Abstract
Anthropogenic O₃ and CO₂-induced declines in soil N availability could counteract greater plant growth in a CO₂-enriched atmosphere, thereby reducing net primary productivity (NPP) and the potential of terrestrial ecosystems to sequester anthropogenic CO₂. Presently, it is uncertain how increasing atmospheric CO₂ and O₃ will alter plant N demand and the acquisition of soil N by plants as well as the microbial supply of N from soil organic matter. To address this uncertainty, we initiated an ecosystem-level ⁱ⁵N tracer experiment at the Rhinelander (Wisconsin, USA) free air CO₂-O₃ enrichment (FACE) facility to understand how projected increases in atmospheric CO₂ and O₃ alter the distribution and flow of N in developing northern temperate forests. Tracer amounts of ⁱ⁵NH₄ were applied to the forest floor of developing Populus tremuloides and P. tremuloides—Betula papyrifera communities that have been exposed to factorial CO₂ and O₃ treatments for seven years. One year after isotope addition, both forest communities exposed to elevated CO₂ obtained greater amounts of ⁱ⁵N (29%) and N (40%) from soil, despite no change in soil N availability or plant N-use efficiency. As such, elevated CO₂ increased the ability of plants to exploit soil for N, through the development of a larger root system. Conversely, elevated O₃ decreased the amount of ⁱ⁵N (-15%) and N (-29%) in both communities, a response resulting from lower rates of photosynthesis, decreases in growth, and smaller root systems that acquired less soil N. Neither CO₂ nor O₃ altered the amount of N or ⁱ⁵N recovery in the forest floor, microbial biomass, or soil organic matter. Moreover, we observed no interaction between CO₂ and O₃ on the amount of N or ⁱ⁵N in any ecosystem pool, suggesting that O₃ could exert a negative effect regardless of CO₂ concentration. In a CO₂-enriched atmosphere, greater belowground growth and a more thorough exploitation of soil for growth-limiting N is an important mechanism sustaining the enhancement of NPP in developing forests (0—8 years following establishment). However, as CO₂ accumulates in the Earth's atmosphere, future O₃ concentrations threaten to diminish the enhancement of plant growth, decrease plant N acquisition, and lessen the storage of anthropogenic C in temperate forests.