Nitrogen inputs and losses in response to chronic CO<sub>2</sub> exposure in a subtropical oak woodland

Abstract

Abstract. Rising atmospheric CO 2 concentrations may alter the nitrogen (N) content of ecosystems by changing N inputs and N losses, but responses vary in field experiments, possibly because multiple mechanisms are at play. We measured N fixation and N losses in a subtropical oak woodland exposed to 11 years of elevated atmospheric CO 2 concentrations. We also explored the role of herbivory, carbon limitation, and competition for light or nutrients in shaping the response of N fixation to elevated CO 2 . Elevated CO 2 did not significantly alter gaseous N losses, but lower recovery and deeper distribution in the soil of a long-term 15 N tracer indicated that elevated CO 2 increased leaching losses. Elevated CO 2 had no effect on nonsymbiotic N fixation, and had a transient effect on symbiotic N fixation by the dominant legume. Elevated CO 2 tended to reduce soil and plant concentrations of iron, molybdenum, phosphorus, and vanadium, nutrients essential for N fixation. Competition for nutrients and herbivory likely contributed to the declining response of N fixation to elevated CO 2 . These results indicate that positive responses of N fixation to elevated CO 2 may be transient and that chronic exposure to elevated CO 2 can increase N leaching. Models that assume increased fixation or reduced N losses with elevated CO 2 may overestimate future N accumulation in the biosphere.

Variability of projected terrestrial biosphere responses to elevated levels of atmospheric CO<sub>2</sub> due to uncertainty in biological nitrogen fixation

“…Experimental field studies on BNF under eCO 2 are rare and inconclusive, presumably owing to the regulatory impacts of micronutrients and vegetation dynamics. Field experiments have found very large eCO 2 responses of BNF in fertilized grasslands (Hartwig et al, 2000;Lüscher et al, 2000), but also moderate responses that declined and became negative over time in subtropical oak woodlands (Hungate et al, 2004(Hungate et al, , 2014. Heterotrophic fixation was shown not to be affected by eCO 2 at the Duke FACE experiment (Hofmockel and Schlesinger, 2007).…”

Variability of projected terrestrial biosphere responses to elevated levels of atmospheric CO<sub>2</sub> due to uncertainty in biological nitrogen fixation

“…Experimental field studies on BNF under eCO 2 are rare and inconclusive, presumably owing to the regulatory impacts of micronutrients and vegetation dynamics. Field experiments have found very large eCO 2 responses of BNF in fertilized grasslands but also moderate responses that declined and became negative over time in subtropical oak woodlands (Hungate et al, 2004(Hungate et al, , 2014. Heterotrophic fixation was shown not to be affected by eCO 2 at the Duke FACE experiment (Hofmockel and Schlesinger, 2007).…”

Elevated CO2 promotes long‐term nitrogen accumulation only in combination with nitrogen addition

“…Effects of elevated CO 2 on N pools are equivocal, with some empirical evidence supporting models that predict ecosystem N accumulation (Iversen et al ., ) while other evidence indicates mixed (Reich & Hobbie, ) or even negative effects (Hungate et al ., ). Many CO 2 enrichment studies focus on particular pools that may not necessarily capture the trajectory of total ecosystem N (e.g.…”