Bacterial salt tolerance is unrelated to soil salinity across an arid agroecosystem salinity gradient

In arid and semi-arid ecosystems, salinization is a major threat to the productivity of agricultural land. While the influence of other physical and chemical environmental factors on decomposer microorganisms have been intensively studied in soil, the influence of salinity has been less exhaustively assessed. We investigated the influence of soil salinity on soil bacterial communities in soils covering a range of salt levels. We assessed tolerance of the bacterial communities from Libyan agricultural soils forming a salinity gradient to salt (NaCl), by extracting bacterial communities and instantaneously monitoring the concentration-response to added NaCl with the Leucine incorporation technique for bacterial growth. To maximise our ability to detect differences in bacterial salt tolerance between the soils, we also repeated the assessment of bacterial growth tolerance after one month incubation with 1 or 2% added organic matter additions to stimulate microbial growth levels. We could establish clear concentration-response relationships between bacterial growth and soil salinity, demonstrating an accurate assessment of bacterial tolerance. The in situ soil salinity in the studied soils ranged between 0.64 and 2.73 mM Na (electrical conductivities of 0.74-4.12 mS cm⁻¹; cation exchange capacities of 20-37 mmol_c kg⁻¹) and the bacterial tolerance indicated by the concentration inhibiting 50% of the bacterial growth (EC₅₀) varied between 30 and 100 mM Na or between electrical conductivities of 3.0 and 10.7 mS cm⁻¹. There was no relationship between in situ soil salinity and the salt tolerance of the soil bacterial communities. Our results suggest that soil salinity was not a decisive factor for bacterial growth, and thus for structuring the decomposer community, in the studied soils.     

Pasture and forest soil microbial communities show distinct patterns in their catabolic respiration responses at a landscape scale

Catabolic responses to specific substrates can be used to differentiate soil microbial communities. We hypothesized that the catabolic respiration responses of microbial communities from pastures would differ from those of forest soils, and that the differences would be consistent at a landscape scale, due to inherent differences in litter quality and management regimes. We analysed respiration responses to 25 different substrates of 20 pasture soils (dominated by rye grass and white clover) and 20 forest soils (indigenous forest species or the plantation species Pinus radiata) over a wide geographical range in New Zealand.Within each pasture or forest category, the catabolic responses showed a similar pattern, suggesting similarities in functional catabolic capability and microbial community Indigenous forests and pine forests microbial communities did not differ in their responses. Pasture soil communities had significantly higher relative responses to carbohydrate and amino-acid substrates and significantly lower relative response to carboxylic acid substrates, than microbial communities from forest soils. Forest soils had relatively greater responses to carboxylic acids as a group, as well as citric acid, α-ketobutyric acid, α-ketoglutaric acid, and α-ketovaleric acid, than did the pasture soils. A subset of 6 substrates was equally as effective at differentiating the microbial catabolic response of pasture soils from forest soils as the entire set of 25 substrates. The results demonstrated distinct differences in the respiration responses of the soil microbial communities of pastures and forests, but showed strong similarities within each vegetation class, despite the wide geographical spread, different soils and plant species.     

Microbial biomass and activity in salt affected soils under arid conditions

The effects of salinity on the size, activity and community structure of soil microorganisms in salt affected arid soils were investigated in Shuangta region of west central Anxi County, Gansu Province, China. Eleven soils were selected which had an electrical conductivity (EC) gradient of 0.32–23.05 mS cm⁻¹. There was a significant negative exponential relationship between EC and microbial biomass C, the percentage of soil organic C present as microbial biomass C, microbial biomass N, microbial biomass N to total N ratio, basal soil respiration, fluorescein diacetate (FDA) hydrolysis rate, arginine ammonification rate and potentially mineralizable N. The exponential relationships with EC demonstrate the highly detrimental effect that soil salinity had on the microbial community. In contrast, the metabolic quotient (qCO₂) was positively correlated with EC, and a quadratic relationship between qCO₂ and EC was observed. There was an inverse relationship between qCO₂ and microbial biomass C. These results indicate that higher salinity resulted in a smaller, more stressed microbial community which was less metabolically efficient. The biomass C to biomass N ratio tended to be lower in soils with higher salinity, reflecting the bacterial dominance in microbial biomass in saline soils. Consequently, our data suggest that salinity is a stressful environment for soil microorganisms.     

Microbial eco-physiology of degrading aral sea wetlands: Consequences for C-cycling

Abstract

The effects of elevated salinity level on the physiology of microbial communities and C-cycling were evaluated in soils located in areas near the degraded Aral Sea. One site on a freshwater wetland (FW) and two sites on a salt water wetland on the eastern (EW) and northern shores (NW) with salt contents of 1.1, 34.3, and 78.9 g L^-1, respectively, were selected. The total microbial biomass estimated by fumigation-extraction and the respiration rate were lower at the two sites with elevated salinity level (EW and NW) than in FW while the amount of extractable organic carbon (EOC) was higher. The EOC/biomass-C ratio as an index of carbon supply was highest in NW where the lowest microbial activity was detected, indicating the restriction of microbial metabolism. Thus elevated salinity level appeared to restrict microbial colonization of organic substrates and C transformation rate but to increase the amount of soil available C. Difference in precipitation (rain events) between 2002 and 2003 did not affect the respiration rate and surface CO₂ flux in FW. In NW, the respiration rate increased by more than 5 times, suggesting that in the ecosystems with a high content of available C, elevated CO₂ emissions may occur under changing environmental conditions.     

Development of pollution-induced community tolerance is linked to structural and functional resilience of a soil bacterial community following a five-year field exposure to copper

Copper (Cu) is accumulating in agricultural soils worldwide creating concern for adverse impacts on soil microbial communities and associated ecosystem services. In order to evaluate the structural and functional resilience of soil microbial communities to increasing Cu levels, we compared a Cu-adapted and a corresponding non-adapted soil microbial community for their abilities to resist experimental Cu pollution. Laboratory soil microcosms were set-up with either High-Cu soil from Cu-amended field plots (63 g Cu m⁻²) or with Low-Cu control soil from the same five-year field experiment. Laboratory treatments consisted of Cu amendments in the presence or absence of pig manure. Microbial activities (soil respiration, substrate-induced respiration, [³H]leucine incorporation), bacterial community structure (terminal restriction fragment length polymorphism, T-RFLP), community-level physiological profiles, and pollution-induced bacterial community tolerance (PICT detected using the [³H]leucine incorporation technique) were monitored for 12 weeks. The High-Cu and Low-Cu soil microbial communities initially exhibited almost identical structure and function and could only be distinguished from each other by their differential Cu tolerance. Experimental Cu pollution inhibited microbial activities, affected bacterial community structure, and induced further bacterial community tolerance to Cu. However, Low-Cu and High-Cu soil microbial communities showed essentially identical responses. Manure amendment did not protect against Cu toxicity and slightly increased Cu bioavailability as measured by a Cu-specific whole-cell bacterial biosensor. Our results indicate convergence of bacterial community structure and function in the High-Cu and Low-Cu soils during the five-year field experiment. We conclude that soil bacterial communities can exhibit structural and functional resilience to a five-year Cu exposure by virtue of their ability to develop Cu tolerance without affecting overall community structure. The observed increased Cu tolerance may involve phenotypic adaptation or selection at the micro-diversity level, for example an increased proportion of Cu-resistant strains within each bacterial species, which go undetected by T-RFLP community fingerprinting. Finally, our results indicate that Cu-dissolved organic matter complexes contribute to microbial toxicity in manure-amended soils implying that free Cu may comprise a poor predictor of metal toxicity.     

Salinity and sodicity effects on respiration and microbial biomass of soil

An understanding of the effects of salinity and sodicity on soil carbon (C) stocks and fluxes is critical in environmental management, as the areal extents of salinity and sodicity are predicted to increase. The effects of salinity and sodicity on the soil microbial biomass (SMB) and soil respiration were assessed over 12weeks under controlled conditions by subjecting disturbed soil samples from a vegetated soil profile to leaching with one of six salt solutions; a combination of low-salinity (0.5dSm⁻¹), mid-salinity (10dSm⁻¹), or high-salinity (30dSm⁻¹), with either low-sodicity (sodium adsorption ratio, SAR, 1), or high-sodicity (SAR 30) to give six treatments: control (low-salinity low-sodicity); low-salinity high-sodicity; mid-salinity low-sodicity; mid-salinity high-sodicity; high-salinity low-sodicity; and high-salinity high-sodicity. Soil respiration rate was highest (56–80mg CO₂-C kg⁻¹ soil) in the low-salinity treatments and lowest (1–5mg CO₂-C kg⁻¹ soil) in the mid-salinity treatments, while the SMB was highest in the high-salinity treatments (459–565mg kg⁻¹ soil) and lowest in the low-salinity treatments (158–172mg kg⁻¹ soil). This was attributed to increased substrate availability with high salt concentrations through either increased dispersion of soil aggregates or dissolution or hydrolysis of soil organic matter, which may offset some of the stresses placed on the microbial population from high salt concentrations. The apparent disparity in trends in respiration and the SMB may be due to an induced shift in the microbial population, from one dominated by more active microorganisms to one dominated by less active microorganisms.     

酸雨对土壤呼吸的影响机制研究进展与展望

土壤呼吸是陆地生态系统与大气之间进行碳交换的主要途径,其动态变化直接影响着全球碳平衡。由于人类活动的影响,酸雨成为人类当前面临的最严重的生态环境问题之一,但其对土壤呼吸的影响及其机理尚无定论。本文综述了不同生态系统土壤呼吸对酸雨的响应特征,多数文献表明,高强度的酸雨抑制土壤呼吸,而在低强度的酸雨作用下土壤呼吸的响应存在差异。从影响土壤呼吸的4个关键生物因子,即光合作用、微生物、凋落物和根系生物量,重点讨论了酸雨对土壤呼吸的影响机制。在此基础上,提出了以下研究展望:①开展土壤呼吸对不同组成类型酸雨的响应研究;②开展与土壤碳排放相关的功能微生物对酸雨的响应研究;③开展不同物候期土壤呼吸对酸雨的响应研究;④开展土壤呼吸各过程对酸雨的响应研究;⑤建立全球酸雨地区土壤碳排放监测研究网络。     

黄河三角洲退化湿地微生物群落特性研究

Five different sites with a soluble salt gradient of 3.0--17.7 g kg-1 dry soil from the coast to the inland were selected, and the microbial population size, activity and diversity in the rhizospheres of five common plant species and the adjacent bulk soils (non-rhizosphere) were compared in a degraded wetland of the Yellow River Delta, Shandong Province, China to study the effects of soil environment (salinity, seasonality, depth, and rhizosphere) on microbial communities and the wetland’s ecological function, thus providing basic data for the bioremediation of degraded wetlands. There was a significant negative linear relationship between the salinity and the total number of microorganisms, overall microbial activity, or culturable microbial diversity. Salinity adversely affected the microbial community, and higher salinity levels resulted in smaller and less active microbial communities. Seasonal changes were observed in microbial activity but did not occur in the size and diversity. The microbial size, activity and diversity decreased with increasing soil depth. The size, activity and diversity of culturable microorganisms increased in the rhizospheres. All rhizospheres had positive effects on the microbial communities, and common seepweed had the highest rhizosphere effect. Three halophilic bacteria (Pseudomonas mendocina, Burkholderia glumae, and Acinetobacter johnsonii) were separated through BIOLOG identification, and common seepweed could be recommended for bioremediation of degraded wetlands in the Yellow River Delta.     

The adverse effects of soil salinization on the growth of Trifolium alexandrinum L. and associated microbial and biochemical properties in a soil from Iran

The aim of this study was to determine the effects of increasing concentrations of salt solutions (including 0.12, 2, 6, and 10 dS m⁻¹) on the growth of berseem clover (Trifolium alexandrinum L.) and related soil microbial activity, biomass and enzyme activities. Results showed that the dry weights of root and shoot decreased with an increase in the concentrations of salt solutions. Soil salinization depressed the microbiological activities including soil respiration and enzyme activities. Substrate-induced respiration was consistently lower in salinized soils, whereas microbial biomass C did not vary among salinity levels. Higher metabolic quotients (qCO₂) and unaffected microbial biomass C at high EC values may indicate that salinity is a stressful factor, inducing either a shift in the microbial community with less catabolic activity or reduced efficiency of substrate utilization. Acid phosphatase and alkaline phosphatase activities decreased with increasing soil salinity. We found significant, positive correlations between the activities of phosphatase enzymes and plant's root mass, suggesting that any decrease in the activities of the two enzymes could be attributed to the reduced root biomass under saline conditions.     

北疆绿洲不同盐分浓度梯度下土壤的生物活性及其功能多样性

土壤盐分类型与含量对土壤生物活性和功能结构多样性会产生重要影响。本文模拟新疆北疆典型硫酸盐盐土,通过设置不同盐分浓度梯度(3、6、10、20和40 g kg-1),研究了土壤5种主要功能酶、土壤微生物量和土壤呼吸等生物性状的变化特征。结果表明:过氧化氢酶、蔗糖酶、β-葡萄糖苷酶和多酚氧化酶4种酶对盐分梯度响应非常敏感,其中过氧化氢酶、蔗糖酶和β-葡萄糖苷酶这3种酶活性与盐分浓度呈负相关,当盐分浓度达到40 g kg-1时,以上3种酶分别较对照降低25.7%、69.6%、30.0%;在3~6 g kg-1盐分浓度范围,多酚氧化酶随盐分浓度而升高,但在10~40 g kg-1时却显著降低,表现出"低盐促进,高盐抑制"的效应;纤维素酶对盐分浓度变化不敏感。土壤盐分与微生物量和微生物商呈显著负相关,与土壤基础呼吸和呼吸商则显著正相关,如40 g kg-1的盐分浓度导致土壤微生物量和微生物商分别较对照降低73.0%和69.5%,呼吸商升高了6.4倍。土壤过氧化氢酶、蔗糖酶、土壤微生物量、微生物商和呼吸商等可作为硫酸盐盐土早期预警生物指标。在40 g kg-1的含盐量下土壤生物功能结构明显区别于其他处理,且多样性指数显著下降。     

Effect of temperature on metabolic activity of intact microbial communities: Evidence for altered metabolic pathway activity but not for increased maintenance respiration and reduced carbon use efficiency

We used metabolic tracers and modeling to analyze the response of soil metabolism to a sudden change in temperature from 4 to 20 °C. We hypothesized that intact soil microbial communities would exhibit shifts in pentose phosphate pathway and glycolysis activity in the same way as is regularly observed for individual microorganisms in pure culture. We also hypothesized that increased maintenance respiration at higher temperature would result in greater energy production and reduced carbon use efficiency (CUE). Two hours after temperature increase, respiration increased almost 10-fold. Although all metabolic processes were increased, the relative activity of metabolic processes, biosynthesis, and energy production changed. Pentose phosphate pathway was reduced (17-20%), while activities of specific steps in glycolysis (51%) and Krebs cycle (7-13%) were increased. In contrast, only small but significant changes in biosynthesis (+2%), ATP production (−3%) and CUE (+2%) were observed. In a second experiment, we compared the metabolic responses to temperature increases in soils from high and low elevation. The shift in activity from pentose phosphate pathway to glycolysis with higher temperature was confirmed in both soils, but the responses of Krebs cycle, biosynthesis, ATP production, and CUE were site dependent. Our results indicate that 1) in response to temperature, communities behave biochemically similarly to single species and, 2) our understanding of temperature effects on CUE, energy production and use for maintenance and growth processes is still incomplete.     

Responses of soil microbial respiration to thermal stress in alpine steppe on the Tibetan plateau

Extensive research has focused on the temperature sensitivity of microbial respiration in tundra and alpine meadows. However, the response of microbial respiration to thermal stress in alpine steppe soils, which have less organic matter and greater aeration, has received less attention. We investigated spatial and temporal variations in microbial respiration using an incubation experiment under different temperature (0, 15 and 30°C) and different percentages of water‐holding capacity (WHC) (50 and 100%) conditions in the alpine steppe, and using a subsequent cooling experiment determined the ‘thermal stress' of soil microorganisms in response to increased temperature in the alpine steppe ecosystem. Microbial respiration rates decreased with increasing temperature at both 50 and 100% WHC for three sampling locations. Thermal stress of soil microorganisms under increased temperatures was found in the alpine steppe because subsequent cooling led to an increase in microbial respiration rates. Soil moisture did not affect microbial respiration, but the temperature sensitivity (Q₁₀) of microbial respiration varied with the interactive effect between soil moisture and sampling location. Our findings suggest that the response of microbial respiration to high temperature may not be always positive as bacteria may experience thermal stress in the alpine steppe. Therefore, it is necessary to include thermal stress responses in alpine steppe models, as they may represent an important negative feedback effect on microbial respiration.     

Precipitation modifies the effects of warming and nitrogen addition on soil microbial communities in northern Chinese grasslands

Terrestrial ecosystems experience simultaneous shifts in multiple drivers of global change, which can interactively affect various resources. The concept that different resources co-limit plant productivity has been well studied. However, co-limitation of soil microbial communities by multiple resources has not been as thoroughly investigated. Specifically, it is not clearly understood how microbial communities respond to shifts in multiple interacting resources such as water, temperature, and nitrogen (N), in the context of global change. To test the effects of these various resources on soil microorganisms, we established a field experiment with temperature and N manipulation in three grasslands of northern China, where there is a decrease in precipitation from east to west across the region. We found that microbial responses to temperature depended upon seasonal water regimes in these temperate steppes. When there was sufficient water present, warming had positive effects on soil microorganisms, suggesting an interaction between water and increases in temperature enhanced local microbial communities. When drought or alternating wet–dry stress occurred, warming had detrimental effects on soil microbial communities. Our results also provide clear evidence for serial co-limitation of microorganisms by water and N at the functional group and community levels, where water is a primary limiting factor and N addition positively affects soil microorganisms only when water is sufficient. We predict that future microbial responses to changes in temperature and N availability could be seasonal or exist only in non-drought years, and will strongly rely on future precipitation regimes.     

Community composition and activity of microbes from saline soils and non-saline soils respond similarly to changes in salinity

Saline soils are wide-spread and characterised by poor plant growth and low microbial activity but salinity fluctuates seasonally or with irrigation water quality. Therefore it is important to understand the response of soil microbial communities to changes in soil salinity. We carried out an experiment to test the hypothesis that microbial communities from soils with medium to high salinity respond differently to salinity than microbes from non-saline soils or soils with low salinity. We prepared a microbial inoculum from field soils of different salinity (EC_1:5 0.3, 1.1, 2.7, 4.6 and 6.0 dS m⁻¹). This inoculum was added to quartz sand adjusted to EC_1:5 0.3, 1.1, 2.9, 4.6, 6.0 and 8.0 dS m⁻¹ and amended with finely ground wheat straw and basal nutrients. The sand mix was incubated at 80% water holding capacity for 27 days. Soil respiration was measured continuously, microbial community composition (based on phospholipid fatty acid analysis) and particulate organic carbon (POC) were determined at the start and the end of the incubation. Irrespective of inoculum EC, cumulative respiration decreased with increasing adjusted EC with no differences among inocula. The POC concentration was always lowest at adjusted EC 0.3 and highest at EC 8.0. Up to adjusted EC 4.6, the POC concentration was lower with inoculum EC 0.3 than with the inocula of higher EC. The inocula had distinct microbial community composition at all adjusted ECs, but the changes induced by the adjusted EC were similar in all inocula. The results are contrast to our hypothesis because increasing salinity decreased soil respiration of all inocula to a similar extent. In fact, the lower POC concentration with inoculum from the non-saline soil up to an adjusted EC of 4.6 suggests that the microbial communities from the non-saline soil are able to decompose the added wheat straw under low to moderate salinity to a greater extent than those from saline soils. On the other hand, even microbes from highly saline soils can respond quickly with an increase in activity if the salinity is reduced, e.g. after heavy rainfall which leaches the salts out of the top soil.     

Biotic community shifts explain the contrasting responses of microbial and root respiration to experimental soil acidification

Soil respiration is comprised primarily of root and microbial respiration, and accounts for nearly half of the total CO₂ efflux from terrestrial ecosystems. Soil acidification resulting from acid deposition significantly affects soil respiration. Yet, the mechanisms that underlie the effects of acidification on soil respiration and its two components remain unclear. We collected data on sources of soil CO₂ efflux (microbial and root respiration), above- and belowground biotic communities, and soil properties in a 4-year field experiment with seven levels of acid in a semi-arid Inner Mongolian grassland. Here, we show that soil acidification has contrasting effects on root and microbial respiration in a typical steppe grassland. Soil acidification increases root respiration mainly by an increase in root biomass and a shift to plant species with greater specific root respiration rates. The shift of plant community from perennial bunchgrasses to perennial rhizome grasses was in turn regulated by the decreases in soil base cations and N status. In contrast, soil acidification suppresses microbial respiration by reducing total microbial biomass and enzymatic activities, which appear to result from increases in soil H⁺ ions and decreases in soil base cations. Our results suggest that shifts in both plant and microbial communities dominate the responses of soil respiration and its components to soil acidification. These results also indicate that carbon cycling models concerned with future climate change should consider soil acidification as well as shifts in biotic communities.     

Experimental warming effects on the microbial community of a temperate mountain forest soil

Soil microbial communities mediate the decomposition of soil organic matter (SOM). The amount of carbon (C) that is respired leaves the soil as CO₂ (soil respiration) and causes one of the greatest fluxes in the global carbon cycle. How soil microbial communities will respond to global warming, however, is not well understood. To elucidate the effect of warming on the microbial community we analyzed soil from the soil warming experiment Achenkirch, Austria. Soil of a mature spruce forest was warmed by 4 °C during snow-free seasons since 2004. Repeated soil sampling from control and warmed plots took place from 2008 until 2010. We monitored microbial biomass C and nitrogen (N). Microbial community composition was assessed by phospholipid fatty acid analysis (PLFA) and by quantitative real time polymerase chain reaction (qPCR) of ribosomal RNA genes. Microbial metabolic activity was estimated by soil respiration to biomass ratios and RNA to DNA ratios. Soil warming did not affect microbial biomass, nor did warming affect the abundances of most microbial groups. Warming significantly enhanced microbial metabolic activity in terms of soil respiration per amount of microbial biomass C. Microbial stress biomarkers were elevated in warmed plots. In summary, the 4 °C increase in soil temperature during the snow-free season had no influence on microbial community composition and biomass but strongly increased microbial metabolic activity and hence reduced carbon use efficiency.     

Potential of plant growth-promoting rhizobacteria-plant interactions in mitigating salt stress for sustainable agriculture: A review

Soil salinization affecting different crops is one of the serious threats to global food security.Soil salinity affects 20%and 33%of the total cultivated and irrigated agricultural lands,respectively,and has been reported to caused a global crop production loss of 27.3 billion USD.The conventional approaches,such as using salt-tolerant varieties,saline soil scrapping,flushing,leaching,and adding supplements (e.g.,gypsum and lime),often fail to alleviate stress.In this context,developing diverse arrays of microbes enhancing crop productivity under saline soil conditions without harming soil health is necessary.Various advanced omics approaches have enabled gaining new insights into the structure and metabolic functions of plant-associated beneficial microbes.Various genera of salt-tolerating rhizobacteria ameliorating biotic and abiotic stresses have been isolated from different legumes,cereals,vegetables,and oil seeds under extreme alkaline and saline soil conditions.Rapid progress in rhizosphere microbiome research has revived the belief that plants may be more benefited from their association with interacting diverse microbial communities as compared with individual members in a community.In the last decade,several salt-tolerating plant growth-promoting rhizobacteria (PGPR) that improve crop production under salt stress have been exploited for the reclamation of saline agrosystems.This review highlights that the interaction of salt-tolerating microbes with plants improves crop productivity under salinity stress along with potential salt tolerance mechanisms involved and will open new avenues for capitalizing on cultivable diverse microbial communities to strengthen plant salt tolerance and,thus,to refine agricultural practices and production under saline conditions.     

Soil microbial response following wildfires in thermic oak-pine forests

The ecosystem response to wildfire is often linked to fire severity, with potentially large consequences for belowground biogeochemistry and microbial processes. While the impacts of wildfire on belowground processes are generally well documented, it remains unclear how fire affects the fine-scale composition of microbial communities. Here, we investigate the composition of soil bacterial and fungal communities in burned and unburned forests in an attempt to better understand how these diverse communities respond to wildfire. We explored the belowground responses to three wildfires in Linville Gorge, NC, USA. Wildfires generally increased soil carbon content while simultaneously reducing soil respiration. We employed amplicon sequencing to describe soil microbial communities and found that fires decreased both bacterial and fungal diversity. In addition, wildfires resulted in significant shifts in both bacterial and fungal community composition. Bacterial phylum-level distributions in response to fire were mixed without clear patterns, with members of Acidobacteria being representative of both burned and unburned sites. Fungal communities showed consistent increases in Ascomycota dominance and concurrent decreases in Basidiomycota and Zygomycota dominance in response to burning. Indicator species analysis confirmed shift to Ascomycota in burned sites. These shifts in microbial communities may reflect differences in the quality and quantity of soil organic matter following wildfires.     

Plant roots and species moderate the salinity effect on microbial respiration,biomass, and enzyme activities in a sandy clay soil

The aim of this study was to determine the effects of plant absence or presence on microbial properties and enzyme activities at different levels of salinity in a sandy clay soil. The treatments involved five salinity levels—0.5 (control), 2.5, 5, 7.5, and 10 dS m^?1 which were prepared using a mixture of chloride salts—and three soil environments (unplanted soil, and soils planted with either wheat or clover) under greenhouse conditions. Each treatment was replicated three times. At the end of the experiment, soil microbial respiration, substrate-induced respiration (SIR), microbial biomass C (MBC), and enzyme activities were determined after plant harvest. Increasing salinity decreased soil microbial properties and enzyme activities, but increased the metabolic quotient (qCO₂) in both unplanted and planted soils. Most microbial properties of planted soils were greater than those of unplanted soils at low to moderate salinity levels, depending upon plant species. There was a small or no difference in soil properties between the unplanted and planted treatments at the highest salinity level, indicating that the indirect effects of plant presence might be less important due to significant reduction of plant growth. The lowered microbial activity and biomass, and enzyme activities were due to the reduction of root activity and biomass in salinized soils. The lower values of qCO₂ in planted than unplanted soils support the positive influence of plant root and its exudates on soil microbial activity and biomass in saline soils. Nonetheless, the role of plants in alleviating salinity influence on soil microbial activities decreases at high salinity levels and depends on plant type. In conclusion, cultivation and growing plant in abandoned saline environments with moderate salinity would improve soil microbial properties and functions by reducing salinity effect, in particular planting moderately tolerant crops. This helps to maintain or increase the fertility and quality of abandoned saline soils in arid regions.     

Carbon and nitrogen limitation to microbial respiration and biomass in an acidic solfatara field

The solfatara field is a unique ecosystem characterized by harsh conditions such as acidic soils. We examined the respiration rate and phospholipid fatty acid (PLFA) content of solfatara soils and their responses to carbon and nitrogen addition to determine whether soil microbial respiration and biomass in a solfatara field are limited by substrate availability. Soil samples were collected from locations along a transect across a solfatara field in Oita Prefecture, Japan. The soil in the central part of the solfatara field was highly acidic (pH 2.4) and contained low amounts of carbon and nitrogen. Low basal respiration rates were detected in these soil samples. Measurements of substrate-induced respiration (SIR) and PLFA contents suggested that it was partly attributable to low microbial biomass. Addition of a carbon source (glucose) to the solfatara soil engendered a marked increase in the microbial respiration rate, whereas the nitrogen source (ammonium nitrate) application had no marked effect. Addition of both carbon and nitrogen caused a nearly eightfold increase in the microbial respiration rate and a threefold increase in the total PLFA contents. These results suggest that some acidophilic and/or acid-tolerant microorganisms exist in solfatara soil, but that their respiration and biomass are limited by low substrate availability.