Transferring soils from high- to low-elevation forests increases nitrogen cycling rates: climate change implications

We assessed the potential impact of global warming resulting from a doublin g of preindustrial atmospheric CO2 on soil net N transformations by transfe rring intact soil cores (0-15 cm) from a high-elevation old-growth forest t o a forest about 800 m lower in elevation in the central Oregon Cascade Mou ntains, USA. The lower elevation site had mean annual air and soil (10-cm m ineral soil depth) temperatures about 2.4 and 3.9 degrees C higher than the high-elevation site, respectively. Annual rates of soil net N mineralizati on and nitrification more than doubled in soil transferred to the low-eleva tion site (17.2-36.0 kg N ha(-1) and 5.0-10.7 kg NO3--N ha(-1), respectivel y). Leaching of inorganic N from the surface soil tin the absence of plant uptake) also increased. The reciprocal treatment (transferring soil cores f rom the low- to the high-elevation site) resulted in decreases of about 70, 80, and 65% in annual rates of net N mineralization, nitrification, and in organic N leaching, respectively. Laboratory incubations of soils under con ditions of similar temperature and soil water potential suggest that the qu ality of soil organic matter is higher at the high-elevation site. Similar in situ rates of soil net N transformations between the two sites occurred because the lower temperature counteracts the effects of greater substrate quantity and quality at the high elevation site. Our results support the hy pothesis that high-elevation, old-growth forest soils in the central Cascad es have higher C and N storage than their low-elevation analogues primarily because low temperatures limit net C and N mineralization rates at higher elevations.