Staccone Anika, Liao Wenying, Perakis Steven, Compton Jana, Clark Christopher, Menge Duncan
Columbia University, Ecology, Evolution, and Environmental Biology Department.
Princeton University, Department of Ecology and Evolutionary Biology.
Global Biogeochem Cycles. 2020 Feb 7;42(2). doi: 10.1029/2019GB006241.
Quantifying human impacts on the N cycle and investigating natural ecosystem N cycling depend on the magnitude of inputs from natural biological nitrogen fixation (BNF). Here, we present two bottom-up approaches to quantify tree-based symbiotic BNF based on forest inventory data across the coterminous US plus SE Alaska. For all major N-fixing tree genera, we quantify BNF inputs using (1) ecosystem N accretion rates (kg N ha yr) scaled with spatial data on tree abundance and (2) percent of N derived from fixation (%N) scaled with tree N demand (from tree growth rates and stoichiometry). We estimate that trees fix 0.30-0.88 Tg N yr across the study area (1.4-3.4 kg N ha yr). Tree-based N fixation displays distinct spatial variation that is dominated by two genera, (64% of tree-associated BNF) and (24%). The third most important genus, , accounted for 5%. Compared to published estimates of other N fluxes, tree-associated BNF accounted for 0.59 Tg N yr, similar to asymbiotic (0.37 Tg N yr) and understory symbiotic BNF (0.48 Tg N yr), while N deposition contributed 1.68 Tg N yr and rock weathering 0.37 Tg N yr. Overall, our results reveal previously uncharacterized spatial patterns in tree BNF that can inform large-scale N assessments and serve as a model for improving tree-based BNF estimates worldwide. This updated, lower BNF estimate indicates a greater ratio of anthropogenic to natural N inputs, suggesting an even greater human impact on the N cycle.
量化人类对氮循环的影响并研究自然生态系统的氮循环，取决于自然生物固氮（BNF）的输入量。在此，我们提出两种自下而上的方法，基于美国本土及阿拉斯加东南部的森林清查数据，对基于树木的共生BNF进行量化。对于所有主要的固氮树木属，我们使用以下两种方法来量化BNF输入量：（1）根据树木丰度的空间数据进行缩放的生态系统氮积累率（千克氮/公顷·年），以及（2）根据树木氮需求（来自树木生长速率和化学计量学）进行缩放的来自固氮的氮百分比（%N）。我们估计，研究区域内树木每年固定0.30 - 0.88太克氮（1.4 - 3.4千克氮/公顷·年）。基于树木的固氮显示出明显的空间变化，主要由两个属主导，（占与树木相关的BNF的64%）和（占24%）。第三重要的属，占5%。与已发表的其他氮通量估计值相比，与树木相关的BNF每年为0.59太克氮，与非共生（0.37太克氮/年）和林下共生BNF（0.48太克氮/年）相似，而氮沉降贡献了1.68太克氮/年，岩石风化贡献了0.37太克氮/年。总体而言，我们的结果揭示了树木BNF中以前未被描述的空间模式，可为大规模氮评估提供信息，并作为改进全球基于树木的BNF估计的模型。这个更新后的、较低的BNF估计值表明人为氮输入与自然氮输入的比例更高，这表明人类对氮循环的影响更大。
