Frost damage and winter nitrogen uptake by the grass Poa pratensis: Consequences for vegetative versus reproductive growth

Background/Question/Methods

Frost damage can decrease nitrogen uptake by grasses over the short term, and can also decrease grass biomass production over the following growing season.  However, to what extent is reduced nitrogen uptake over winter merely a symptom of root damage, as opposed to being of functional significance to grass production?  The answer depends on the broader question of whether nitrogen uptake by grasses over winter is of any consequence to summer grass production.  We examined the growth response of the grass Poa pratensis (Kentucky blue grass) to variation in the timing of freezing and nitrogen availability over winter.  Tillers were exposed to severe freezing events (-10 degrees C) in either early or late winter, or left as controls, and tillers from each temperature treatment were grown either in the presence or absence of supplementary nitrogen.  All tillers were transplanted into untreated soil in early spring, and at peak seed maturation, root, shoot and reproductive biomass were measured.

Results/Conclusions

Overall, both enhanced freezing in late winter and increased winter nitrogen availability had significant effects on grass production.  However, there was a significant interaction between enhanced freezing and nitrogen addition, whereby tillers frozen in early or late winter did not respond significantly to increased nitrogen availability.  Enhanced freezing and winter nitrogen availability also had differing effects on vegetative versus reproductive growth.  Tillers frozen in early and late spring produced very few inflorescences, as compared will tillers grown under ambient temperature.  Although increased winter nitrogen availability did not affect final root or shoot production, increased nitrogen availability increased inflorescence production in ambient temperature tillers.  Late winter freezing was the only factor that significantly reduced final vegetative biomass production.  Our results, while complicated by variation in effects on vegetative versus reproductive growth, support the hypothesis that the disruption of nitrogen uptake over winter as a result of severe freezing is of functional significance in P. pratensis.  These results have important implications for grass responses to climate change induced changes in soil freezing dynamics.