Effects of bryophytes on soil microbial processes

In high latitude ecosystems bryophytes (mainly mosses) play an important role in ecosystem processes and they often constitute a high proportion of total biomass and productivity. Abundance of bryophytes, however, has been observed to decline as a result of global change, which may strongly influence carbon and nitrogen cycling and hence feedback on climate. Little is known, however, about the effects of bryophytes on microbial decomposition processes and nutrient transformations in soils. This project thus aims at elucidating how and by what mechanisms bryophytes influence microbial processes and microbial community composition in soils compared to higher plants and how these interactions affect nutrient availability for higher plants. We will investigate this by a transplant experiment in a tundra heath in northern Sweden, as well as by a field manipulation experiment simulating effects of bryophytes on soil temperature and moisture and soil N availability. We will determine effects of bryophytes on various microbial processes, such as extracellular enzyme activities, N mineralization and nitrification rates, as well as effects on C and N availability and microbial community composition. The results of this project will contribute to a better insight into plant soil interactions apart from higher plants and will thus help to predict changes in ecosystem processes resulting from future shifts in plant community composition in tundra and boreal forest ecosystems.

Effects of bryophytes on soil microbial processes and nitrogen cycling In high-latitude ecosystems bryophytes (mosses) play an important role in ecosystem processes, as they constitute a high proportion of total biomass and productivity. The abundance of mosses, however, has been observed to decline as a result of global change, which may strongly impact on carbon and nitrogen cycling and hence yield feedback on climate. The effects of mosses on activities of soil microbes are, however, still poorly understood. This project thus aimed at elucidating how and by what mechanisms mosses influence microbial decomposition processes of soil organic matter and microbial community composition compared to higher plants and how these interactions affect soil nutrient availability. We investigated this by a field study and a transplant experiment in a tundra heath in northern Sweden, as well as by a field manipulation experiment in a subarctic birch forest where we simulated effects of bryophytes on soil temperature and moisture by means of an artificial soil cover. The results of this study demonstrate that the effects of mosses on soil microbial processes and carbon and nitrogen cycling vary strongly between ecosystems, depending on the vegetation type and the moss species present and on the factors which are most strongly constraining microbial activity under the respective conditions. In a subarctic birch forest, the thick continuous moss cover reduced soil nitrogen availability and retarded nitrogen cycling. This was linked to the thermal insulation capacity of the moss layer causing low soil temperature in summer, as well as reduced soil temperature fluctuations and thus decreased frequency of freeze-thaw events in autumn and spring. On the other hand, results from a tundra heath with patchy distribution of mosses and dwarf shrubs revealed relatively high nitrogen availability in soil under mosses, which was also reflected in the pattern of extracellular enzyme activities and a high abundance of bacteria. These differences between moss and dwarf shrub sites were mainly explained by the variation in the chemical composition of the plant litter and soil organic matter, as well as differences in soil pH-value. Finally, the results of a transplant experiment showed that under temporarily dry conditions and low input of fresh organic matter mosses may also stimulate microbial activity via their moisture conserving capacity and leaching of substances from the moss biomass. This project elucidated the complex relationships between mosses and soil microbial processes and microbial communities. The results emphasize the crucial role of mosses in carbon and nitrogen cycling of high-latitude ecosystems and the potential strong impact of future changes in plant community composition on ecosystem processes and carbon storage in arctic and subarctic ecosystems.