Nutrient dynamics in a warmer world: Nitrogen uptake by trees

Background/Question/Methods

Climate models project a reduced winter snowpack depth and duration in the northeastern United States over the next 100 years. Reduction of the snowpack leaves soils exposed to freezing air temperatures, which can induce soil frost and reduce soil biotic activity in winter. Repeated freezing damage and subsequent heightened production of fine roots in the following growing season may deplete nutrient and carbon reserves within trees. These changes may leave fine roots more susceptible to frost damage in the following winter, which could decrease nutrient uptake by trees in future years.   In order to assess the effects of increased soil frost on nutrient uptake by trees in mixed temperate forests, a snow removal experiment was conducted at Harvard Forest, MA.   We induced soil frost by removing snow from treatment plots for the first four weeks of winter in 2010 and 2011 in three 13 X 13 m² plots of a mixed temperate deciduous forest. We measured nitrogen uptake by excised fine roots during the summer following two consecutive winters of snow removal.

Results/Conclusions

In the winter of 2010-2011 snow removal induced significantly greater depth and duration of soil freezing in the treatment plots relative to the reference plots.  However, due to low snowfall, there were no significant differences in soil freezing depth between reference and snow removal plots for the mild winter of 2011-2012.  In August 2012, fine roots from snow removal plots had greater rates of NH₄⁺ (P = 0.038) and NO₃^- uptake (P = 0.065) than fine roots from reference plots. Uptake of NH₄⁺ was significantly greater than NO₃^- in both reference (P = 0.011) and snow-removal (P = 0.003) plots. Differences in nutrient uptake by fine roots between the reference and snow-removal plots could be occurring for multiple reasons. Roots in treatments plots may have been damaged during winter #1 (2010-2011), leading to diminished uptake capacity of roots in the following growing season, which we did not measure. This may have lead to increased carbon allocation to roots in the following growing season, and as such the roots in treatment plots were not as affected by soil freezing in winter #2 (2011-2012) resulting in  greater nutrient uptake capacity. There is a possibility that roots in treatments plots were better acclimated to freezing after experiencing winter #1, and roots in reference plots were more susceptible to damage in winter #2.