Norby Richard J, Iversen Colleen M
Environmental Sciences Division, Oak Ridge National Laboratory, Tennessee 37831-6422, USA.
Ecology. 2006 Jan;87(1):5-14. doi: 10.1890/04-1950.
The Progressive Nitrogen Limitation (PNL) hypothesis suggests that ecosystems in a CO2-enriched atmosphere will sequester C and N in long-lived biomass and soil organic pools, thereby limiting available N and constraining the continued response of net primary productivity to elevated [CO2]. Here, we present a six-year record of N dynamics of a sweetgum (Liquidambar styraciflua) stand exposed to elevated [CO2] in the free-air CO2 enrichment (FACE) experiment at Oak Ridge, Tennessee, USA. We also evaluate the concept of PNL for this ecosystem from the perspective of N uptake, content, distribution, and turnover, and N-use efficiency. Leaf N content was 11% lower on a leaf mass basis (NM) and 7% lower on a leaf area basis (NA) in CO2-enriched trees. However, there was no effect of [CO2] on total canopy N content. Resorption of N during senescence was not altered by [CO2], so NM of litter, but not total N content, was reduced. The NM of fine roots was not affected, but the total amount of N required for fine-root production increased significantly, reflecting the large stimulation of fine-root production in this stand. Hence, total N requirement of the trees was higher in elevated [CO2], and the increased requirement was met through an increase in N uptake rather than increased retranslocation of stored reserves. Increased N uptake was correlated with increased net primary productivity (NPP). N-use efficiency, however, did not change with CO2 enrichment because increased N productivity was offset by lower mean residence time of N in the trees. None of the measured responses of plant N dynamics in this ecosystem indicated the occurrence of PNL, and the stimulation of NPP by elevated [CO2] was sustained for the first six years of the experiment. Although there are some indications of developing changes in the N economy, the N supply in the soil at this site may be sufficient to meet an increasing demand for available N, especially as the roots of CO2-enriched trees explore deeper in the soil profile.
渐进性氮限制（PNL）假说表明，处于二氧化碳浓度升高环境中的生态系统会将碳和氮固存于长寿生物量和土壤有机库中，从而限制有效氮，并抑制净初级生产力对升高的[CO₂]的持续响应。在此，我们展示了在美国田纳西州橡树岭进行的自由空气二氧化碳富集（FACE）实验中，一片暴露于升高的[CO₂]环境下的胶皮糖香树（Liquidambar styraciflua）林分的六年氮动态记录。我们还从氮吸收、含量、分布、周转以及氮利用效率的角度，评估了该生态系统的PNL概念。在以叶质量为基础（NM）时，二氧化碳浓度升高的树木叶片氮含量低11%，以叶面积为基础（NA）时低7%。然而，[CO₂]对冠层总氮含量没有影响。衰老期间氮素的再吸收不受[CO₂]影响，因此凋落物的NM降低，但总氮含量未变。细根的NM不受影响，但细根生产所需的总氮量显著增加，这反映了该林分中细根生产受到的巨大刺激。因此，二氧化碳浓度升高时树木的总氮需求更高，增加的需求是通过增加氮吸收来满足的，而不是通过增加储存养分的再转运。氮吸收增加与净初级生产力（NPP）增加相关。然而，氮利用效率并未随二氧化碳富集而改变，因为氮生产力的增加被氮在树木中的平均停留时间缩短所抵消。该生态系统中植物氮动态的所有测量响应均未表明发生了PNL，并且在实验的前六年中，升高的[CO₂]对NPP的刺激持续存在。尽管有一些迹象表明氮经济在发生变化，但该地点土壤中的氮供应可能足以满足对有效氮不断增加的需求，特别是随着二氧化碳浓度升高的树木根系在土壤剖面中更深地探索。
