Microbial response to Na2SO4 additions in a volcanic soil

We studied the effects of increased electrical conductivity (EC) of a soil on the activity and structure of its microbial community. Dry soil samples were added with 0, 11, 22, and 45 g kg−1 of Na2SO4 and left to incubate for 40 d before microbial respiration, microbial biomass C (MBC), microbial biomass N (MBN), K2SO4-extractable C (Ext-C), K2SO4-extractable N (Ext-N), and potentially mineralizable N (PMN) were determined. Amplified ribosomal DNA restriction analysis (ARDRA) and denaturing gradient gel electrophoresis (DGGE) were applied on α-, β-proteobacterial, and actinomycete 16S rDNA fragments amplified by PCR from total DNA in order to better understand the effect of osmotic stress on the soil microbial communities. The increase in EC significantly reduced respiratory activity of the microbial biomass and lowered microbial C; moreover, it increased the soluble fraction of both C and N. Greater N mineralization was found in soils to which 11 and 22 g kg−1 of Na2SO4 had been added as compared with both the untreated soil, and that receiving 45 g kg−1 of Na2SO4. The two former soils were also richer in aerobic bacteria (107 CFU g−1 soil) than the other two soils (106 CFU g−1 soil), and the ARDRA and DGGE analyses showed an activation of the α-proteobacteria. No significant differences were found in the ARDRA and DGGE patterns of the β-proteobacteria and actinomyces groups, suggesting a no-detectable response of these microorganisms to the Na2SO4 addition within the concentration range in this study.