Climate models project higher growing season air temperatures and decreased depth and duration of winter snowpack for the northeastern United States over the next century. Together, these climatic changes will warm soils in the growing season and induce greater frequency of soil freeze/thaw cycles (FTCs) in winter. Past research indicates that soil warming increases nitrogen (N) mineralization rates and N uptake by plants, while soil FTCs increase root injury and mortality, leading to reduced plant N uptake. Therefore, changes in climate could alter plant foliar chemistry, plant photosynthetic capacity, and forest carbon storage. However, it remains unknown how the combined effects of warmer growing season soil temperatures and winter soil FTCs affect N cycling and foliar N in northern hardwood forests.
We created the Climate Change Across Seasons Experiment (CCASE) at Hubbard Brook Experimental Forest in Woodstock, NH in 2012. Two plots serve as reference, two are warmed five degrees Celsius above ambient temperatures during the growing season, and two are both warmed in the growing season and have soils FTCs induced in winter via snow removal. We measured foliar N of sun-lit foliage in 2014, 2015 and 2017 and in situ N mineralization throughout the 2017 growing season.
Results/Conclusions
Heating cables warmed treatment plots 5 degrees Celsius above ambient temperatures during 2015, 2016, and 2017 growing seasons. We induced four successful soil FTCs in the 2014/2015 and two in 2015/2016 and 2016/2017 winters via snow removal and soil warming with heating cables. Decreased snow depth and increased frequency of soil FTCs increased rates of N mineralization in the peak and late growing season in 2017. Soil warming in the growing season increased rates of in situ N mineralization in plots that experienced both reference winter conditions and increased soil FTCs. Finally, soil FTCs combined with growing season warming increased foliar N relative to reference and warming alone. We conclude that the combined effects of warmer growing season temperatures and greater frequency of soil FTCs for the northeastern U.S. will influence forest biogeochemical cycling and should be considered when projecting the ability of these forests to retain soil nutrients and carbon in the future.