Background and aims

The amount of nitrogen (N) derived from symbiotic N₂ fixation by legumes in grasslands might be affected by anthropogenic N and phosphorus (P) inputs, but the underlying mechanisms are not known.

Methods

We evaluated symbiotic N₂ fixation in 17 natural and semi-natural grasslands on four continents that are subjected to the same full-factorial N and P addition experiment, using the ¹⁵N natural abundance method.

Results

N as well as combined N and P (NP) addition reduced aboveground legume biomass by 65% and 45%, respectively, compared to the control, whereas P addition had no significant impact. Addition of N and/or P had no significant effect on the symbiotic N₂ fixation per unit legume biomass. In consequence, the amount of N fixed annually per grassland area was less than half in the N addition treatments compared to control and P addition, irrespective of whether the dominant legumes were annuals or perennials.

Conclusion

Our results reveal that N addition mainly impacts symbiotic N₂ fixation via reduced biomass of legumes rather than changes in N₂ fixation per unit legume biomass. The results show that soil N enrichment by anthropogenic activities significantly reduces N₂ fixation in grasslands, and these effects cannot be reversed by additional P amendment.

中文翻译：

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氮而不是磷的添加影响位于四大洲的天然和半天然草原中豆科植物的共生 N2 固定

背景和目标

草地中豆科植物共生固氮产生的氮（N）量_可能受到人为输入的氮和磷（P）的影响，但其潜在机制尚不清楚。

方法

^{我们使用15} N 自然丰度法评估了四大洲 17 个天然和半天然草原的共生 N _{2固定，这些草原进行了相同的全因子 N 和 P 添加实验。}

结果

与对照相比，N 以及 N 和 P (NP) 联合添加分别使地上豆科植物生物量减少了 65% 和 45%，而 P 添加没有显着影响。N和/或P的添加对每单位豆科植物生物量的共生N ₂固定没有显着影响。因此，无论优势豆科植物是一年生植物还是多年生植物，与对照和P添加相比，N添加处理中每个草地每年固定的N量不到一半。

结论

我们的研究结果表明，N 添加主要通过减少豆科植物的生物量而不是改变每单位豆科植物生物量的 N _{2固定来影响共生的 N 2固定。}结果表明，人为活动引起的土壤N富集显着降低了草地的N ₂固定，并且这些影响不能通过额外的P修正来逆转。