How delayed soil warming affects nitrogen fixation, plant performance and growth in the nitrogen fixing shrub green alder (Alnus alnobetula subsp. crispa (Aiton) Raus)
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Date
2019-12-18
Authors
Anderson, Paige
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Abstract
The short growing season and cold climate of the Canadian boreal forest can result in restricted amounts of available soil nitrogen, limited plant growth and low ecosystem productivity. Nitrogen fixing plants, which form symbiotic relationships with specialised bacteria, should have an advantage in low nitrogen environments, yet, their abundance in the boreal forest is low. Cool temperatures limit nitrogen fixation activity. How nitrogen fixation is affected when temperature differences occur between the soil and air, especially in the spring when soil temperatures remain cool, have not been well documented. To investigate this, two studies were conducted on the nitrogen fixing shrub Alnus alnobetula subsp. crispa (Aiton) Raus. In 2017, nitrogen fixation was monitored in both open and forested sites in the southern boreal forest. Nitrogen fixation activity began in mid June, was highest in early August at a soil temperature of 20˚C, and ceased in early October. The difference between soil and air temperature was largest in the spring. Soil temperature did not have an effect on nitrogen fixation until after bud burst and leaf development. In the open site, soil temperature warmed at an earlier date compared to the forested site. However there was no difference in nitrogen fixation rate between the sites. Soil moisture had no effect on nitrogen fixation. To examine the effects of extended delays in soil warming, past leaf development, a lab study was performed. For a period of thirteen weeks, soil was cooled to 10˚C, 14˚C and 16˚C, independently of shoot temperature (20˚C). On average, soils at 14˚C and 10˚C inhibited nitrogen fixation (by 29% and 62%, respectively) and photosynthesis (by 43% and 39%). Reductions in photosynthetic rate were mainly attributed to a reduction in nitrogen supply and the formation of new chlorophyll pigment. Although this effect was not immediate, suggesting some utilization of stored nitrogen. Reduced amounts of fixed nitrogen and photosynthates resulted in lower growth and biomass production when exposed to low soil temperatures. The advantages of nitrogen fixation may be constrained by soil temperature in cool environments, restricting the abundance of nitrogen fixing species in the boreal forest.
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Soil temperature, Nitrogen fixation, Frankia, Alder