Soil salinity as a driver of microbial community structure and functioning

Abstract: Soil salinization is a pressing agricultural problem in many areas of the world, particularly in areas heavily reliant on irrigation agriculture. While the negative effects of salinity on crop plants have been widely studied, its effects on soil microorganisms have received less attention, and the impact of soil salinity on both microbial community structure and functioning is not well understood. Soil microorganisms are important in regulating terrestrial biogeochemical processes and changes in microbial community structure and functioning in response to salinization could have far-reaching consequences for carbon and nutrient cycling. The main aims of this thesis were to (a) assess and compare the impact of salinity on different microbial processes in soil, (b) study the ability of soil microbial communities to adapt to increasing salinity and connect increased tolerance to changes in community composition and functioning, and (c) study the combined impact of salinity and another important environmental disturbance, drought. The impact of salinity on soil microorganisms was studied at three different time scales: (i) acute salt exposure, (ii) exposure over one month and (iii) naturally saline soils having experienced high salt concentrations over long time periods. I found that processes such as growth, respiration and N transformation rates were strongly negatively reduced in response to acute salt exposure. However, increased salt tolerance of the microbial community could be induced quickly, combined with a partial recovery of process rates. A strong relationship between soil salinity and community salt tolerance was also found along natural gradients of salinity. Higher community salt tolerance was reflected in shifts in the composition of the bacterial community. Fungi were less sensitive to salt exposure than bacteria, which manifested in less inhibition of fungal growth both after short-term and long-term exposure to salinity. When salinity was combined with drying of soils, the effects of the individual factors were exacerbated. Increased soil salinity also modified the microbial response to rewetting of died soil.In summary, soil salinity was found to be an important regulator of both microbial functioning and community composition. While microbial communities have a strong potential to adapt to increasing salt concentrations which can offset to some defree the loss of functioning of the microbial community, process rates are still reduced in saline soils compared to non-saline soils. Thus, as salt-affected soils increase in area, the ability of microorganisms in saline soils to sustain biogeochemical cycles need to be considered.