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 chlorophyll formation.
Photosynthesis was not reduced immediately, suggesting some utilization of
a nitrogen source not supplied from current fixation. Reduced amounts of
fixed nitrogen and photosynthates resulted in diminished biomass
production and relative growth rate. Conclusion: The assumed
advantages of being a nitrogen-fixing plant in a low nitrogen environment
may be constrained by soil temperature to a larger extent than previously
considered. This may restrict the abundance of nitrogen-fixing species in
the boreal forest.