Plant-microbe-soil interactions and soil nitrogen dynamics in boreal forests

Abstract: Widespread nitrogen (N) limitation of plant growth in boreal forests is a well recognized phenomenon. Yet, the mechanisms responsible for the development of N limitation are unknown. By exploring the linkage between N cycling and microbial community structure, this thesis examines the role of soil microorganisms in N limitation development. The first part of the thesis addresses effects of long-term N additions on microbial communities and N cycling in Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.) forests and if the effects are reversible after termination of N addition. The second part of the thesis explores the role soil microbes play in the development of N limitation in primary forest ecosystems. The general structure and biomass of soil microbial communities was assessed by phospholipid fatty acid analyses. Soil and ecosystem N cycling were inferred from gross N mineralisation measurements and 15N natural abundance in soil and foliage. Retention of the 15N label by soil microorganisms was used to infer N retention capacity of the ecosystems. Despite unique responses in microbial communities and gross N mineralisation to long-term N additions between the two studies, some common patterns emerged. Gross N mineralisation, microbial community structure, and N retention were strongly linked. Microbial biomass decreased but gross N mineralisation increased after N addition. The increased biotic N retention after termination of N addition coincided with increased functional role ectomycorrhizal fungi play in ecosystem N cycle as inferred from changes in 15N natural abundance. In the land uplift chronosequence, large inputs of N through N2-fixation resulted in soil N accumulation but a decline in N supply rates. This coincided with increasing microbial N-immobilisation and increasing abundance of ectomycorrhizal fungi suggesting their importance in N retention. I suggest that the strong N limitation typical of boreal forests can develop in about 150 years. This thesis provides strong evidence that ectomycorrhizal fungi played an important role both in the return of N limitation two decades after termination of N addition and in the development of N limitation in a primary boreal forest.