Effect of soil temperature on nitrogen fixation in Alnus

Purpose: The short growing season and cold climate of the boreal forest can restrict soil nitrogen availability, limiting plant growth and ecosystem productivity. Vascular nitrogen-fixing plants should have an advantage in low nitrogen environments. Yet, their abundance in the boreal forest is low. How nitrogen fixation is affected when temperature differences occur between the soil and air, especially in the spring when soil temperatures remain cool, has not been documented in actinorhizal shrubs.

Methods: A lab study was performed on Alnus alnobetula subsp. crispa (Aiton) Raus. For 13 weeks, soil was kept at either 10˚C, 14˚C or 16˚C, independently of shoot temperature, at 20˚C.

Results: Soils at 14˚C and 10˚C inhibited whole-plant nitrogen fixation (by 53% and 68%) and photosynthesis (by 43% and 39%), respectively, compared to soils at 16˚C. Reductions in photosynthetic rate were mainly attributed to a reduction in the fixed nitrogen supply and subsequent reduction in chlorophy...