Comparative Toxicities of Salts on Microbial Processes in Soil

Abstract

Soil salinization is a growing threat to global agriculture and carbon sequestration, but to date it remains unclear how microbial processes will respond. We studied the acute response to salt exposure of a range of anabolic and catabolic microbial processes, including bacterial (leucine incorporation) and fungal (acetate incorporation into ergosterol) growth rates, respiration, and gross N mineralization and nitrification rates. To distinguish effects of specific ions from those of overall ionic strength, we compared the addition of four salts frequently associated with soil salinization (NaCl, KCl, Na2SO4, and K2SO4) to a nonsaline soil. To compare the tolerance of different microbial processes to salt and to interrelate the toxicity of different salts, concentration-response relationships were established. Growth-based measurements revealed that fungi were more resistant to salt exposure than bacteria. Effects by salt on C and N mineralization were indistinguishable, and in contrast to previous studies, nitrification was not found to be more sensitive to salt exposure than other microbial processes. The ion-specific toxicity of certain salts could be observed only for respiration, which was less inhibited by salts containing SO4(2-) than Cl(-) salts, in contrast to the microbial growth assessments. This suggested that the inhibition of microbial growth was explained solely by total ionic strength, while ion-specific toxicity also should be considered for effects on microbial decomposition. This difference resulted in an apparent reduction of microbial growth efficiency in response to exposure to SO4(2-) salts but not to Cl(-) salts; no evidence was found to distinguish K(+) and Na(+) salts.

FIG 1

Concentration-response relationships between salt (NaCl) exposure and bacterial growth measured as leucine incorporation (A), fungal growth measured as acetate incorporation into ergosterol (B), and soil respiration (C). Soil samples were collected from the same site at different time points (represented by different symbols), and data from repeated runs of the experiment were combined into a single inhibition curve. Error bars indicate the standard errors (n = 2).

FIG 2

Concentration-response relationships between salt (NaCl) exposure and gross N mineralization (A) and gross nitrification (B). Error bars indicate standard errors (n = 2).

FIG 3

Dose-response relationships between bacterial growth (A), fungal growth (B), and soil respiration (C) and short-term exposure to different salts (NaCl, KCl, Na₂SO₄, and K₂SO₄). Data from repeated runs of the experiment were combined into a single inhibition curve. In the control treatments without added salt, the bacterial growth was 42 pmol Leu g⁻¹ h⁻¹, the fungal growth rate was 15 pmol Ac g⁻¹ h⁻¹, and the respiration rate was 1.8 μg CO₂ g⁻¹ h⁻¹. Error bars indicate the standard errors (n = 2).