Sub-lethal effects of freezing on growth and nitrogen uptake in Poa pratensis

Background/Question/Methods

Climate change driven increases in winter temperatures may decrease snow cover and increase the frequency and intensity of soil freeze thaw cycles (FTC) in temperate regions. Reduced plant cold hardiness in late fall and early spring may also make plants more vulnerable to cold spells. FTC can alter soil nitrogen (N) dynamics and cause root damage, affecting the ability of plants to take up N, and leading to changes in productivity during the following growing season. Therefore, the ecological consequences of changes in soil freezing dynamics need to be studied to understand climate change impacts in temperate ecosystems. To test if FTC inhibit plant N uptake by causing sub-lethal affects on roots, we collected P. pratensis tillers in late fall, mid winter, and early spring of 2009 and exposed them to FTC of varying minimum freezing temperatures, rates and lengths in a refrigerated circulator. We then immersed the roots in an aerated solution of 100 µM ¹⁵N for 20 minutes to measure N uptake ability. To assess the direct affects of FTC on plant growth, we planted P. pratensis tillers in untreated soil and grew them in the field following exposure to FTC of varying minimum temperatures and lengths in late fall, mid winter, and early spring. We recorded above and below ground biomass at the end of July. 

Results/Conclusions

P. pratensis root ¹⁵N uptake declined significantly following freezing below -5 °C in late fall and early spring.  Furthermore, longer soil freezing (3 days) at -5 °C significantly decreased root ¹⁵N uptake in the fall and in the spring seasons compared to shorter freezing (1 day) at -5 °C. Freezing rate and season had no significant effects on ¹⁵N uptake. Exposure of P. pratensis to freezing in winter and spring at -15°C and -5 °C  for 5 and 3 days respectively resulted in significantly reduced above ground plant biomass in the summer, with spring FTC hindering plant productivity the most. Our results suggest that despite increased plant growth in response to climate warming over summer, increased soil freezing over winter and reduced fall and spring plant cold hardiness may hinder plant N acquisition and growth.