Environmental Sciences Division , Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA.
Proc Natl Acad Sci U S A. 2010 Nov 9;107(45):19368-73. doi: 10.1073/pnas.1006463107. Epub 2010 Oct 25.
Stimulation of terrestrial plant production by rising CO(2) concentration is projected to reduce the airborne fraction of anthropogenic CO(2) emissions. Coupled climate-carbon cycle models are sensitive to this negative feedback on atmospheric CO(2), but model projections are uncertain because of the expectation that feedbacks through the nitrogen (N) cycle will reduce this so-called CO(2) fertilization effect. We assessed whether N limitation caused a reduced stimulation of net primary productivity (NPP) by elevated atmospheric CO(2) concentration over 11 y in a free-air CO(2) enrichment (FACE) experiment in a deciduous Liquidambar styraciflua (sweetgum) forest stand in Tennessee. During the first 6 y of the experiment, NPP was significantly enhanced in forest plots exposed to 550 ppm CO(2) compared with NPP in plots in current ambient CO(2), and this was a consistent and sustained response. However, the enhancement of NPP under elevated CO(2) declined from 24% in 2001-2003 to 9% in 2008. Global analyses that assume a sustained CO(2) fertilization effect are no longer supported by this FACE experiment. N budget analysis supports the premise that N availability was limiting to tree growth and declining over time--an expected consequence of stand development, which was exacerbated by elevated CO(2). Leaf- and stand-level observations provide mechanistic evidence that declining N availability constrained the tree response to elevated CO(2); these observations are consistent with stand-level model projections. This FACE experiment provides strong rationale and process understanding for incorporating N limitation and N feedback effects in ecosystem and global models used in climate change assessments.
大气中 CO2 浓度升高刺激陆地植物生长，预计会减少人为 CO2 排放的气载部分。耦合气候-碳循环模型对大气 CO2 这种负反馈非常敏感，但模型预测存在不确定性，因为预计氮（N）循环的反馈会降低这种所谓的 CO2 施肥效应。我们评估了在田纳西州一个落叶枫香（Liquidambar styraciflua）林分的自由空气 CO2 富集（FACE）实验中，11 年间 N 限制是否会降低大气 CO2 浓度升高对净初级生产力（NPP）的刺激作用。在实验的头 6 年，与当前大气 CO2 浓度下的 NPP 相比，暴露在 550 ppm CO2 下的森林样地的 NPP 显著增强，且这种增强是一致且持续的。然而，在 CO2 升高条件下，NPP 的增强作用从 2001-2003 年的 24%下降到 2008 年的 9%。不再有全球分析假设持续的 CO2 施肥效应。这个 FACE 实验不再支持假设持续的 CO2 施肥效应的全球分析。N 预算分析支持这样的前提，即 N 供应是限制树木生长并随时间推移而减少的，这是林分发育的预期结果，而 CO2 升高则加剧了这一结果。叶片和林分水平的观测为 N 供应减少限制树木对 CO2 升高响应的机制提供了证据，这些观测结果与林分水平模型预测一致。这个 FACE 实验为在气候变化评估中使用的生态系统和全球模型中纳入 N 限制和 N 反馈效应提供了有力的理由和过程理解。
