Potential impacts of climate change on nitrogen transformations and greenhouse gas fluxes in forests: a soil transfer study

Relatively little research has been conducted on how climate change may affect the structure and function of arid to semiarid ecosystems of the American Southwest. Along the slopes of the San Francisco Peaks of Arizona, USA, I transferred intact soil cores from a spruce-fir to a ponderosa pine forest 730 m lower in elevation to assess the potential impacts of climate change on soil N cycling and trace gas fluxes. The low-elevation site has a mean annual soil temperature about 2.5°C higher than the high-elevation site. Net rates of N transformations and trace gas fluxes were measured in high-elevation soil cores incubated in situ and soil cores transferred to the low-elevation site. Over a 13-month period, volumetric soil water content was similar in transferred soil cores relative to soil cores incubated in situ. Net N mineralization and nitrification increased over 80% in transferred soil cores compared with in situ soil cores. Soil transfer significantly increased net CO₂ efflux (120%) and net CH₄ consumption (90%) relative to fluxes of these gases from soil cores incubated in situ. Soil net N₂O fluxes were relatively low and were not generally altered by soil transfer. Although the soil microbial biomass as a whole decreased in transferred soil cores compared with in situ soil cores after the incubation period, active bacterial biomass increased. Transferring soil cores from the low-elevation to the high-elevation site (i.e. simulated global cooling) commonly, but not consistently, resulted in the opposite effects on soil pools and processes. In general, soil containment (root trenching) did not significantly affect soil measurements. My results suggest that small increases in mean annual temperature can have large impacts on soil N cycling, soil-atmosphere trace gas exchanges, and soil microbial communities even in ecosystems where water availability is a major limiting resource.