Plants and their soil microbial symbionts influence ecosystem productivity and nutrient cycling, but the controls on these symbioses remain poorly understood. This is particularly true for plants in the Fabaceae family (hereafter legumes), which can associate with both arbuscular mycorrhizal fungi (AMF) and nitrogen (N) -fixing bacteria. Here we report results of the first manipulated field experiment to explore the abiotic and biotic controls of this tripartite symbiosis in Neotropical canopy gaps (hereafter gaps). We grew three species of Neotropical N-fixing legume seedlings under different light (gap–full light, gap–shadecloth, and understory) and soil nitrogen (20 g N·m⁻²·yr⁻¹ vs. 0 g N·m⁻²·yr⁻¹) conditions across a lowland tropical forest at La Selva Biological Station, Costa Rica. We harvested the seedlings after 4 months of growth in the field and measured percent AMF root colonization (%AMF), nodule and seeding biomass, and seedling aboveground:belowground biomass ratios. Our expectation was that seedlings in gaps would grow larger and, as a result of higher light, invest more carbon in both AMF and N-fixing bacteria. Indeed, seedlings in gaps had higher total biomass, nodule biomass (a proxy for N-fixing bacteria investment) and rates of AMF root colonization, and the three were significantly positively correlated. However, we only found a significant positive effect of light availability on %AMF when seedlings were fertilized with N. Furthermore, when we statistically controlled for treatment, species, and site effects, we found %AMF and seedling biomass had a negative relationship. This was likely driven by the fact that seedlings invested relatively less in AMF as they increased in biomass (lower %AMF per gram of seedling). Taken together, these results challenge the long-held assumption that high light conditions universally increase carbon investment in AMF and demonstrate that this tripartite symbiosis is influenced by soil nutrient and light conditions.

中文翻译：

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新热带冠层间隙中豆科植物-根瘤菌-菌根三方关系受光和土壤氮的影响

植物及其土壤微生物共生体影响生态系统生产力和养分循环，但对这些共生体的控制仍然知之甚少。对于豆科植物（以下简称豆科植物）尤其如此，它们可以与丛枝菌根真菌 (AMF) 和固氮 (N) 细菌相关联。在这里，我们报告了第一次操纵野外实验的结果，以探索新热带冠层间隙（以下简称间隙）中这种三方共生的非生物和生物控制。我们在不同的光照（间隙-全光、间隙-遮阳布和林下）和土壤氮（20 g N·m ^-2 ·yr ^-1对 0 g N·m ^-）下种植了三种新热带固氮豆科植物幼苗² ·年^-1) 在哥斯达黎加 La Selva 生物站的低地热带森林中的条件。我们在田间生长 4 个月后收获了幼苗，并测量了 AMF 根定植百分比 (%AMF)、根瘤和播种生物量以及地上幼苗：地下生物量比率。我们的预期是，间隙中的幼苗会长得更大，并且由于光照强度更高，会在 AMF 和固氮细菌中投入更多的碳。事实上，间隙中的幼苗具有更高的总生物量、根瘤生物量（固氮细菌投资的代表）和 AMF 根定植率，并且三者显着正相关。然而，我们只发现当幼苗用 N 施肥时，光照对 %AMF 有显着的积极影响。 此外，当我们对处理、物种和场地影响进行统计控制时，我们发现 %AMF 和幼苗生物量呈负相关。这可能是由于幼苗在 AMF 上的投资相对较少，因为它们增加了生物量（每克幼苗的 AMF 百分比较低）。总之，这些结果挑战了长期存在的假设，即强光条件普遍增加 AMF 中的碳投资，并证明这种三方共生受土壤养分和光照条件的影响。