Foliar uptake of nitrogen from ant faecal droplets: an overlooked service to ant-plants

Nutrient supplies to plants from ants are well known from specialised myrmecophytic symbioses and from plants growing in soil close to ant nests. However, above-ground nutrient pathways may play a largely unrecognised role also in less specialised ant–plant interactions—the numerous facultative relationships, where ants forage on plants.
In a laboratory experiment, weaver ants (Oecophylla smaragdina) were confined to the canopies of coffee (Coffea arabica) seedlings, excluding any ant-to-plant transfer of nutrients via the soil strata. When ants were fed 15N-labelled glycine and subsequently deposited faecal droplets on the seedlings, coffee leaves showed increased levels of 15N and total nitrogen compared to control plants without ants. This was evident for both exposed leaves and leaves covered in plastic bags (i.e. not directly exposed to ants). Thus, nitrogen from ant excretions was absorbed through the coffee leaves, and subsequently, translocated within the plants and possibly leading to the observed higher shoot/root (wet weight) ratios observed on ant-plants compared to controls.
Synthesis. These results reveal an undescribed foliar uptake of ant-provided nutrients. If this is a general mechanism, a vast amount of ant–plant interactions involves fertilisation. Foliar fertilisation by ants may be an important steady benefit to plants, which has driven the evolution of ant–plant mutualisms in parallel with the well-recognised but probably more fluctuating benefit from herbivore protection. Given the world-wide abundance of plant canopies foraged by ants, this nutrient pathway may be of high ecological significance.

蚂蚁为植物提供养分的现象，已在特化蚁栖共生（myrmecophytic symbioses）以及生长在蚁巢附近土壤中的植物中得到广泛证实。然而，在特化程度更低的蚁植互作——即蚂蚁在植物上觅食的众多兼性共生关系中，地上养分传递路径的作用可能长期未被认知。
本研究通过室内实验，将织叶蚁（Oecophylla smaragdina）限定在阿拉伯咖啡（Coffea arabica）幼苗的冠层范围内，排除了通过土壤层实现蚂蚁向植物养分转移的可能。实验中，研究人员向织叶蚁投喂氮15标记的甘氨酸（15N-labelled glycine），当其在幼苗体表沉积粪便液滴后，与无蚁对照组相比，咖啡叶片的氮15与总氮含量均显著提升。这一现象在直接暴露于蚂蚁的叶片以及用塑料袋覆盖（即未直接接触蚂蚁）的叶片中均有体现。由此可见，蚂蚁排泄物中的氮可通过咖啡叶片被植物吸收，并在植株体内进行转运，这或许正是蚁植组幼苗地上/地下（鲜重）比值高于对照组的原因。
综合分析表明，本研究结果揭示了一种此前未被报道的、蚂蚁来源养分的叶片吸收机制。若该机制具有普遍性，则大量蚁植互作本质上都属于养分施肥过程。蚂蚁通过叶片为植物提供养分，可能是植物获得的一项重要且稳定的收益，这与广为人知但波动程度可能更高的植食动物防御收益共同推动了蚁植互利共生的演化。考虑到全球范围内蚂蚁觅食的植物冠层分布广泛，这一养分传递路径可能具有极高的生态学意义。