Department of Biology, Villanova University, 800 Lancaster Avenue, Villanova, PA, 19085, USA.
Smithsonian Environmental Research Center, 647 Contees Wharf Road, Edgewater, MD, 21037, USA.
Glob Chang Biol. 2016 Jan;22(1):391-403. doi: 10.1111/gcb.13112. Epub 2015 Nov 18.
Biogeochemical models that incorporate nitrogen (N) limitation indicate that N availability will control the magnitude of ecosystem carbon uptake in response to rising CO2 . Some models, however, suggest that elevated CO2 may promote ecosystem N accumulation, a feedback that in the long term could circumvent N limitation of the CO2 response while mitigating N pollution. We tested this prediction using a nine-year CO2 xN experiment in a tidal marsh. Although the effects of CO2 are similar between uplands and wetlands in many respects, this experiment offers a greater likelihood of detecting CO2 effects on N retention on a decadal timescale because tidal marshes have a relatively open N cycle and can accrue soil organic matter rapidly. To determine how elevated CO2 affects N dynamics, we assessed the three primary fates of N in a tidal marsh: (1) retention in plants and soil, (2) denitrification to the atmosphere, and (3) tidal export. We assessed changes in N pools and tracked the fate of a (15) N tracer added to each plot in 2006 to quantify the fraction of added N retained in vegetation and soil, and to estimate lateral N movement. Elevated CO2 alone did not increase plant N mass, soil N mass, or (15) N label retention. Unexpectedly, CO2 and N interacted such that the combined N+CO2 treatment increased ecosystem N accumulation despite the stimulation in N losses indicated by reduced (15) N label retention. These findings suggest that in N-limited ecosystems, elevated CO2 is unlikely to increase long-term N accumulation and circumvent progressive N limitation without additional N inputs, which may relieve plant-microbe competition and allow for increased plant N uptake.
纳入氮（N）限制的生物地球化学模型表明，N 供应将控制生态系统碳吸收对不断上升的 CO2 的响应幅度。然而，一些模型表明，升高的 CO2 可能会促进生态系统 N 积累，这种反馈在长期内可能会规避 CO2 响应中的 N 限制，同时减轻 N 污染。我们使用潮汐沼泽中的九年 CO2 xN 实验来检验这一预测。尽管在许多方面，CO2 的影响在旱地和湿地之间相似，但由于潮汐沼泽具有相对开放的 N 循环并且可以迅速积累土壤有机质，因此该实验更有可能检测到 CO2 对 N 保留的影响在十年时间尺度上。为了确定升高的 CO2 如何影响 N 动态，我们评估了潮汐沼泽中 N 的三个主要命运：（1）保留在植物和土壤中，（2）反硝化到大气中，以及（3）潮汐出口。我们评估了 N 库的变化，并跟踪了 2006 年添加到每个地块的（15）N 示踪剂的命运，以量化添加的 N 在植被和土壤中的保留分数，并估计侧向 N 移动。单独升高的 CO2 不会增加植物 N 质量、土壤 N 质量或（15）N 标记保留。出乎意料的是，CO2 和 N 相互作用，使得尽管减少（15）N 标记保留表明 N 损失增加，但 N+CO2 处理增加了生态系统的 N 积累。这些发现表明，在 N 限制的生态系统中，升高的 CO2 不太可能在没有额外 N 输入的情况下增加长期 N 积累并规避逐渐的 N 限制，这可能会减轻植物-微生物竞争并允许增加植物 N 吸收。
