Collaborative Research: Does a transition in nitrogen fixation strategy explain the latitudinal distribution of nitrogen-fixing trees?

氮是植物生长所必需的营养物质。大气主要由氮气（N2）组成，而这种形式的氮是非常稳定的。大多数植物不能直接吸收N2，它们的生长速度受到土壤中有效氮的限制。因此，一些种类的树木已经进化出一种与一类特殊细菌的共生关系，这种细菌可以固定大气中的N2并将其转化为植物可利用的形式。这些固氮树有充足的途径获取氮，但这种共生关系是有代价的：这些树木为细菌提供碳，因此牺牲了一些自己的生长来维持共生关系。因此，固氮树只占地球上树种的一小部分。理论上，固氮只在土壤中氮含量非常有限的生态系统中是有利的。限制氮的森林往往在高纬度地区比在低纬度地区（热带和亚热带）更常见，但奇怪的是，固定氮的树木实际上在低纬度地区最常见。事实上，固氮树木在高纬度地区的数量比在低纬度地区少十倍。为什么会这样呢？这个提议将检验一种可能的解释。“差异调节假说”指出，固氮树调节固氮的程度的变化可以解释这种模式。快速调整固氮以满足氮需求的固氮树应该更丰富，在低纬度森林中就是这样。相反，在高纬度森林中观察到，不能调节固氮的物种应该较少。该项目将测量不同纬度对固氮调节的实际变化程度，以及调节固氮的代谢成本以及这些成本与温度的关系。这些结果将为地球上固氮树木的分布提供一个长期寻求的解释，这是全球森林生长的重要组成部分。该项目将包括在不同纬度的森林进行一次实地试验以及两次温室试验。田间试验将向森林施用标记氮，改变氮素限制程度，测定田间固氮规律。实验地点为俄勒冈州（桤木）、纽约（刺槐）和夏威夷（木麻黄、木麻黄、金合欢和金槐），跨越固氮树木丰度的纬度阈值（35°N）。固氮共生有两种主要类型（放线根菌和根瘤菌），我们选择了这两种固氮共生的田间地点。温室实验将测试更多的物种，并涉及更严格控制的条件。温室试验还将评估植物调节氮固定的速度。实验和实验室分析将涉及纽约州，加利福尼亚州和俄勒冈州的第一高中学生以及本科生和研究生的参与。

Nitrogen (N) is a necessary nutrient for plant growth. The atmosphere consists mainly of nitrogen gas (N2), and this form of N is extremely stable. Most plants can't directly access N2 and their rate of growth is instead limited by the presence of available N in the soil. Therefore, some species of trees have evolved a symbiosis with a special class of bacteria that fix atmospheric N2 and convert it into a plant-available form. These nitrogen-fixing trees have abundant access to nitrogen, but there is a cost to this symbiotic association: these trees provide carbon to bacteria and therefore sacrifice some of their own growth to maintain the symbiosis. As a result, nitrogen-fixing trees comprise only a small proportion of tree species on earth. In theory, nitrogen fixation is only advantageous in ecosystems that have very limited amounts of N in the soil. N-limited forests tend to be more common at high latitudes than at lower latitudes (the tropics and subtropics), but curiously, N-fixing trees are actually most common at lower latitudes. In fact, N-fixing trees are ten times less abundant at higher latitudes than at lower latitudes. Why is this the case? This proposal will test a potential explanation. The "differential regulation hypothesis" states that variation in the degree to which N-fixing trees regulate N fixation can explain the pattern. N-fixing trees that rapidly adjust N-fixation to meet N demand should be more abundant, as is the case in lower-latitude forests. In contrast, species that cannot regulate N-fixation should be less abundant, as observed in higher-latitude forests. This project will measure the extent to which the regulation of N-fixation actually varies across latitudes, as well as the metabolic costs of regulating N-fixation and the relationship of these costs to temperature. The results will provide a long sought after explanation for the distribution of N-fixing trees on earth, which is an essential component of the growth of forests worldwide.The project will involve a field experiment in forests at different latitudes as well as two greenhouse experiments. The field experiment will fertilize forests with labeled N to vary the degree of N limitation and measure the regulation of N fixation in the field. The experimental locations are Oregon (with Alnus rubra), New York (with Robinia pseudoacacia), and Hawaii (with Morella faya, Casuarina equisetifolia, Acacia koa, and Sophora chrysophylla) and span the latitudinal threshold for N-fixing tree abundance (35° N). There are two major types of N fixing symbioses (actinorhizal and rhizobial) and the field sites were chosen to capture both. The greenhouse experiments will test a larger number of species and involve more highly controlled conditions. The greenhouse experiments will also evaluate how quickly plants can regulate N fixation. The experiments and laboratory analyses will involve participation of Title 1 high school students as well as undergraduate and graduate students in New York, California and Oregon.