Impact of salinity on soil microbial communities and the decomposition of maize in acidic soils

Soil salinity, as an increasingly important process of land degradation, is a major threat to microbial communities and thus strongly alters organic matter turnover processes. This study was conducted to determine the influence of salinity on the decomposition of maize and on the response of soil microbial communities. Soil samples were collected from two pasture sites in Heringen (Germany). One of the sites has previously been influenced by salinity caused by saline effluent from a potassium mine. These sandy soils were washed, resulting in equal levels of electrical conductivity. Moist soils were then incubated with 2% incorporated maize straw and at three levels of salinity (0, 15, 50 mg NaCl g− 1 soil) for almost 7 weeks at 25 °C. The amount of recovered maize derived particulate organic matter (POM) increased with increasing salinity, exhibiting reduced decomposition of substrate. Furthermore, inorganic N, which consisted almost exclusively of NH4+, increased with increasing levels of salinity. Corresponding to this, biological indices like soil respiration and microbial biomass decreased with increasing levels of salinity, underlining the detrimental effect of salinity on soil microorganisms. This effect was reduced after addition of maize straw, documenting the importance of organic matter amendment in counteracting the negative effects of salinity on microbial communities and related mineralisation processes. Addition of organic matter also led to a spatial differentiation of the microbial community in the soil, with bacteria dominating the surface of the substrate, indicated by a low glucosamine-to-muramic acid ratio. This ratio, however, was not altered by salinity. On the other hand, the ergosterol-to-microbial biomass C ratio was an evidence of fungal dominance in the soil. The ratio increased with elevated salt content, either showing a shift towards fungi, a change in fungal cell morphology, or accumulation of ergosterol in the soil. The metabolic quotient qCO2 was higher in the soil previously subjected to osmotic stress, showing a physiologically more active population that is using substrate less efficiently. We assume that it might further reflect adaptation mechanisms to the increased osmotic pressure.