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, Na 2 SO 4 , and K 2 SO 4 ) to a nonsaline soil. To compare the tolerance of different microbial processes to salt and to interrelate the toxicity of different salts, concentrationresponse 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 SO 4 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 SO 4 2؊ salts but not to Cl ؊ salts; no evidence was found to distinguish K ؉ and Na ؉ salts. Soil salinization affects a large area of land globally and has become a major threat to agricultural productivity and food security (1). Due to the wide distribution of salt-affected soils around the world (2, 3), it is important to understand the influence of salinity on the soil microbial community. The soil microbial decomposer community plays an essential role in the decomposition and stabilization of soil organic matter (SOM), as well as the cycling of nutrients vital for plant growth. How substrate during decomposition is allocated to either microbial biomass production or respiration determines the microbial growth efficiency (MGE), which is an important parameter for the C sequestration potential of a soil (4). The potential for soil C storage could be compromised by disturbances or unfavorable environmental conditions that reduce microbial growth efficiencies due to the metabolic burden they place on microbial cells (5).It is generally held that fungus-dominated communities have a higher MGE than communities dominated by bacteria (4). Thus, changes in the relative contribution of bacteria and fungi to the soil microbial community are thought to reflect changes in ecosystem processes, such as decomposition, C sequestration potential, and nutrient cycling (6, 7). It is unclear whether fungi and bacteria are affe...

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