Department of Earth System Science, University of California Irvine, Irvine, CA, USA.
Department of Ecology and Evolutionary Biology, University of California Irvine, Irvine, CA, USA.
Nature. 2019 Feb;566(7743):205-211. doi: 10.1038/s41586-019-0911-2. Epub 2019 Feb 13.
Uncertainty in the global patterns of marine nitrogen fixation limits our understanding of the response of the ocean's nitrogen and carbon cycles to environmental change. The geographical distribution of and ecological controls on nitrogen fixation are difficult to constrain with limited in situ measurements. Here we present convergent estimates of nitrogen fixation from an inverse biogeochemical and a prognostic ocean model. Our results demonstrate strong spatial variability in the nitrogen-to-phosphorus ratio of exported organic matter that greatly increases the global nitrogen-fixation rate (because phytoplankton manage with less phosphorus when it is in short supply). We find that the input of newly fixed nitrogen from microbial fixation and external inputs (atmospheric deposition and river fluxes) accounts for up to 50 per cent of carbon export in subtropical gyres. We also find that nitrogen fixation and denitrification are spatially decoupled but that nevertheless nitrogen sources and sinks appear to be balanced over the past few decades. Moreover, we propose a role for top-down zooplankton grazing control in shaping the global patterns of nitrogen fixation. Our findings suggest that biological carbon export in the ocean is higher than expected and that stabilizing nitrogen-cycle feedbacks are weaker than previously thought.
海洋固氮的全球模式存在不确定性，这限制了我们对海洋氮和碳循环对环境变化的响应的理解。由于原位测量的局限性，固氮的地理分布和生态控制因素难以得到有效限制。本研究通过反演生物地球化学模型和预测性海洋模型，得出了固氮的收敛估计值。研究结果表明，输出有机物质的氮磷比具有很强的空间变异性，这极大地增加了全球固氮速率（因为当磷供应不足时，浮游植物可以用更少的磷来维持生长）。研究发现，微生物固定和外部输入（大气沉降和河流通量）新固定氮的输入，占亚热带环流中碳输出的 50%之多。研究还发现，固氮和反硝化在空间上是解耦的，但在过去几十年中，氮的来源和汇似乎达到了平衡。此外，研究还提出了浮游动物摄食控制在塑造全球固氮模式中的作用。研究结果表明，海洋生物碳输出高于预期，氮循环反馈的稳定性比之前认为的要弱。
