Methane uptake in salt-affected soils shows low sensitivity to salt addition

Methane oxidation in aerated soils is a significant sink for atmospheric methane (CH₄). Salt-affected soils are extensively present and constitute about 7% of total land surface. However, our knowledge about CH₄ turnover between the atmosphere and the saline soils is very limited. In order to evaluate the potential of CH₄ consumption in saline soils, CH₄ fluxes were measured in intact cores of the slightly (ECe = 3.2 mS cm⁻¹), moderately (ECe = 7.1 mS cm⁻¹) and extremely (ECe = 50.7 mS cm⁻¹ and 112.6 mS cm⁻¹) saline soils from the Yellow River Delta, China. CH₄ uptake of cores from the slightly saline soil ranged from 14 to 24 μg CH₄-C m⁻² h⁻¹, comparable to those in the non-saline forest soils with similar texture. CH₄ uptake of cores from the moderately saline soil was only about 6% of that in the slightly saline soil. CH₄ uptake was too low to be measurable in the extremely saline soil. Compared with the non-saline soil, CH₄ uptake in the saline soils was much less sensitive to salt, suggesting the higher salt-tolerance of CH₄ oxidizers in the saline soil. The result also indicated an underestimate in CH₄ uptake for the naturally-occurring saline soils by adding salt to non-saline soils. These results should be useful to study the global CH₄ budget and to explore the physiological and ecological characteristics of methanotrophic bacteria in the salt-affected soils.     

Modeling salinity effects on soil reflectance under various moisture conditions and its inverse application: A laboratory experiment

Soil salinization is an important desertification process that threatens the stability of ecosystems, especially in arid lands. Quantifying and mapping soil salinity to monitor soil salinization is difficult because of its large spatial and temporal variability. There has been a growing interest in the use of hyperspectral reflectance as a rapid and inexpensive tool for soil salinity characterization in the recent past. However, as soil moisture often jointly affects soil reflectance, a moisture-insensitive reflectance model is needed to provide the base for soil salinity monitoring from soil reflectance. In this paper, we developed an exponent reflectance model to estimate soil salt contents inversely under various soil moisture conditions, based on a control laboratory experiment on the two factors (soil salinity and soil moisture) to soil reflectance. Main soil salt types (Na₂SO₄, NaCl, Na₂CO₃) with wide soil salinity (0% to 20%) and soil moisture (1.75% to 20%) levels (in weight base) from Western China were examined for their effects on soil reflectance through a model based approach. Moisture resistant but salt sensitive bands of reflected spectra have been identified for the model before being applied to inversely estimate soil salt content. Sensitive bands for Na₂SO₄ type of salt affected soils were identified as from 1920 to 2230 nm, and 1970 to 2450 nm for NaCl, 350 to 400 nm for Na₂CO₃ type of salt affected soils, respectively. The sensitive bands focused on ranged from 1950 to 2450 nm when all data were considered when ignoring salt types. The model was then applied to inversely estimate soil salt contents. High R² of 0.87, 0.79, and 0.66, and low mean relative error (MRE) of 16.42%, 21.17%, and 27.16%; have been obtained for NaCl, Na₂SO₄ and Na₂CO₃, respectively. Performance of the inverse model dropped but remained significant when ignoring salt types with an R² of 0.56 and a MRE of 33.25%. The approach proposed in this study should thus provide a new direction for estimating salinity from reflectance under various soil moisture conditions and should have wide applications in future monitoring of soil salinization.     

Response of microbial activity and biomass to increasing salinity depends on the final salinity,not the original salinity

Salinization is a global land degradation issue which inhibits microbial activity and plant growth. The effect of salinity on microbial activity and biomass has been studied extensively, but little is known about the response of microbes from different soils to increasing salinity although soil salinity may fluctuate in the field, for example, depending on the quality of the irrigation water or seasonally. An incubation experiment with five soils (one non-saline, four saline with electrical conductivity (EC_e) ranging from 1 to 50 dS m⁻¹) was conducted in which the EC was increased to 37 EC_e levels (from 3 to 119 dS m⁻¹) by adding NaCl. After amendment with 2% (w/w) pea straw to provide a nutrient source, the soils were incubated at optimal water content for 15 days, microbial respiration was measured continuously and chloroform-labile C was determined every three days. Both cumulative respiration and microbial biomass (indicated by chloroform-labile C) were negatively correlated with EC. Irrespective of the original soil EC, cumulative respiration at a given adjusted EC was similar. Thus, microorganisms from previously saline soils were not more tolerant to a given adjusted EC than those in originally non-saline soil. Microbial biomass in all soils increased from day 0 to day 3, then decreased. The relative increase was greater in soils which had a lower microbial biomass on day 0 (which were more saline). Therefore the relative increase in microbial biomass appears to be a function of the biomass on day 0 rather than the EC. Hence, the results suggest that microbes from originally saline soils are not more tolerant to increases in salinity than those from originally non-saline soils. The strong increase in microbial biomass upon pea straw addition suggests that there is a subset of microbes in all soils that can respond to increased substrate availability even in highly saline environments.     

Microbial biomass and activity in alkalized magnesic soils under arid conditions

The effects of salinity and Mg²⁺ alkalinity on the size and activity of the soil microbial communities were investigated. The study was conducted along the border area of the alluvial fan of the Taolai River. Thirty soil samples were taken which had an electrical conductivity (EC) gradient of 0.93-29.60 mS cm⁻¹. Soil pH ranged from 8.60 to 9.33 and correlated positively with Mg²⁺/Ca²⁺ ratio, exchangeable Mg²⁺ percentage and HCO₃⁻+CO₃²⁻. Mg²⁺/Ca²⁺ varied considerably from 3.04 to 61.31, with an average of 23.03. Exchangeable Mg²⁺ percentage generally exceeded 60% and had a positive correlation with Mg²⁺/Ca²⁺. HCO₃⁻+CO₃²⁻ averaged 1.63 cmol kg⁻¹ and usually did not exceed 2.0 cmol kg⁻¹.Microbial biomass, indices of microbial activity and the activities of the hydrolases negatively correlated with Mg²⁺/Ca²⁺ or exchangeable Mg²⁺ percentage. Biomass C, biomass N, microbial quotient (the percentage of soil organic C present as biomass C), biomass N as a percentage of total N, potentially mineralizable N, FDA hydrolysis rate and arginine ammonification rate decreased exponentially with increasing EC. The biomass C/N tended to be lower in soils with higher salinity and Mg²⁺ alkalinity, probably reflecting the bacterial dominance in microbial biomass in alkalized magnesic soils. The metabolic quotient (qCO₂) positively correlated with salinity and Mg²⁺ alkalinity, and showed a quadratic relationship with EC, indicating that increasing salinity and Mg²⁺ alkalinity resulted in a progressively smaller, more stressed microbial communities which was less metabolically efficient. Consequently, our data suggest that salinity and Mg²⁺ alkalinity are stressful environments for soil microorganisms.     

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.     

Rhizobial and bradyrhizobial symbionts of mesquite from the Sonoran Desert: salt tolerance, facultative halophily and nitrate respiration

Rhizobial symbionts were isolated from the surface (0-0.5 M) and phreatic (3.9-5.0 M) root environments of a mature mesquite woodland in the Sonoran Desert of Southern California, and from variable depths (0-12 m) of non-phreatic mesquite ecosystems in the Chihuahuan Desert of New Mexico. They were tested for their ability to tolerate high salinity, and respire NO₃⁻ as mechanisms of free-living survival. Sixteen of 25 isolates were grown in yeast-extract mannitol (YEM) broth at NaCl concentrations of 2 (basal concentration), 100, 300, 500 and 600 mM, and their specific growth rates, cell dry weight and lag times were determined. Twenty of the 25 isolates were also grown in YEM broth under anaerobic conditions with or without 10 mM KNO₃. Three categories of NaCl salinity responses were observed: (1) eight isolates showed decreased specific growth rates at NaCl concentrations of 100, 300 and 500 mM, but they nevertheless remained viable at 500 mM NaCl concentration; (2) the specific growth rate of six isolates increased significantly at 100 and 300 mM NaCl; and (3) specific growth rates of two isolates were significantly greater than the base-rate at all concentrations of NaCl. Five of 11 of the Bradyrhizobium isolates tested respired NO₃⁻, but showed no growth. Seven Rhizobium isolates, three from the deep (3.9-5 m) phreatic rhizobial community, and four from the surface community denitrified NO₃⁻ but only the isolates from the phreatic community displayed anaerobic growth. Long-term interactions between rhizobial and bradyrhizobial communities and the surface and phreatic root environments of the mature Sonoran Desert mesquite woodland appear to have selected for strains of NO₃⁻ respiring rhizobia, general salt tolerance of both rhizobial and bradyrhizobial symbionts, and strains of weak facultative halophilic bradyrhizobia. These survival characteristics of mesquite rhizobia may be important regarding mesquite's establishment and long-term productivity in marginal desert soils, and may provide novel types of rhizobia for food crops growing in harsh environments.     

盐条件下产胞外多糖植物促生细菌研究

Salt-tolerant plant growth-promoting rhizobacteria (PGPR) can play an important role in alleviating soil salinity stress during plant growth and bacterial exopolysaccharide (EPS) can also help to mitigate salinity stress by reducing the content of Na + available for plant uptake.In this study,native bacterial strains of wheat rhizosphere in soils of Varanasi,India,were screened to identify the EPS-producing salt-tolerant rhizobacteria with plant growth-promoting traits.The various rhizobacteria strains were isolated and identified using 16S rDNA sequencing.The plant growth-promoting effect of inoculation of seedlings with these bacterial strains was evaluated under soil salinity conditions in a pot experiment.Eleven bacterial strains which initially showed tolerance up to 80 g L -1 NaCl also exhibited an EPS-producing potential.The results suggested that the isolated bacterial strains demonstrated some of the plant growth-promoting traits such as phosphate solubilizing ability and production of auxin,proline,reducing sugars,and total soluble sugars.Furthermore,the inoculated wheat plants had an increased biomass compared to the un-inoculated plants.     

Absorption and transport of Na and Cl in rice cultivars differing in their tolerance to salinity: An examination of the effects of ammonium and potassium salts

The absorption and transport of Na and Cl from 0.1 mM and 10 mM ²²Na labelled NaCl or ³⁶Cl labelled KCl were examined in 15 days old seedlings of 3 cultivars of rice differing in their tolerance to salinity. Furthermore, the effects of 10, 100 and 1000 ppm (N)₂S on their uptake were studied. It was found that in general, the salt‐tolerant cultivars BR and PNL‐1 absorbed more Na and translocated a lesser proportion of it to the shoot, compared to the salt‐sensitive IR‐8, from 0.1 mM NaCl. The presence of (N)₂S reduced the uptake of Na in all the cultivars. It was also found that the presence of 100 ppm K, KN or NNreduced Na absorption from 0.1 mM NaCl significantly in all the cultivars, and the translocation to shoot in BR‐ Chloride transport from 0.1 mM NaCl was reduced by (N)₂S in all the cultivars. The 3 cultivars differed significantly in the rates of absorption and transport of Na and Cl. The results indicate that PNL‐1 which is a cross of IR‐8 X BR, has inherited the salt tolerance trait from BR. Lower rates of Na translocation to the shoot can be used as an index of salt tolerance in rice.     

Effects of salinity on partitioning, uptake and toxicity of zinc in the earthworm Eisenia fetida

Salinisation of soil is a problem in many parts of the world especially in agricultural lands that could also be contaminated with metals from pesticide use. This study aimed to derive a quality criterion standard in a defined substrate with the eventual aim of protecting earthworms against salinity, and to assess the influence of salinity on partitioning of, uptake in and toxicity of zinc to earthworms. To achieve this, two experiments were conducted with specimens of Eisenia fetida exposed in the laboratory for 28 days using OECD artificial soil spiked with either NaCl (experiment 1) or combination of Zn and NaCl (experiment 2). In the first experiment, NaCl was added in the following concentrations: 0, 1000, 2000, 4000, 6000 and 8000 mg kg⁻¹ NaCl. Mortality, growth and cocoon production were assessed at day 28. The results showed 28-day LC₅₀ of 5436 mg kg⁻¹ for NaCl. The EC₅₀s for growth and cocoon production were 4985 and 2020 mg kg⁻¹ NaCl, respectively. In the second experiment, Zn, added as ZnCl₂ in a range of sub-lethal concentrations (0, 250, 500, 750 mg kg⁻¹ Zn) was combined with 0, 2000 or 4000 mg kg⁻¹ NaCl. The endpoints: mortality, weight change, and the internal zinc concentration were assessed at day 1, 7, 14 and 28 while cocoon production was assessed only at day 28. Apart from the total zinc concentration in the substrates, DTPA and CaCl₂ extractable Zn concentrations were also determined at day 1 and 28 to assess how salinity influenced the partitioning of this metal in the substrates. There was a significant increase in CaCl₂ and DTPA extractable Zn in the substrates as salinity increased suggesting that salinity influenced the partitioning of Zn in the substrates. Weight and mortality of worms were not significantly affected by NaCl and Zn as individual substances, but in combination both had significant effects on these parameters. In contrast, cocoon production was significantly affected by increased NaCl and Zn administered as individual substances, and the effects were more severe when both substances were present. The apparent synergy between Zn and NaCl could not be fully explained by internal zinc concentrations in the worms. It could, however, be partly explained by Cl⁻ effect from the addition of Zn as ZnCl₂. It is concluded that salinity resulting from increased NaCl had an additive to synergistic effect in combination with Zn, in influencing toxicity to these earthworms.     

Salinity effects on carbon mineralization in soils of varying texture

In salt-affected soils, soil organic carbon (SOC) levels are usually low as a result of poor plant growth; additionally, decomposition of soil organic matter (SOM) may be negatively affected. Soil organic carbon models, such as the Rothamsted Carbon Model (RothC), that are used to estimate carbon dioxide (CO₂) emission and SOC stocks at various spatial scales, do not consider the effect of salinity on CO₂ emissions and may therefore over-estimate CO₂ release from saline soils. Two laboratory incubation experiments were conducted to assess the effect of soil texture on the response of CO₂ release to salinity, and to calculate a rate modifier for salinity to be introduced into the RothC model. The soils used were a sandy loam (18.7% clay) and a sandy clay loam (22.5% clay) in one experiment and a loamy sand (6.3% clay) and a clay (42% clay) in another experiment. The water content was adjusted to 75%, 55%, 50% and 45% water holding capacity (WHC) for the loamy sand, sandy loam, sandy clay loam and the clay, respectively to ensure optimal soil moisture for decomposition. Sodium chloride (NaCl) was used to develop a range of salinities: electrical conductivity of the 1:5 soil: water extract (EC_1:5) 1, 2, 3, 4 and 5 dS m⁻¹. The soils were amended with 2% (w/w) wheat residues and CO₂ emission was measured over 4 months. Carbon dioxide release was also measured from five salt-affected soils from the field for model evaluation. In all soils, cumulative CO₂-C g⁻¹ soil significantly decreased with increasing EC_1:5 developed by addition of NaCl, but the relative decrease differed among the soils. In the salt-amended soils, the reduction in normalised cumulative respiration (in percentage for the control) at EC_1:5 > 1.0 dS m⁻¹ was most pronounced in the loamy sand. This is due to the differential water content of the soils, at the same EC_1:5; the salt concentration in the soil solution is higher in the coarser textured soils than in fine textured soils because in the former soils, the water content for optimal decomposition is lower. When salinity was expressed as osmotic potential, the decrease in normalised cumulative respiration with increasing salinity was less than with EC_1:5. The osmotic potential of the soil solution is a more appropriate parameter for estimating the salinity effect on microbial activity than the electrical conductivity (EC) because osmotic potential, unlike EC, takes account into salt concentration in the soil solution as a function of the water content. The decrease in particulate organic carbon (POC) was smaller in soils with low osmotic potential whereas total organic carbon, humus-C and charcoal-C did not change over time, and were not significantly affected by salinity. The modelling of cumulative respiration data using a two compartment model showed that the decomposition of labile carbon (C) pool is more sensitive to salinity than that of the slow C pool. The evaluation of RothC, modified to include the decomposition rate modifier for salinity developed from the salt-amended soils, against saline soils from the field, suggested that salinity had a greater effect on cumulative respiration in the salt-amended soils. The results of this study show (i) salinity needs to be taken into account when modelling CO₂ release and SOC turnover in salt-affected soils, and (ii) a decomposition rate modifier developed from salt-amended soils may overestimate the effect of salinity on CO₂ release.     

Spatial distribution of soil salinity and potential implications for soil management in the Manas River watershed,China

Understanding the watershed-scale spatial distribution of soil salinity and its compositions is important for soil management. Here, we present the first study on the Manas River watershed in northwest China. In this study, we took soil samples in upper 20 cm of soil from 186 locations across the watershed and measured total salt concentration (TSC), salt ion composition and soil particle size distribution (PSD). We found that on average topsoil TSC tended to increase, from 3.55 g kg⁻¹ in upstream regions to 19.40 g kg⁻¹ in downstream regions. The stoichiometric analysis showed that the equivalence ratio of soil Cl^- to SO₄²⁻ increased from 0.53 in upstream regions to 2.12 in midstream regions, and further to 3.76 in downstream regions; thus, the soil types were classified into chloride–sulfate, sulfate–chloride and chloride soils types, respectively. Additionally, proportions of small (<2 μm in diameter) and large (>2,000 μm) soil particles increased, while that of medium sizes (2–50 μm) decreased from upstream to downstream, with an increasing coefficient of variance (CV) in PSD. Taken together, watershed-scale topsoil salinity may be horizontally characterized by increased TSC and Na⁺ & Cl⁻ proportions, greater equivalence ratio of Cl⁻ vs. SO₄²⁻ and more balanced distribution of PSD along with surface water flow. Results demonstrated that soil salinity and its ions compositions showed a great variation across the watershed scale, suggesting that soil management may consider the spatial heterogeneity of saline–alkaline soil types, and our results provided scientific guidance for local soil management and restoration.     

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.     

Microbial populations immobilizing NH4-N and NO3-N differ in their sensitivity to sodium chloride salinity in soil

An incubation experiment was conducted to study the response to sodium chloride (NaCl) salinity of microbial population immobilizing NH₄⁺- and NO₃⁻-N using glucose as an easily oxidizable C source. Immobilization of NH₄⁺-N was faster than that of NO₃⁻-N and was complete within 12 h of -incubation. Presence of NaCl retarded the process of N immobilization; that of NO₃⁻-N being more affected. Remineralization of immobilized N started within 48 h in case of both NH₄⁺- and NO₃⁻-N and was faster for the latter. Both remineralization and nitrification were significantly delayed in the presence of NaCl; inhibition being more at 4000 mg NaCl kg⁻¹ soil. The inhibitory effect of NaCl on remineralization of N was relatively more for NH₄⁺-treated soil. The results of the study suggested a higher sensitivity to NaCl of microorganisms assimilating NO₃⁻. However, remineralization of N from NO₃⁻-assimilating microbial population was less affected by NaCl salinity compared to NH₄⁺-assimilating population.     

Effect of Piriformospora indica and Azospirillum strains from saline or non-saline soil on mitigation of the effects of NaCl

Salinity toxicity is a worldwide agricultural and eco-environmental problem. The intent of this study was to determine the salt tolerance of Piriformospora indica and strains of Azospirillum, isolated from non-saline and saline soil, as well as to determine their affect on the tolerance of wheat to soil salinity. In this study, an experiment was conducted to investigate the salt stress tolerance abilities of the endophytic fungi, P. indica, and Azospirillum strains, isolated from non-saline and saline soil, at five NaCl levels (0, 0.1, 0.2, 0.3, 0.4, 0.5 mol L^?1). Additionally, a greenhouse experiment was conducted to test the effects of these selected microorganisms under increasing salinity levels on seedling growth, solute accumulation (proline and sugars), and photosynthetic pigments (Chl a, b, ab) of seedling wheat. Azospirillum strains were isolated in Iran from the root of field-grown maize from non-saline soil with an EC = 0.7 dS m^?1 and from saline soil with an EC = 4.7 dS m^?1. Plants were irrigated with non-saline water–tap water with an electrical conductivity water (ECw) value of 0.2 dS m^?1, as well as low, moderate and severe saline water-irrigation with saline water with an ECw of 4 dS m^?1, 8 dS m^?1 and 12 dS m^?1, respectively. The upper threshold of P. indica salinity tolerance was 0.4 mol L^?1 NaCl in both liquid and solid broth medium. The upper thresholds of the salt adapted and non-adapted Azospirillum strains were 0.2 and 0.4 mol L^?1 NaCl, respectively. The results indicated a positive influence of the organisms on salinity tolerance, more with the saline-adapted Azospirillum strains than the non-adapted strains. P. indica was more effective than the Azospirillum strains. These results could be related to a better water status, higher photosynthetic pigment contents and proline accumulation in wheat seedlings inoculated with P. indica. The benefits of both isolates and P. indica depended on two factors: water salinity and growth stage of the host plant. Inoculation with the two isolates increased salinity tolerance of wheat plants; the saline-adapted Azospirillum strains showed better performance with respect to improved fresh and dry weights at 80 and 100 days after sowing under both non-saline and saline conditions. When compared to plants inoculated with non-saline-adapted Azospirillum strains, those inoculated with adapted Azospirillum strains had much better performance with respect to the presence of photosynthetic pigment (Chl a, b and ab) and proline accumulation. Overall, these results indicate that the symbiotic association between P. indica fungus and wheat plants improved wheat growth, regardless of the salinity. It is concluded that the mechanisms for protecting plants from the detrimental effects of salinity by P. indica fungus and Azospirillum strains may differ in their salinity tolerance and influence the uptake of water, photosynthetic pigment contents and proline accumulation in wheat seedlings.     

Effect of salinity on the decomposition of soil organic carbon in a tidal wetland

Purpose

Climate warming and sea level rise have the potential to change the salt level of soil in tidal wetlands. And it is important to clarify the process and the mechanism of decomposition of soil organic carbon in a tidal wetland under varying salinities. The aim of this study was to evaluate the impacts of soil salinity on the decomposition rate of organic carbon (DROC) and dissolved organic carbon (DOC) in a tidal wetland.

Materials and methods

Two types of soil (surface soil in Suaeda salsa and bare tidal flat) were collected, air-dried, and homogenized. After adding different content of salt (0 g/L, 3 g/L, 6 g/L, 9 g/L, and 12 g/L), the soils were incubated for 28 days at stable room temperature (25?±?2 °C) and added by deionized water to maintain the stability of soil moisture. The gases (CO₂ and CH₄) emission rates of each salt treatment were measured during 28-day incubation. DROC was determined by the sum of daily CO₂-C emission rates and daily CH₄-C emission rates in this study.

Results and discussion

Salt addition inhibited the process of gas emissions and DROC. Gases emission rates and DROC of two types of soil showed similar temporal trends, with distinctive drop in the beginning of experiment and no significant decrease followed. Significant difference of DOC among salt treatments was found in the bare tidal flat soil. Variations of partial correlation between DROC and soil salinity and DOC showed similar trends (e.g., in days 9–18, the positive effect of DOC on DROC was greatly promoted (R²?=?0.80, p?<?0.001), and the negative effect of soil salinity was highly improved (R²?=?0.93, p?<?0.001)). Soil properties, in particular DOC, may be primary factors accounting for the discrepancy of gases emission rates and DROC of two types of soil.

Conclusions

Increased soil salinity had a negative effect on DROC during 28-day incubation. The impact of soil salinity and DOC on DROC were varied in different phases of laboratory experiment (soil salinity generally had increasingly negative relationship with DROC, but DOC showed most significantly positive relationship in the middle stage of incubation). Both the formation and consumption of DOC may be valuable for more detail research regarding to decomposition of soil organic carbon.

     

Changes of arbuscular mycorrhizal traits and community structure with respect to soil salinity in a coastal reclamation land

A comprehensive knowledge on the relationship between soil salinity and arbuscular mycorrhizal fungi (AMF) is vital for a deeper understanding of ecosystem functioning under salt stress conditions. The objective of this study was to determine the effects of soil salinity on AMF root colonization, spore count, glomalin related soil protein (GRSP) and community structure in Saemangeum reclaimed land, South Korea. Soil samples were collected and grouped into five distinct salt classes based on the electrical conductivity of soil saturation extracts (EC_se). Mycorrhizal root colonization, spore count and GRSP were measured under different salinity levels. AMF community structure was studied through three complementary methods; spore morphology, terminal restriction fragment length polymorphism (T-RFLP) and denaturing gradient gel electrophoresis (DGGE). Results revealed that root colonization (P < 0.01), spore count (P < 0.01) and GRSP (P < 0.01) were affected negatively by soil salinity. Spore morphology and T-RFLP data showed predominance of AMF genus Glomus in Saemangeum reclaimed land. T-RFLP and DGGE analysis revealed significant changes in diversity indices between non (EC_se < 2 dS/m) and extremely (EC_se > 16 dS/m) saline soil and confirmed dominance of Glomus caledonium only in soils with EC_se < 8 dS/m. However, ribotypes of Glomus mosseae and Glomus proliferum were ubiquitous in all salt classes. Combining spore morphology, T-RFLP and DGGE analysis, we could show a pronounced effect in AMF community across salt classes. The result of this study improve our understanding on AMF activity and dominant species present in different salt classes and will substantially expand our knowledge on AMF diversity in reclaimed lands.     

Interactive effects of salinity and two phosphorus fertilizers on growth and grain yield of Cicer arietinum L.

Chickpea is considered among the most sensitive grain legumes to salinity. The improvement of tolerance of lines in combination with tolerant rhizobial strains depends on various environmental and cultural conditions such as soil properties. This investigation was undertaken to evaluate the effect of phosphorus fertilization (0, 90 and 200 kg ha^?1 of P₂O₅) on biomass, nodular traits and grain yield (GY) of chickpea (cv. Flip 84-79C) growing under salinity (0 and 150 mM NaCl). The trial was laid out following a randomized block design with three replicates during 2010–2012, at the experimental farm of Oued Smar (Algiers). Salinity did not significantly decrease the dry biomass of the plants but the relative shoot growth was more affected than control, P and SP1 treatments. Besides, salinity significantly reduced GY (?20%) and nodulation traits compared to the control plants while an inversely proportional relationship was found between protein, leghemoglobin and MDA content, K/Na ratio and the increase in salt concentration. Application of two P levels to saline soil enhanced growing conditions of plants. Particularly, the (90?kg?ha^–1 of P ×?150?mM?NaCl) combination significantly increased leghemoglobin (92%), reduced proline content (?69%) and protected membranes against peroxydation compared to saline conditions. A significant increase was observed in the GY (about 30%) of plants at both P doses combined with salt stress compared to other cases. Statistically, the low P level combined with salinity induced similar responses of plants and sometimes better responses to control plants. Finally, our results support the roles of phosphorus fertilizer in the alleviation of salt stress and enhancing the soil quality for better symbiosis efficiency and yield of chickpea.     

溶质类型与矿化度对半干旱盐渍化地区果园土壤水分有效性的影响

盐渍化土壤水分有效性是制约土地生产能力的关键因素之一。研究不同盐分类型及矿化度的盐溶液对土壤水分有效性的影响, 可为微咸水合理灌溉以及促进土壤生产潜力的发挥提供科学依据。本研究采用离心法在室内研究了脱水过程中灌溉水的溶质类型(NaCl和Na₂SO₄)与矿化度(0、1 g·L^-1、3 g·L^-1、5 g·L^-1、10 g·L^-1)对半干旱盐渍化地区果园土壤水分有效性的影响。结果表明: 不同矿化度的NaCl和Na2SO4处理均可使田间持水量、暂时萎蔫系数、永久萎蔫系数、迟效水和无效水较对照有所降低。不同矿化度的NaCl处理以及1 g·L^-1的Na₂SO₄处理土壤全有效水和速效水都较对照增加, 3 g·L^-1、5 g·L^-1和10 g·L^-1的Na₂SO₄处理土壤全有效水和速效水都较对照减小。不同矿化度的NaCl和Na₂SO₄处理均可使土壤通气孔隙和毛管孔隙相对减少, 非活性孔隙增大, 其中矿化度为5 g·L^-1的NaCl和Na₂SO₄处理对其影响最为明显, 通气孔隙分别较对照减小16.8%和14.8%, 毛管孔隙分别较对照减小5.2%和6.5%, 非活性孔隙分别较对照增加15.7%和14.4%。NaCl对于土壤比水容量和毛管断裂的延迟效果比NaSO₄明显。且土壤溶液盐分含量增加, 土壤持水能力下降、供水性能增加而土壤抗旱性降低。     

新疆玛纳斯河流域土壤盐分特征研究

以新疆玛纳斯河流域为研究区,结合面域土壤性状调研,利用相关分析和主成分分析方法对区域土壤盐分特征进行研究.结果表明,研究区域土壤盐渍化类型以硫酸盐为主,剖面土壤盐分含量呈现底聚分布特征;土壤盐分含量与SO42-、Ca2+离子含量呈极显著正相关;各层土壤盐分阴离子以SO42-为主,阳离子以Ca2+为主;SO42-、Ca2+、Mg2+及土壤盐分含量(St)是表征玛纳斯河流域土壤盐渍化的主要特征因子.本研究将为新疆玛纳斯河流域土壤资源可持续利用提供重要的理论依据.     

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.