Soil microbial responses to mild winters and seasonal change

Sammanfattning: The aim with this thesis work was to study microorganisms response to milder winters, different seasons, and altered temperature. Winter warming treatments were established in a semi-natural temperate grassland, simulating mild winters for two years. We found that arbuscular mycorrhizal (AM) fungi received more carbon (C) from the plants after experiencing milder winters. The amount of plant-derived C found in AM fungi also varied over the year, peaking during the growing season. Surprisingly, also saprotrophic fungi received considerable amounts of C from the plants, however, the C flow was un-affected by both mild winters and time of the year.Temperature is known to strongly affect the rate of decomposition. We showed that microorganisms could rapidly adapt to altered temperatures when exposed to temperatures above their optimum, in a laboratory study. However, when we assessed responses of microbial temperature adaptation in the dynamic natural environment that microorganisms inhabit in the field, we did not find that microorganism adapted to increased winter temperatures, or due to the environmental temperatures in summer compared to winter.Seasonal variation in microbial biomass, growth of bacteria and saprotrophic fungi, and microbial community composition were analysed along a natural gradient varying in pH and nutrients, factors known to strongly influence microorganisms, over one year. The influence of seasonal variation showed to be less than the variation in pH and nutrient availability within the gradient, for the microorganisms. In conclusion, mild winters seem to promote an AM fungal dominated microbial community, likely at the expense of other microbial groups. Saprotrophic fungi also received considerable amounts of recently plant-fixed C, particularly during the winter. Furthermore, we found that microorganisms could adapt to altered temperatures within two months, however, no effect on the microbial temperature relationship was seen by warmer winters in field. Hence, events of extreme temperatures during summer are likely more important for shaping the microbial community than winter temperatures.