Understanding belowground processes in a multifactor world: Interactive effects of atmospheric carbon dioxide, temperature, and soil moisture

Rising atmospheric temperatures may alter ecosystem nutrient processes, but the effects are likely moderated by rising atmospheric [CO₂] and changes in precipitation regimes. We used an ongoing (4 years), multifactor (ambient or elevated [CO₂], ambient or elevated temperature, and moist or dry soil), experiment in a constructed oldfield plant community (including C3 and C4 grasses, forbs, and legumes) to better understand how interactions among global change factors alter soil nutrient cycling. Symbiotic N-fixation, potential soil N-mineralization, soil nutrient availability, and soil enzyme activities were measured during the 2005 and or 2006 growing season. After four years, the OCCAM (Old-field Community, Climate and Atmospheric Manipulation) project has resulted in three main results with respect to soil nutrient cycling: 1) symbiotic N₂-fixation was an important process in the N budget of the constructed old-fields and contributed 44 to 51% of the aboveground community N stocks, but there were no detectable effects of elevated [CO₂], temperature, or soil moisture on symbiotic N₂-fixation by legumes; 2) there were also no treatment effects on potential soil nitrogen mineralization after 60 days or monthly soil nutrient availability over two years; 3) there was, however, a significant interaction between temperature and moisture on total soil microbial enzyme activity. In this ecosystem, changes in species composition over time may have a larger effect on soil nutrient cycling than the response of the microbial community to changes in [CO₂], temperature, or soil water availability.