Abstract
AbstractNitrogen (N) supply often limits the productivity of temperate forests and is regulated by a complex mix of biological and climatic drivers. In excess, N is linked to a variety of soil, water, and air pollution issues. Here, we use results from an elevation gradient study and historical data from the long‐term Hubbard Brook Ecosystem Study (New Hampshire,USA) to examine relationships between changes in climate, especially during winter, and N supply to northern hardwood forest ecosystems. Low elevation plots with less snow, more soil freezing, and more freeze/thaw cycles supported lower rates of N mineralization than high elevation plots, despite having higher soil temperatures and no consistent differences in soil moisture during the growing season. These results are consistent with historical analyses showing decreases in rates of soil N mineralization and inorganic N concentrations since 1973 that are correlated with long‐term increases in mean annual temperature, decreases in annual snow accumulation, and a increases in the number of winter thawing degree days. This evidence suggests that changing climate may be driving decreases in the availability of a key nutrient in northern hardwood forests, which could decrease ecosystem production but have positive effects on environmental consequences of excess N.
Highlights
Nitrogen (N) is an essential nutrient for all biota
We hypothesized that: (1) N mineralization and nitrification rates are slower in warmer, drier low elevation plots than in colder, wetter high elevation plots; (2) N mineralization rates have decreased over the past four decades along with climate change-d riven variations in both dormant and growing season conditions; and (3) these decreases are linked to the historical, unexplained decrease in nitrate export in northeastern U.S forests in recent decades
The elevation gradient at the Hubbard Brook Experimental Forest (HBEF) was effective in providing a wide range of climatic conditions
Summary
Nitrogen (N) is an essential nutrient for all biota. In most temperate forest ecosystems, N is often a limiting nutrient that constrains primary production (Vitousek and Howarth 1991, LeBauer and Treseder 2008). In recent decades there have been remarkable, unexplained declines in nitrate concentrations and export from these forests (Goodale et al 2003, Bernhardt et al 2005, Bernal et al 2012) Proposed explanations for these declines include recovery from past disturbances, decreases in atmospheric N deposition, increased losses of dissolved organic N, a longer growing season, increases in the N content of mineral soil, increased plant uptake or denitrification, and changes in hydrologic flow paths (Bernhardt et al 2005, Bernal et al 2012, Yanai et al 2013, Morse et al 2015). We exploited a natural elevation gradient that has little variation in soils or vegetation in a northern hardwood forest at the HBEF, but encompasses the variation in temperature that is projected to occur with climate change over the 50–100 yr (Hayhoe et al 2007) With this approach we evaluated the expected long-term effects of climate variation on soil N pools and transformation rates in northern hardwood forests. We hypothesized that: (1) N mineralization and nitrification rates are slower in warmer, drier low elevation plots than in colder, wetter high elevation plots; (2) N mineralization rates have decreased over the past four decades along with climate change-d riven variations in both dormant (i.e., winter) and growing (i.e., summer) season conditions; and (3) these decreases are linked to the historical, unexplained decrease in nitrate export in northeastern U.S forests in recent decades
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