Assessing soil microbial tolerance to salt

Abstract

Salinisation is a global issue which leads to desertification and soil degradation and exposure to salt is also known to inhibit microbial activity and plant growth. This study was conducted to determine the influence of salinity on microbial activity of a non- saline soil. By using Salt Induced Community Tolerance (SICT) concept I found that microbial community tolerance to salt can be induced in a previously non- saline soil. The SICT concept is based on the fact that microorganisms in a saline environment survive only if they are tolerant to the salt i.e. the most tolerant microorganism will outgrow more sensitive one, leading to an increase in average community tolerance. Non- saline soil samples were collected from Vomb in Southern Sweden. The soil was exposed to different types of salts (NaCl, KCl, Na2SO4 and K2SO4), and a range of microbial processes were measured to characterize the acute microbial susceptibility to salt exposure. There was overall negative impact of different types of salts on microbial processes in a non- saline soil. Fungi were found to be more tolerant than bacteria to salt exposures. C- use efficiency changes upon salt exposure will depend upon which microbial group (fungi or bacteria) are dominating respiration. I found evidence for an ionspecific toxicity for catabolic processes whereas the toxicity was due to general ionic strength toxicity for anabolic processes in non- saline soil. An incubation experiment with six treatments in replicates (3.4, 1.1, 0.38, 0.13, 0.04 and 0 mmol NaCl g-1 soil) was conducted. After amendment with 1:1 straw- alfalfa (25 mg g-1 soil) to provide nutrient source, the treatments were incubated at optimal water content for 40 days, microbial respiration, bacterial growth and fungal growth were measured continuously. An early appearance of fungal growth at high salt concentration than bacterial growth suggested a competitive release of fungi. SICT was induced over short time scales. Its values increased with higher salinization, showing more tolerant communities at higher salt concentrations than low salt concentration. These NaCl- induced tolerant microbial communities were also found to be tolerant to other types of salts (KCl, Na2SO4 and K2SO4). The tolerance increased with higher salt concentrations. The toxicity that induced tolerance was due to an ion- specific tolerance at high salt concentration and due to general ionic strength at intermediate salt concentrations. Another incubation experiment was conducted to study the effect of dual stress (salinity and drought) on bacterial growth of a non- saline soil. The non-saline soil with water content 25% dry wt. was adjusted at five different salinity levels by adding different amount of NaCl (1.1, 0.38, 0.13, 0.04, and 0 mmol NaCl g-1 soil). The soils were then incubated with addition of 1:1 straw –alfalfa (25 mg g-1 soil) at 20°C in dark for 32 days. After the incubation, soil was dried in oven at 21°C and at different time intervals (0, 20, 22, 24, 26, 42, 44, 46, 48 and 50 h) the bacterial growth and soil water content were estimated. The results were supporting our hypothesis i.e. there was a faster decline in bacterial growth of a saline soil as compare to non- saline soil upon drying.

Popular Abstract

Salt exposure changes microbial community composition of a non- saline soil

Salinisation is the process that leads to an increase of water-soluble salts in the soil. Worldwide about 20% of agricultural land especially in economically challenged regions are largely affected by salinization. This leads to desertification and soil degradation, endangering the future use of the soil.

Soil is the habitat for many organisms among which microorganisms dominate. When the salt concentration in the soil pore water outside the microbial cells increases, microbial cells loose water, leading to shrinking and dehydration of the cells. This is why salt can kill plants; it leaches the water from the cells. Similarly, the movement of water to leave an animal cell will also cause those cells to shrink and cause dehydration. This is why a person could die from dehydration if he drinks enough sea water. In order to cope with this stress, microorganisms use different strategies to adapt themselves. If they fail to adapt then they die or become inactive. Therefore, microorganisms that can cope with high salt concentrations will become more abundant resulting in a community composed of more tolerant members in the soil.

In this study I found that salt exposure has over all a negative impact on microbial activity and growth of a non- saline soil. I have tested different salts commonly found in saline soils (chloride and sulfate salts). Salts containing chloride ions were more toxic to processes related to breakdown of organic matter, whereas processes related to build- up of microbial biomass were more affected by the overall concentration of ions in a non-saline soil. Fungi were found to be more tolerant to all types of salts than bacteria, both to short term (few hours) and during the long term salt exposure (one month).

I found that microbial community tolerance to salt can be induced in a previously non- saline soil. This means that microbial communities from saline soils cope better with table- salt (NaCl) than those in non- saline soil. Consequently, the salt stress has induced a change in the microbial community composition of the non- saline soil to a more tolerant community. The reason for this is that microorganisms in a saline environment survive only if they cope with the salt i.e. the most tolerant microorganism will outgrow more sensitive one. I found that a more salt tolerant microbial community developed at high salt concentrations compared to low salt concentration in a non-saline soil. These table salt- induced tolerant microbial communities were also found to be tolerant to other types of salts.

When the non- saline soil was exposed to the combined stresses of salinity and drought I found that bacterial growth declines faster in saline soil than in non- saline soil. During drying of soil the salt in the pore water of the soil become more concentrated, which worsens the effect of salinity on the soil microorganisms.

Supervisors: Johannes Rousk and Kristin Rath
Master´s Degree Project 30 credits 2014/2015
Department of Biology, Lund University, Sweden.

• author: Maheshwari, Arpita
• supervisor: Johannes Rousk; Kristin Rath
• organization: Degree Projects in Biology
• course: BIOM01 20142
• year: 2015
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 5435852
• date added to LUP: 2015-05-29 14:48:57
• date last changed: 2015-06-01 16:25:40