoir in the biosphere, and microorganisms have been recognized as significant material contributors to the formation of SOM. However, the turnover of microbial biomass residues with respect to their detailed microbial food web remains elusive. To elucidate this turnover process, we traced the fate of Gram-negative (Gram−) microbial biomass carbon through the microbial food web over time, using the concentrations and isotopic compositions of biomarker phospholipid fatty acids (PLFA) in a soil incubation with isotopically (13C) labelled model bacterial cells (Escherichia coli). We found that after 120 days 46.5% of the recovered E. coli derived 13C remained in the soil, whereas 53.5% was emitted as 13CO2 (recovery 77.0% of the initially added 13C). The 13C in microbial biomass decreased to 5.3% of the initial value. This indicates that about 41% of the E. coli biomass in our experiment was transformed to SOM. The PLFA patterns over time demonstrate the pathway of 13C transformation associated with different groups of microorganisms along the incubation. The E. coli-derived 13C was utilized by fungi first, then the label was shifted from fungi to Gram-positive (Gram+) bacteria, arbuscular mycorrhizal fungi (AMF), actinomycetes, and to Gram− bacteria other than E. coli. Finally the carbon was transferred from all consumers to the next consumer level; this is reflected in ongoing loss of 13C-PLFA without a shift in the 13C-PLFA pattern. In summary, our study details the turnover process of microbial biomass residues via the microbial food web to necromass and finally to SOM. This supports the soil microbial carbon pump concept, i.e. carbon channelling and the assimilation of easily degradable carbon into microbial biomass and a significant contribution of these residues in SOM.

The drivers of soil microbial communities structure on forest stands along the altitudinal gradient in western carpathians

This study examines the effect of the soil physicochemical properties and the vegetation diversity and composition on the soil microorganism community structure on the temperate forest stands along the altitudinal gradient. The studied stands were located at three altitudes (600, 900, and 1200 m a.s.l.) on five separate mountain slopes (treated as independent replicates) in the Western Carpathians Mountains (Poland). The soil physicochemical characteristics varied widely between the stands on these same altitude, and only the organic matter content, water-holding capacity and dissolved organic nitrogen differed significantly between the different altitudes, exhibited highest values in the stands at 1200 m a.s.l. The vascular plant species number, including trees, shrubs and understorey decreased with an increase in the altitude. Multivariate analysis (CCA, ANOSIM) indicated that the plant diversity and composition differed significantly between 600 m a.s.l. and 1200 m a.s.l. and was related to altitude, soil pH and fertility, defined as P and K content. Soil microbial biomass and community structure was measured using PLFA (phospholipid fatty acids) analysis. PLFA pattern did not differ between altitudes but was related to the number of plant species, soil pH and C:N ratio. Taking all stands altogether, we showed clear relationship between the plant diversity and composition and the PLFA pattern (Mantel test). Additionally, we indicated that the main factor affecting both the plant diversity and composition as well as microbial community structure was the soil pH.

Fatty acid metabolism in an oribatid mite: de novo biosynthesis and the effect of starvation.

The fatty acid (FA) composition of lipids in animals is influenced by factors such as species, life stage, availability and type of food, as well as the ability to synthesize certain FAs de novo. We investigated the effect of starvation on the neutral lipid (NLFA) and phospholipid (PLFA) fatty acid patterns of the oribatid mite Archegozetes longisetosus Aoki. Furthermore, we performed stable-isotope labeled precursors feeding experiments under axenic conditions to delineate de novo FA synthesis by profiling 13C and deuterium incorporation via single-ion monitoring. Starvation of mites resulted in a decline in the total amount of NLFAs and significantly changed the fatty acid patterns, indicating that NLFAs were metabolized selectively. Biochemical tracer experiments confirmed that oribatid mites, like other animals, can produce stearic (18:0) and oleic acid (18:1ω9) de novo. Mass spectrometric data also revealed that they appear to synthesize linoleic acid [18:2ω6,9 = (9Z,12Z)-octadeca-9,12-dienoic acid]-an ability restricted only to a few arthropod taxa, including astigmatid mites. The physiological and biosynthesis processes revealed here are crucial to understand the potential biomarker function of fatty acids-especially 18:2ω6,9-in oribatid mites and their applicability in soil animal food web studies.

Key factors controlling microbial community responses after a fire: Importance of severity and recurrence

Wildfires are a major problem in Mediterranean forest ecosystems, affecting the same area year after year. Their severity is increasing, partly due to climate change and hence, every now and then, virulent fires of high severity spread ravage this region. The aim of this study was to evaluate the influence of fire regime (recurrence, severity) in soil microbial community structure analyzing the phospholipid fatty acid (PLFA) and the microbial functional diversity assessing the level physiological profiling technique (CLPP). Samples of a soil affected by a high severities wildfire and a soil affected by a low severity experimental fire were heated under laboratory conditions at different temperatures to simulate different fire severity. To simulate fire recurrence, the heating treatment was repeated after one month of incubation. The fire severity was estimated as the amount of heat supplied to samples by degree-hour methodology. A marked impact of fire regime on soil microorganisms was detected; the microbial community response varied depending on previous history of fire and the magnitude of changes in PLFA pattern and CLPP, was related to the amount of heat supplied to the samples. Wildfires had a greater impact on microbial community structure than subsequent soil heating in the laboratory. The total biomass and the biomass of specific groups of microorganisms decreased notably as a consequence of wildfire and minor changes were detected due to the experimental fire and soil heating under laboratory conditions. The results clearly showed the usefulness of PLFA pattern to study the effect of fire regimes and associated direct and indirect changes in soil microorganisms and in soil quality. The data also indicated that the degree-hour methodology rather than maximum temperature is adequate to simulate fire severity and evaluate the impact of thermal shock on soil ecosystems.

Tracing elevational changes in microbial life and organic carbon sources in soils of the Atacama Desert

Abstract The Atacama Desert frequently serves as model system for tracing life under extremely dry conditions. We hypothesized that traces of life in the Atacama Desert follow distinct micro- and macro-scale gradients such as soil depth and elevation, respectively. Different depth intervals of surface soils (0–1, 1–5, and 5–10 cm) were sampled at five sites along an elevational transect near the Quebrada Aroma, spanning from the hyperarid core of the desert towards the arid Western Andean Precordillera (1300 to 2700 m a.s.l.), and from one additional site in the hyperarid core near Yungay. We determined the contents of major elements, pedogenic minerals and oxides, organic carbon (OC), and its δ13C and δ15N isotopic composition. The presence of living microorganisms was assessed by cultivation, and bacterial community composition was analyzed based on 16S rRNA gene sequencing. Additional information about past and present plant and microbial life was obtained from lipid biomarker analysis. We did not detect consistent micro-scale distributions for most of these proxies within the soils. However, concentrations of OC and of long-chain, plant wax-derived n-alkanes increased in soils along the aridity gradient towards the wetter sites, indicating the presence of past life at places presently not covered by vegetation. Likewise, bacterial abundance and diversity decreased as hyperaridity increased and the microbial community composition changed along the transect, becoming enriched in Actinobacteria. The distributional patterns of phospholipid fatty acids (PLFAs) confirmed the larger bacterial diversity at the higher, more humid sites compared to the drier ones. Archaeal isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) and bacterial branched (br)GDGTs, which can also indicate past life, did not follow a clear elevational trend and were absent at the driest site. Taken together, plant-derived and microbiological markers follow primarily the macro-scaled elevation and aridity gradient. Viable bacteria are present even at the driest sites, while detected biomolecules also indicate past life. The detection of past plant life in nowadays apparently lifeless regions suggests that conditions for life were less hostile in former times.

Plasma phospholipid fatty acid patterns are associated with adiposity and the metabolic syndrome in black South Africans: a cross-sectional study.

Diets rich in n-6 polyunsaturated fatty acids (PUFAs) and saturated fatty acids (SFA) have been associated with increased risk of obesity and the metabolic syndrome (MetS), but the evidence is inconsistent, whereas diets high in n-3 long-chain (LC) -PUFAs are associated with lower risk. There is limited information about the association of plasma phospholipid fatty acids (FAs) with obesity and the MetS among black South Africans. To investigate the association of dietary FAs and plasma phospholipid FA patterns, respectively, with measures of adiposity (body mass index, waist circumference, waist-to-height ratio) and the MetS in black South Africans. Factor analysis was used to identify FA patterns from 11 dietary FAs and 26 individual plasma phospholipid FAs. Cross-sectional association of the identified patterns with measures of adiposity and the MetS was investigated. A random sample of 711 black South African adults aged 30 to 70 years (273 men, 438 women) from the North West Province was selected from the South African leg of the Prospective Urban and Rural Epidemiology (PURE) study. Sequential regression models adjusted for confounders were applied to investigate the association between dietary FAs and plasma phospholipid FA patterns with measures of adiposity and the MetS. Two patterns were derived from dietary FAs and six patterns from plasma phospholipid FAs that explained the cumulative variance of 89 and 73%, respectively. The association of FA patterns with adiposity and the MetS was weaker for dietary FA patterns than for plasma phospholipid FA patterns. The plasma phospholipid FA pattern with high loadings of saturated FAs (high-Satfat) and another with high loadings of n-3 very-long-chain PUFAs (n-3 VLC-PUFAs) were positively associated with measures of adiposity and the MetS, while patterns with positive loadings of LC monounsaturated fatty acids (n-9 LC-MUFA) and a positive loading of n-3 essential FAs (n-3 EFA) showed inverse associations with the MetS and some measures of adiposity. The n-9 LC-MUFA and n-3 EFA patterns seemed to provide possible protective associations with adiposity and the MetS, whereas the high-Satfat and n-3 VLC-PUFA patterns were associated with adiposity and the MetS in our study participants. The results are reflective of the metabolic difference between overweight and obese compared to lean individuals.

Towards sulfide removal and sulfate reducing bacteria inhibition: Function of biosurfactants produced by indigenous isolated nitrate reducing bacteria

The effectiveness of nitrate-mediated souring control highly depends on the interactions of sulfate reducing bacteria (SRB) and nitrate reducing bacteria (NRB). Biosurfactants produced by natural NRB are promising bio-agents for enhancing NRB competence towards SRB. However, the function of NRB-produced biosurfactants in NRB-SRB interactions remains unexplored due to the rarely successful isolation of natural biosurfactant-producing NRB. Hereby, biosurfactant-aided inhibitory control of SRB strain Desulfomicrobium escambiense ATCC 51164 by biosurfactant-producing NRB strain Pseudomonas stutzeri CX3, reported in our previous work, was investigated. Under non-sour conditions, insufficient nitrate injection resulted in limited SRB inhibition. Phospholipid fatty acid (PLFA) biomarkers traced the overall bacterial responses. Compositional PLFA patterns revealed biosurfactant addition benefitted both SRB and NRB towards stressful conditions. Under sour conditions, nitrite oxidation of sulfide proved to be the primary mechanism for sulfide removal. The subsequent elevation of redox potential and pH inhibited SRB activities. NRB-produced biosurfactants significantly enhanced SRB inhibition by NRB through more efficient sulfide removal and effective duration of nitrate in the microcosms. Biosurfactants specially produced by the NRB strain are for the first time reported to significantly strengthen SRB inhibition by NRB via reduced nitrate usage and prolonged effective duration of nitrate, which has encouraging potential in nitrate-dependent souring control.

Associations of plasma total phospholipid fatty acid patterns with feeding practices, growth, and psychomotor development in 6-month-old South African infants.

The objective of this study was to assess plasma fatty acid (FA) patterns of 6-month-old South African infants and to determine their association with feeding practices, growth, and psychomotor development. Plasma total phospholipid FA composition (% of total FAs) of 6-month-old infants (n=353) from a peri-urban township was analysed, and principal component and factor analysis were performed to identify plasma FA patterns. Feeding practices, anthropometric measurements, and psychomotor development scores were determined. Four major plasma phospholipid FA patterns were identified: A plant-based C18 FA, a high n-6 long-chain polyunsaturated fatty acids (LCPUFA), a C16:1 and long-chain saturated fatty acid (SFA), and a high n-3 and low n-6 LCPUFA pattern. Formula feeding was associated with higher, whereas breastfeeding was associated with lower scores for the plant-based C18 FA and C16:1 and long-chain SFA patterns. On the other hand, breastfeeding, the consumption of cow's milk, and the consumption of semisolid foods were associated with higher scores, whereas formula feeding was associated with lower scores for the high n-6 LCPUFA pattern. Breastfeeding and the consumption of semisolids were also associated with higher high n-3 and low n-6 LCPUFA pattern scores. The C16:1 and long-chain SFA and high n-3 and low n-6 LCPUFA patterns were positively associated with psychomotor development scores. In 6-month-old South African infants, we identified distinct plasma FA patterns that presumably represent the FA quality of their diet and that are associated with psychomotor development. Our results suggest that breast milk is an important source of n-6 LCPUFAs and formula-fed infants may be at risk of inadequate LCPUFA intake.

Medium-term impact of post-fire emergency rehabilitation techniques on a shrubland ecosystem in Galicia (NW Spain)

The aim of this work was to study the effectiveness of two post-fire emergence<em> </em>rehabilitation techniques (seeding and mulching) for reducing soil erosion as well as their effects on the soil quality; therefore in the field, experimental plots of unburnt soil, burnt soil, burnt soil plus seeding and burnt soil plus mulching were established. Soil samples were collected from the A horizon and a wide range of physical, chemical and biological soil properties were analyzed to evaluate soil quality. The effect of fire on the vegetation cover was observed after one year and changes in soil properties persisted even after four years. The phospholipid fatty acids pattern showed that in the medium-term (8-48 months after the fire), the fire may modify the soil microbial communities by altering the plant community via plant-induced changes in the soil environment. No effect of seeding or mulching on the vegetation cover was observed. The mean efficiency in preventing soil erosion between 8 and 12 months after the fire and the application of the treatments was 11% for seeding and 65% for mulching. These stabilization treatments had a minor influence on the post-fire soil quality in the medium-term (48 months); therefore, taking into account its effectiveness for reducing soil erosion, the mulching treatment is recommended as the best post-fire stabilization technique.

Organic matter dynamics along a salinity gradient in Siberian steppe soils

Abstract. Salt-affected soils will become more frequent in the next decades as arid and semiarid ecosystems are predicted to expand as a result of climate change. Nevertheless, little is known about organic matter (OM) dynamics in these soils, though OM is crucial for soil fertility and represents an important carbon sink. We aimed at investigating OM dynamics along a salinity and sodicity gradient in the soils of the southwestern Siberian Kulunda steppe (Kastanozem, non-sodic Solonchak, Sodic Solonchak) by assessing the organic carbon (OC) stocks, the quantity and quality of particulate and mineral-associated OM in terms of non-cellulosic neutral sugar contents and carbon isotopes (δ13C, 14C activity), and the microbial community composition based on phospholipid fatty acid (PLFA) patterns. Aboveground biomass was measured as a proxy for plant growth and soil OC inputs. Our hypotheses were that (i) soil OC stocks decrease along the salinity gradient, (ii) the proportion and stability of particulate OM is larger in salt-affected Solonchaks compared to non-salt-affected Kastanozems, (iii) sodicity reduces the proportion and stability of mineral-associated OM, and (iv) the fungi : bacteria ratio is negatively correlated with salinity. Against our first hypothesis, OC stocks increased along the salinity gradient with the most pronounced differences between topsoils. In contrast to our second hypothesis, the proportion of particulate OM was unaffected by salinity, thereby accounting for only < 10 % in all three soil types, while mineral-associated OM contributed > 90 %. Isotopic data (δ13C, 14C activity) and neutral sugars in the OM fractions indicated a comparable degree of OM transformation along the salinity gradient and that particulate OM was not more persistent under saline conditions. Our third hypothesis was also rejected, as Sodic Solonchaks contained more than twice as much mineral-bound OC than the Kastanozems, which we ascribe to the flocculation of OM and mineral components under higher ionic strength conditions. Contrary to the fourth hypothesis, the fungi : bacteria ratio in the topsoils remained fairly constant along the salinity gradient. A possible explanation for why our hypotheses were not affirmed is that soil moisture covaried with salinity along the transect, i.e., the Solonchaks were generally wetter than the Kastanozems. This might cause comparable water stress conditions for plants and microorganisms, either due to a low osmotic or a low matric potential and resulting in (i) similar plant growth and hence soil OC inputs along the transect, (ii) a comparable persistence of particulate OM, and (iii) unaffected fungi : bacteria ratios. We conclude that salt-affected soils contribute significantly to the OC storage in the semiarid soils of the Kulunda steppe, while most of the OC is associated with minerals and is therefore effectively sequestered in the long term.

Regional environmental conditions shape microbial community structure stronger than local forest management intensity

Abstract Microorganisms in forest soils provide essential ecosystem services, such as decomposition of organic matter and nutrient mineralization. However, microbial community structure and function can be affected by environmental conditions, such as regional climate and soil properties and, moreover, by human activity through forest management. We examined the biomass and composition of microbial assemblages in 150 forest stands in the organic layer (Oi, Oe, Oa) and upper mineral soil (0–10 cm) in three regions across Germany (Schwabische Alb, Hainich-Dun, Schorfheide-Chorin) by phospholipid fatty acid (PLFA) analysis. Different explanatory environmental variables (total C, N, S, P, organic C, inorganic C, pH, water content) were identified. The intensity of land use was characterized with the Forest Management Intensity Index (ForMI). The total amount of PLFAs, as measure for microbial biomass, was different among the three regions both in the organic layer and mineral soil. In the organic layer, total PLFAs decreased from Schwabische Alb over Hainich-Dun to Schorfheide-Chorin, with the latter comprising a fourfold and twofold lower amount in fungal and bacterial PLFAs, respectively. In contrast, in the mineral soil the forests in the Hainich-Dun showed the highest microbial biomass. Discriminant function analysis of PLFA pattern indicated that Gram-positive bacteria and fungi accounted mainly for the regional differences in the organic layer, whereas in the mineral soil additionally Gram-negative and actinobacteria were important. Redundancy analysis showed that PLFA profiles were predominantly affected by sampling site and environmental variables, with the water content in the organic layer and the soil texture in the mineral soil explaining most of the variability in microbial communities between the three regions. Additionally, forest stands were classified into four management groups (conifer; deciduous with low, medium, and high intensity) based on the ForMI. In the mineral soil, forest management accounted for a small proportion of the observed regional differences. Within regions, fungal biomass in the organic layer decreased with management intensity at the Schwabische Alb and increased in the mineral soils of Hainich-Dun region. Microbial community structure discriminated coniferous and deciduous forests in all three regions, and moreover showed a separation based on forest management intensity in the Schorfheide-Chorin. In conclusion, microbial biomass and community composition in forest organic layer and mineral soil were more influenced by regional conditions, including environmental properties such as moisture, soil texture, C/N ratio and pH, than by forest management intensity. However, within given environments, microbial assemblages can be influenced by forest management, in particular through changes in the tree species composition.

Changes in bacterial community composition and soil respiration indicate rapid successions of protist grazers during mineralization of maize crop residues

Decomposition of organic matter is crucial for ecosystem functioning. Microorganisms, which are responsible for the mineralization of organic matter, are usually treated as a homogeneous functional guild, despite mineralization capacity can differ profoundly between taxa. In addition, a significant part of the microbial community is top-down controlled by microbial grazers, such as protist. Since protist grazing is selective, and selectivity differs among species, we hypothesized that protist taxa complement each other in grazing intensity and thereby affect bacterial community structure and mineralization rate. In a laboratory experiment the species richness of protist communities was manipulated in an arable field soil and the mineralization rate of maize litter residues followed during the decomposition of the labile (4 days) and recalcitrant (3 weeks) carbon (C) fractions. Mineralization rate overall increased in the presence of protists. Changes in microbial function could be related to changes in microbial community composition (measured by phospholipid fatty acids pattern). During microbial decomposition, different protist grazers gained influence on mineralization rates over consecutive time intervals, indicating that a succession of protists caused an enhanced bacterial C-mineralization of plant detritus. Protist identity and species richness affected the microbial community composition, but not the magnitude of its mineralization function. In general, protist identity appeared to be more relevant for the composition of the microbial communities at the beginning of decomposition while the protist species richness appeared to be more critical in the later, slow phase of decomposition. This study provides an example that the overall outcome of ecosystem processes, such as mineralization rate is regulated by the sum of positive and negative effects of complex species interactions operating at a very fine spatial and temporal scales.

Long-term response of soil microbial communities to fire and fire-fighting chemicals

Wildfires are a global problem that can require the application of fire-fighting chemicals for combating them. These compounds can have negative effects on forest ecosystems; however, there are few studies about their influence on the soil microbial community. The aim of this work is to analyse under field conditions the impact of a prescribed fire and the addition at normal field doses of three different fire-fighting chemicals (foaming agent, ammonium polyphosphate, Firesorb) on different soil properties, 10 years after the fire and retardant addition. The study was performed in a Humic Cambisol under scrubland located in Galicia (NW Spain). Several physical, physicochemical, chemical (water holding capacity, pH, electric conductivity, total C and N, hydro-soluble C and carbohydrates), biochemical and microbiological [biomass C, enzyme activities, respiration, bacterial growth, fungal biomass and growth, biomass and community composition by the phospholipid fatty acid (PLFA) pattern] properties were analysed in the 0–2- and 2–5-cm soil layers. A marked effect of soil depth and no effects of prescribed fire on most analysed properties were observed, suggesting that soil chemical quality was recovered after 10 years, although changes in microbial community composition were still detected 10 years after the prescribed fire and the retardant addition. The PLFA pattern combined with principal component analysis allows us to differentiate the microbial communities according to both soil depth and soil treatment. The ammonium polyphosphate was the fire-fighting chemical with the strongest effects on the composition of soil microbial communities, which is in accordance with marked changes observed in vegetation.

Plant community influence on soil microbial response after a wildfire in Sierra Nevada National Park (Spain)

Plant community influence on microbial response after fire has been studied in a Sierra Nevada National Park area affected by a wildfire in 2005. Two different plant communities adapted to different altitudes were selected to analyse possible differences on soil microbial recolonisation process after fire, in oak forest and high mountain shrub communities. Microbial abundance, activity and community composition were monitored to evaluate medium-term changes. Microbial abundance was studied by mean of microbial biomass carbon and plate count methods; microbial activity was analysed by microbial respiration and bacterial growth while microbial community composition was determined by analysing phospholipid fatty acid pattern. Under unburnt conditions oak forest showed higher nutrient content, pH and microbial abundance and activity values than the high mountain shrubs community. Different parameters studied showed different trends with time, highlighting important changes in microbial community composition in high mountain shrubs from first sampling to the second one. Post-fire recolonisation process was different depending on plant community studied. Highlighting fungal response and microbial activity were stimulated in burnt high mountain shrubs community whilst it was negatively affected in oak forest. Fire induced changes in oak forest were almost neutralized 20months after the fire, while high mountain shrubs community still showed fire-induced changes at the end of the study.