The nitrogen (N₂) fixation of marine diazotrophs is critical for providing bioavailable nitrogen to support the carbon export in the broad expanse of N-limited ocean. The availability of iron (Fe) and phosphorus (P) is considered as the limiting factors on the growth of diazotrophs. However, how the future warming will interact with the resource limitation of the N₂ fixation and C export is still largely unknown for the natural diazotroph communities. Here we conducted ship-board incubation experiments to investigate the effects of iron (Fe) and P amendments on the biological N₂ and carbon fixation under future warming conditions on two different diazotroph communities collected from the oligotrophic Western Pacific region. Our results reveal that the effects of both Fe addition and warming are size-dependent. Trace level (0.5 nM) Fe amendments stimulated both N₂ and C fixation, and this effect was magnified with phosphate (0.1 μM) amendments. Although warming in general alleviated the extend of Fe limitation and thus promoted C fixation of both communities, contrast responses of N₂ fixation to warming were determined by the diazotroph community structure. The N₂ fixation rate of diazotroph community dominated by unicellular diazotroph UCYN-B increased with 5ºC of warming; while warming weakened the N₂ fixation of Trichodesium dominated community in spite of nutrient amendments. The size-dependent response of N₂ fixation to Fe availability and warming further suggests the needs of considering size-fractions for the future biogeochemical projection models.

Primary Presenter: Yuanyuan Feng, Shanghai Jiao Tong University (yuanyuan.feng@sjtu.edu.cn)

Authors:

Mark Wells, University of Maine  ()

Yuanli Zhu, Second Institute of Oceanography, Ministry of Natural Resources, China  ()

Qiang Hao, Second Institute of Oceanography, Ministry of Natural Resources, China  ()

Ruifeng Zhang, Shanghai Jiao Tong University  ()

Charles Trick, University of Saskatchewan  ()

Fei Chai, Xiamen University  ()