Theme 2 – Growth and interaction with the environment - current and future

Understanding the interaction

Sustainable forest production requires that the stands use their resources effectively. Carbon and nitrogen are two key components controlling the tree's growth, and their cycles are highly integrated. Trees absorb carbon from the atmosphere, which is used for its own growth and to feed the soil microorganisms. Nitrogen is taken up from the soil by tree roots in an interaction with mycorrhiza fungi. The fungi, in return, receive carbon from the trees.

A basic understanding of these interactions is important to understand how the stand reacts to changes in C and N cycling. The knowledge is important when deciding which sites that should be fertilized, how they should be fertilized, and how the stand reacts on changes in nitrogen deposition and a warmer climate. It is also important for understanding the effects of nitrogen on biodiversity and eutrophication of water.

Why can't the trees utilize all nitrogen?

The activities in Theme 1 are based on the established research on nitrogen uptake and the role of mycorrhiza that is performed at Umeå University and SLU. It is well known that nitrogen is a limiting factor for tree growth in boreal ecosystems. Application of 1 kg of nitrogen can result in an accumulation of 30-50 kg carbon. This research also indicates that the cycles of the two elements are far more integrated than previously thought. One example is that the fluxes of C from tree to soil is restricted on fertile sites, while a greater proportion of the C is accumulated in the soil microorganisms in poor sites. The mechanisms underlying the interaction between microorganisms and trees are largely unknown.

Another example is that only a small proportion of added N will be recovered in the tree. At fertilization, about 10-20% is taken up by the tree, while the rest is stored in soil and the soil microbial community.

Theme 1 aims to answer (1) why is it that trees cannot utilize a greater proportion of the available soil nitrogen; (2) how the limited N uptake still can result in such great effects on tree growth? More specific research questions are:

• What mechanisms drive N-induced shifts in C allocation?
• What mechanisms affect tree acquisition of different N sources?
• How does microbial community structure influence N immobilisation, N mobilisation, N accumulation, and N loss from coniferous forest soils?
• How do microbial community composition and N supply rate interactively determine nitrous oxide emission from forest soils?
• How can linkages between plants and microorganisms drive N-induced vegetation change?