Smart Capillary Barrier (SCB) has been recently promoted to decrease soil salinity and improve water use efficiency and the sustainability of arid land agriculture. In this study, we investigated the effect of SCB on soil microbial diversity, enumeration, and respiration in a tomato field trial. SCB soil and control (unstructured homogenous soils, H) plots were irrigated with four levels of salinity (ECw = 0.8, 3, 6, and 9 dS m−1). Microbial diversity was assessed by ITS and 16S rRNA gene sequencing, enumeration of culturable heterotrophs by agar plates, and microbial respiration by MicroResp™ assays. Salinity was the main driver of the soil microbial diversity, showing a substantial reduction in the number of operational taxonomic units (− 8% for both bacteria and fungi), enumeration of culturable heterotrophs (− 51% for bacteria and − 53% for fungi), and respiration (− 18%) at 9 dS m−1 water salinity. Microbial community composition was significantly different between the SCB and H soils, as evidenced by multivariate analyses and by the appearance of 3352 unique operational taxonomic units at SCB samples that were absent in H plots. The SCB soil showed a steeper metabolic quotient increase in response to soil salinity than the H soils. The abundance of functional microbes such as nitrogen-fixing and nitrifying prokaryotes, as well as mycorrhiza, was also significantly increased in the SCB soils in comparison with the H soils. Our findings suggest that adopting SCB design leads to higher overall soil microbial biodiversity, including those communities unable to withstand extreme soil salinity conditions.
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