Growing season warming and winter freeze-thaw cycles reduce root nitrogen uptake capacity and increase soil solution nitrogen in a northern forest ecosystem

Northern forest ecosystems are projected to experience warmer growing seasons and increased soil freeze-thaw cycles in winter over the next century. Past studies show that warmer soils in the growing season enhance nitrogen uptake by plants, while soil freezing in winter reduces plant uptake and ecosystem retention of nitrogen, yet the combined effects of these changes on plant root capacity to take up nitrogen are unknown. We conducted a 2-year (2014-2015) experiment at Hubbard Brook Experimental Forest in New Hampshire, USA to characterize the response of root damage, nitrogen uptake capacity, and soil solution nitrogen to growing season warming combined with soil freeze-thaw cycles in winter. Winter freeze-thaw cycles damaged roots, reduced nitrogen uptake capacity by 42%, and increased soil solution ammonium in the early growing season (May-June). During the peak growing season (July), root nitrogen uptake capacity was reduced 40% by warming alone and 49% by warming combined with freeze-thaw cycles. These results indicate the projected combination of colder soils in winter and warmer soils in the snow-free season will alter root function by reducing root nitrogen uptake capacity and lead to transient increases of nitrogen in soil solution during the early growing season, with the potential to alter root competition for soil nitrogen and seasonal patterns of soil nitrogen availability. We conclude that considering interactive effects of changes in climate during winter and the snow-free season is essential for accurate determination of the response of nitrogen cycling in the northern hardwood forest to climate change.