Saline stress modifies the effect of cadmium toxicity on soil archaeal communities

The objective of this study was to examine the response of soil archaeal communities to saline stress in different types of Cd-contaminated soils from the North China Plain. Increased soil salinity by addition of 0.5% sodium salts (NaCl: Na₂SO₄: NaHCO₃: Na₂CO₃ = 1:9:9:1) increased available Cd concentration, resulting in decreased ratios of Cd²⁺/Cd_T and CdSO₄/Cd_T and increased ratios of CdCl_n^2-n/Cd_T in soil solution. Soil saline stress decreased archaeal abundance and diversity and changed major soil archaeal taxa. For example, increased saline stress enriched taxa in the archaeal phyla Thaumarchaeota and Euryarchaeota, and these enriched tolerant taxa had much stronger correlations with soil properties, such as soil pH, EC or Na⁺. In addition, some microbes with low abundances like Bathyarchaeia (no rank) and Candidatus Nitrosotenuis were found to closely correlate with soil pH, EC, Na⁺, and Cl^-, indicating they might play disproportionate roles in regulating ecological functions in stressed habitats. These results suggest that saline stress modified the effect of Cd toxicity on soil archaeal communities in different types of Cd-contaminated soils.