| Rising atmospheric CO2 and global warming could alter the functioning of terrestrial ecosystems. A number of experiments have already been established to document these potential changes. These experiments have demonstrated some broad similarities among different ecosystems in their aboveground responses to these global changes, but whether biogeochemical responses belowground exhibit predictable patterns is largely unknown. For example, elevated CO2 and warming can alter nitrogen availability to plants and nitrogen inputs to and losses from ecosystems, but results to date are equivocal, with empirical support for both increases and decreases in nitrogen availability. The proposed work will examine how warming alters nitrogen cycling in grass-dominated vegetations along a 3000-m elevation gradient near Northern Arizona University, and how these changes will feed back to affect plant and ecosystem productivity. The field experiments will document changes in N cycling in response to increased CO2 using a long-term technique that will reveal time-integrated effects of these global changes on N cycling.
The elevation gradient spans 3000 m, and represents a broad range of climatic conditions (from 0-15°C mean annual temperature and 100- 1000 mm annual precipitation). This study of five life zones along the gradient in which grass-dominated interspaces occur, provides a range of climate conditions from 3 to 15 °C mean annual temperature and 100 to 700 mm mean annual precipitation, and allows a comparison among ecosystems where the major limitation to net primary production shifts from precipitation to temperature. This experiment will involve transplanting micro-ecosystems consisting of grasses and soil from these life zones down the gradient to simulate changes in temperature expected with doubled atmospheric CO2 concentrations (Table 1). Because of the central role of water in modulating changes in N cycling in response to warming, and because of uncertainty in changes in precipitation that will accompany warming, the transplants will be combined with watering treatments to simulate the range of possible changes in precipitation.
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From each site, grass/soil monoliths will be removed intact and placed in 30 cm diameter x 40 cm deep plastic cylinders, equipped with leachate collection ports, for transportation by pickup truck to the transplant site. Watering treatments will be applied manually to track weather. We will measure changes in N cycling in each of the experimental microcosms, measuring N content and composition in leaves, stems, roots, and soil fractions, and measuring gaseous and leaching losses of N. |
