The seventh macronutrient: how sodium shortfall ramifies through populations, food webs, and ecosystems

Of the 25 elements required to build most organisms, sodium has a unique set of characteristics that ramify through terrestrial ecology. In plants, sodium is found in low concentrations and has little metabolic function; in plant consumers, particularly animals, sodium is essential to running costly Na-K ATPases. Here I synthesize a diverse literature from physiology, agronomy, and ecology, toward identifying sodium's place as the "7th macronutrient", one whose shortfall targets two trophic levels—herbivores and detritivores. I propose that sodium also plays a central, though unheralded role in herbivore digestion, via its importance to maintaining microbiomes and denaturing tannins. I highlight how sodium availability is a key determinant of consumer abundance and the geography of herbivory and detritivory. And I propose a re-appraisal of the assumption that, because sodium is metabolically unimportant to most plants, it is of little use. Instead, I suggest that sodium's critical role in limiting herbivore performance makes it a commodity used by plants to manipulate their herbivores and mutualists, and by consumers like bison and elephants to generate grazing lawns: dependable sources of sodium.

在绝大多数生物构建所需的25种元素中，钠（sodium）具备一系列独特属性，其影响贯穿陆地生态系统的各个维度。对于植物而言，钠的含量极低且代谢功能微乎其微；而以植物为食的消费者（尤其是动物），钠是维持耗能极高的钠钾ATP酶（Na-K ATPases）运转的必需元素。本文整合了生理学、农艺学与生态学领域的多元文献，旨在明确钠作为“第七大量元素”的定位——其缺乏会直接影响两个营养级：植食动物（herbivores）与腐食动物（detritivores）。本文提出，钠还凭借其在维持微生物组（microbiomes）与使单宁（tannins）变性方面的关键作用，在植食动物的消化过程中发挥着核心却未被充分重视的功能。本文还阐明了钠的可获得性是决定消费者种群丰度，以及植食行为与腐食行为地理分布格局的关键因素。此外，学界此前存在一种假设：由于钠对多数植物的代谢无足轻重，因此其价值有限。本文呼吁对这一假设进行重新评估。恰恰相反，本文认为，钠在限制植食动物生存表现方面的关键作用，使其成为植物用以调控自身植食者与共生伙伴的资源；同时也成为野牛、大象等消费者用以打造放牧草场——稳定的钠源——的工具。