Growing season soil warming may counteract trend of nitrogen oligotrophication in a northern hardwood forest
2020
Harrison, Jamie L. | Schultz, Kyle | Blagden, Megan | Sanders-DeMott, Rebecca | Templer, Pamela H.
Over the next century, many mid and high latitude temperate ecosystems are projected to experience rising growing season temperatures and increased frequency of soil freeze/thaw cycles (FTCs) due to a reduction in the depth and duration of the winter snowpack. We conducted a manipulative field experiment in a northern hardwood forest at the Hubbard Brook Experimental Forest in New Hampshire to determine the interactive effects of climate change across seasons on rates of net N mineralization, foliar N, and natural abundance foliar ¹⁵N (δ¹⁵N) in red maple (Acer rubrum) trees. We warmed soils 5 °C above ambient temperatures and induced winter FTCs to simulate projected changes over the next century. Net N mineralization was dominated by ammonification and increased with warmer soil temperatures, but was not affected by soil FTCs in the previous winter. Similarly, warming led to increased foliar N concentrations and δ¹⁵N, with no effect of soil FTCs. Together, our results show that growing season soil warming increases soil N availability and N uptake by trees, which may offset the previously observed negative effects of a smaller snowpack and more frequent soil freezing on N cycling. We conclude that soil warming in the growing season may counteract the trend of reduced soil N availability relative to plant N demand (i.e. N oligotrophication) observed in northern hardwood forests. This research demonstrates that climate change across seasons affects N cycling in northern hardwood forests in ways that would have not been apparent from examining one season alone.
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