Linking Microbial Community Structure to Trait Distributions and Functions Using Salinity as an Environmental Filter

ABSTRACT The structure and function of microbial communities vary along environmental gradients; however, interlinking the two has been challenging. In this study, salinity was used as an environmental filter to study how it could shape trait distributions, community structures, and the resulting functions of soil microbes. The environmental filter was applied by salinizing nonsaline soil (0 to 22 mg NaCl g−1). Our targeted community trait distribution (salt tolerance) was determined with dose-response relationships between bacterial growth and salinity. The bacterial community structure responses were resolved with Illumina 16S rRNA gene amplicon sequencing, and the microbial functions determined were respiration and bacterial and fungal growth. Salt exposure quickly resulted in filtered trait distributions, and stronger filters resulted in larger shifts. The filtered trait distributions correlated well with community composition differences, suggesting that trait distribution shifts were driven at least partly by species turnover. While salt exposure decreased respiration, microbial growth responses appeared to be characterized by competitive interactions. Fungal growth was highest when bacterial growth was inhibited by the highest salinity, and it was lowest when the bacterial growth rate peaked at intermediate salt levels. These findings corroborated a higher potential for fungal salt tolerance than bacterial salt tolerance for communities derived from a nonsaline soil. In conclusion, by using salt as an environmental filter, we could interlink the targeted trait distribution with both the community structure and resulting functions of soil microbes. IMPORTANCE Understanding the role of ecological communities in maintaining multiple ecosystem processes is a central challenge in ecology. Soil microbial communities perform vital ecosystem functions, such as the decomposition of organic matter to provide plant nutrition. However, despite the functional importance of soil microorganisms, attribution of ecosystem function to particular constituents of the microbial community has been impeded by a lack of information linking microbial processes to community composition and structure. Here, we apply a conceptual framework to determine how microbial communities influence ecosystem processes, by applying a “top-down” trait-based approach. By determining the dependence of microbial processes on environmental factors (e.g., the tolerance to salinity), we can define the aggregate response trait distribution of the community, which then can be linked to the community structure and the resulting function performed by the microbial community.

Tomato Growth in Response to Salinity and Mycorrhizal Fungi from Saline or Nonsaline Soils

Tomato (Lycopersicon esculentum Mill. `Heinz 1350 VF 402') seedlings were inoculated with populations of vesicular–arbuscular mycorrhizal (VAM) fungi collected from saline or nonsaline soil or remained nonmycorrhizal as a control. Plants then were salinated for 8 weeks at 1.0, 2.0, 5.0, or 10.0 dS·m–1 produced by dilutions of 1 m NaCl: 1 m CaCl2 in deionized water. Inoculation with VAM fungi from nonsaline soil enhanced shoot growth, while VAM fungi from saline soil suppressed shoot and root growth. Plants inoculated with VAM fungi from nonsaline soil and non-VAM control plants showed a quadratic increase in leaf Cl– concentration in response to an increased salinity level, whereas plants inoculated with VAM fungi from saline soil showed a linear increase in leaf Cl– concentration. Mycorrhizal-induced growth responses and changes in leaf Cl– concentration were not associated with any apparent alterations in tomato plant P status. Although VAM fungi originating from saline soil did not promote plant growth, reduction in leaf Cl– concentration mediated by these VAM fungi at moderate salinity levels may have beneficial implications for plant survival in saline soil.

Mineral composition of flax (Linum usitatissimum L.) and safflower (Carthamus tinctorius L.) on a saline soil high in sulfate salts

Little information is available on the ion control mechanisms of safflower and flax under the sulfate-salinity conditions typical of the Canadian prairies region. Chemical constituents of Saffire safflower and Andro flax were investigated in the field to evaluate their response to soil salinity. Mean Ca:Mg and K:Na ratios of plant tops, yield, and seed oil-content were lower on saline soil than on nonsaline soil. On saline soil, the total cation content of flax tops decreased less rapidly with age than that of safflower, mainly due to high uptake of Na+ by flax. The pattern of ion regulation in safflower (high K:Na ratio) typifies a more "tolerant" response to salinity than that in flax. Key words: Mineral composition, oilseed crops, saline field soil, Na2SO4 salinity, Na+ uptake, K:Na ratio

Boron tolerance in wheat in relation to soil salinity

SUMMARYA screenhouse study in Haryana in 1986/87 evaluated the effect of boron on the wheat cultivar WH147 grown in pots in a sandy soil at three salinities. Before sowing, 0–9 mg B/kg soil was added. Without added B the crop produced significantly more grain at electrical conductivity of the saturation extract (ECe) 6 dS/m than at other salinities, but at ECe 8 dS/m yields were similar to those in nonsaline (ECe 0·8) soil. Grain yield in nonsaline soil was not affected significantly even at the highest concentration of added B, even though at the boot stage the shoots had accumulated as much as 307 mg B/kg. At ECe 6 and 8 dS/m, grain yield was significantly less with 3 and 1 mg added B/kg, respectively, than without added B, even though the B concentration in the shoots was only 72 and 31 mg/kg, respectively. The results indicated that B tolerance decreased with increasing salinity of the soil, even though B absorption by plants was 24–62% less in saline than in nonsaline soil at similar B concentrations. As B and salinity often occur together, threshold values of B in soil and plants for optimum growth of wheat will differ with the ECe of the soil. The results suggested that Ca:B ratio in wheat straw is not a reliable indicator of B toxicity in the soil.

ROOT PENETRATION AND SHOOT ELONGATION OF TALL WHEATGRASS AND BASIN WILDRYE IN RELATION TO SALINITY

Tall wheatgrass [Elytrigia pontica (Podp.) Holub Syn: Agropyron elongatum (Host) Beauv. ’Jose’] and basin wildrye [Leymus cinereus (Scribn. & Merr.) A. Love Syn: Elymus cinereus Scribn. and Merr. ’Magnar’] may have potential for increasing forage production once established on saline rangelands. The shoot and root elongation, osmotic adjustment, leaf water stress, and turgor at growth cessation of these grasses in response to drought and salinity were compared in a growth chamber experiment. Seedlings were grown in columns of soil initially saturated with solutions with an electrical conductivity of 1.0, 10, and 20 dS∙m−1 and allowed to grow until desiccated. The greater shoot elongation and root penetration of tall wheatgrass than basin wildrye at all soil salinities corresponds with the higher survival of tall wheatgrass than basin wildrye on a saline soil and on a nonsaline soil in central Nevada. As leaf water potential decreased, both species had similar or higher turgor maintenance in saline than nonsaline soil. But salinity decreased growth of both species, even when no water stress was apparent, and plant and soil water potentials in the saline and nonsaline columns were similar. This suggests that salt toxicity or nutritional imbalances due to accumulated ions, rather than water stress, depressed growth. Plant materials for revegetating saline, arid rangelands should be screened not only for seedling vigor and ability to tolerate or avoid water stress but also for tolerance to salinity.Key words: Drought, osmotic adjustment, turgor, ion toxicity, salt tolerance