Smithsonian Environmental Research Center, Edgewater, Maryland 21037, USA.
Nature. 2010 Jul 1;466(7302):96-9. doi: 10.1038/nature09176.
Terrestrial ecosystems gain carbon through photosynthesis and lose it mostly in the form of carbon dioxide (CO(2)). The extent to which the biosphere can act as a buffer against rising atmospheric CO(2) concentration in global climate change projections remains uncertain at the present stage. Biogeochemical theory predicts that soil nitrogen (N) scarcity may limit natural ecosystem response to elevated CO(2) concentration, diminishing the CO(2)-fertilization effect on terrestrial plant productivity in unmanaged ecosystems. Recent models have incorporated such carbon-nitrogen interactions and suggest that anthropogenic N sources could help sustain the future CO(2)-fertilization effect. However, conclusive demonstration that added N enhances plant productivity in response to CO(2)-fertilization in natural ecosystems remains elusive. Here we manipulated atmospheric CO(2) concentration and soil N availability in a herbaceous brackish wetland where plant community composition is dominated by a C(3) sedge and C(4) grasses, and is capable of responding rapidly to environmental change. We found that N addition enhanced the CO(2)-stimulation of plant productivity in the first year of a multi-year experiment, indicating N-limitation of the CO(2) response. But we also found that N addition strongly promotes the encroachment of C(4) plant species that respond less strongly to elevated CO(2) concentrations. Overall, we found that the observed shift in the plant community composition ultimately suppresses the CO(2)-stimulation of plant productivity by the third and fourth years. Although extensive research has shown that global change factors such as elevated CO(2) concentrations and N pollution affect plant species differently and that they may drive plant community changes, we demonstrate that plant community shifts can act as a feedback effect that alters the whole ecosystem response to elevated CO(2) concentrations. Moreover, we suggest that trade-offs between the abilities of plant taxa to respond positively to different perturbations may constrain natural ecosystem response to global change.
陆地生态系统通过光合作用吸收碳，而通过二氧化碳（CO2）的形式失去碳。在当前阶段，生物圈在全球气候变化预测中作为对抗大气中 CO2 浓度升高的缓冲能力的程度仍不确定。生物地球化学理论预测，土壤氮（N）匮乏可能限制自然生态系统对升高的 CO2 浓度的响应，从而减少未受管理的生态系统中陆地植物生产力对 CO2 施肥的影响。最近的模型已经纳入了这种碳氮相互作用，并表明人为氮源可能有助于维持未来的 CO2 施肥效应。然而，确凿的证据表明，添加的 N 会增强自然生态系统中植物对 CO2 施肥的生产力，这一点仍然难以捉摸。在这里，我们在一个草本咸湿地带操纵大气 CO2 浓度和土壤 N 有效性，该湿地的植物群落组成主要由 C3 莎草和 C4 草组成，并且能够快速响应环境变化。我们发现，在多年实验的第一年，N 添加增强了植物生产力对 CO2 的刺激，表明 CO2 响应受到 N 的限制。但我们也发现，N 添加强烈促进了对升高的 CO2 浓度反应较弱的 C4 植物物种的入侵。总的来说，我们发现观察到的植物群落组成的变化最终在第三和第四年抑制了植物生产力对 CO2 的刺激。尽管广泛的研究表明，全球变化因素，如升高的 CO2 浓度和 N 污染，以不同的方式影响植物物种，并且它们可能驱动植物群落变化，但我们证明植物群落的变化可以作为一种反馈效应，改变整个生态系统对升高的 CO2 浓度的响应。此外，我们认为，植物分类群对不同干扰因素做出积极响应的能力之间的权衡可能会限制自然生态系统对全球变化的响应。
