氮添加对中国陆地植被地上-地下生物量分配的影响

大气氮沉降水平持续升高导致的外源氮输入增加，强烈影响了陆地生态系统的碳循环。目前，已有大量报道证实了氮沉降升高对全球陆地植被固碳的积极影响。虽然之前大部分研究将这一结果归因于光合作用增强导致的地上生物量增加，但最近的研究发现长期氮添加对植物地下根系的影响也同样重要。归纳整理了181篇公开发表的我国野外模拟氮沉降试验结果，采用整合分析(Meta-analysis)方法，定量评估了氮添加对我国陆地植被地上-地下生物量分配的影响特征和不同生态系统类型及施氮方式之间的影响差异。通过分析地上-地下生物量分配对氮添加的响应差异来探究植被碳增益对长期大气氮沉降增加的潜在响应机制。结果表明，氮添加显著增强了我国陆地植被的光合作用及碳固存，且植物碳增益在不同生态系统类型及施氮制度间有所差异。植物叶片的氮含量显著增加，使得叶片碳氮比及凋落物碳氮比显著降低，但并未显著影响细根的碳氮比。氮添加总体上显著提高了植物的净光合速率，但降低了光合利用效率。地上生物量，凋落物产量和根生物量平均分别显著增加了38%,17%和18%,总体上植物地上部分对氮添加的响应程度比地下部分更高。然而，不同生态系统类型的地上-地下生物...

Effects of nitrogen addition on plant above- and below-ground biomass allocation in terrestrial ecosystems in China

Increasing exogenous nitrogen (N) input due to the elevated atmospheric N deposition strongly affects the carbon (C) cycle of terrestrial ecosystems. Nowadays, a large number of studies have demonstrated the positive effects of increasing N deposition on globally terrestrial vegetation C storage. While most of previous studies attributed it to the increased plant above-ground biomass due to the enhanced photosynthesis, recent studies have found that the effects of long-term N addition on below-ground biomass were also important for terrestrial C sinks. This study synthesized data from 181 available published papers across the Chinese major terrestrial ecosystems, and performed a meta-analysis to quantitatively evaluate the impact of the elevated N inputs on the biomass allocation of above- and below-ground in the Chinses terrestrial vegetation, as well as the differences between ecosystem types and fertilization regimes. We analyzed the different responses of plant biomass allocation to N enrichment to investigate the potential mechanisms of vegetation C gain response to chronically elevated atmospheric N deposition. Our results showed that N addition significantly enhanced the net photosynthetic rate and C storage in the Chinese terrestrial vegetation, and the responses of plant biomass allocation to N addition differed considerably between the different ecosystem types or fertilization regimes. The N concentration in the leaves of terrestrial plants significantly increased, and hence significantly reduced the C/N ratios in the leaves and litter, but did not significantly affect the C/N ratio in fine roots. N addition significantly increased plant net photosynthetic rate, but decreased photosynthetic nitrogen-use efficiency. On average, the above-ground biomass, litter mass, as well as root biomass increased significantly by 38%, 17%, and 18%, respectively. The magnitude of response of shoot to N addition was higher than that of the root in general. However, the responses of above- and below-ground biomass allocation to N addition were inconsistent in different ecosystem types. We found that the N addition significantly increased above-ground biomass but non-significantly increased root biomass in N-limited temperate forests and grasslands. Alternatively, N addition significantly increased root biomass rather than above-ground biomass in N-rich subtropical forests in China. The results of regression analysis showed that above-ground biomass did not increase linearly with the increase of root biomass, the response ratio of above-ground biomass firstly rising and then falling with the response ratio of root biomass. In addition, the net photosynthetic rate and photosynthetic nitrogen-use efficiency of plants also firstly increased and then decreased with the increase of leaf N concentration. It implies that the plant C partitioning strategies under the increased atmospheric N deposition are evolutionary development. With the continuous exogenous N inputs, plants would invest more C to promote root growth to acquire other resources, instead of preferentially investing more C to shoot to promote the increase of above-ground biomass. In summary, the nonlinear relationship between plant above- and below-ground biomass may affect predictions of C gain in terrestrial ecosystems, and future studies should focus on the changes in plant C partitioning strategy under long-term elevated atmospheric N deposition.