Jill S. Baron Holly A. Ewing Kathleen Weathers Melannie D. Hartman 2014 <p><span>Concurrent changes in climate, atmospheric nitrogen (N) deposition, and increasing levels of atmospheric carbon dioxide (CO</span>₂<span>) affect ecosystems in complex ways. The DayCent-Chem model was used to investigate the combined effects of these human-caused drivers of change over the period 1980–2075 at seven forested montane and two alpine watersheds in the United States. Net ecosystem production (NEP) increased linearly with increasing N deposition for six out of seven forested watersheds; warming directly increased NEP at only two of these sites. Warming reduced soil organic carbon storage at all sites by increasing heterotrophic respiration. At most sites, warming together with high N deposition increased nitrous oxide (N</span>₂<span>O) emissions enough to negate the greenhouse benefit of soil carbon sequestration alone, though there was a net greenhouse gas sink across nearly all sites mainly due to the effect of CO</span>₂<span> fertilization and associated sequestration by plants. Over the simulation period, an increase in atmospheric CO</span>₂<span> from 350 to 600&nbsp;ppm was the main driver of change in net ecosystem greenhouse gas sequestration at all forested sites and one of two alpine sites, but an additional increase in CO</span>₂<span> from 600 to 760&nbsp;ppm produced smaller effects. Warming either increased or decreased net greenhouse gas sequestration, depending on the site. The N contribution to net ecosystem greenhouse gas sequestration averaged across forest sites was only 5–7&nbsp;% and was negligible for the alpine. Stream nitrate (NO</span>₃⁻<span>) fluxes increased sharply with N-loading, primarily at three watersheds where initial N deposition values were high relative to terrestrial N uptake capacity. The simulated results displayed fewer synergistic responses to warming, N-loading, and CO</span>₂<span> fertilization than expected. Overall, simulations with DayCent-Chem suggest individual site characteristics and historical patterns of N deposition are important determinants of forest or alpine ecosystem responses to global change.</span></p> application/pdf 10.1007/s10533-014-9950-9 en Springer Combined global change effects on ecosystem processesin nine U.S. topographically complex areas article