Symbiotic nitrogen fixation (SNF) is a key ecological process whose impact depends on the strategy of SNF regulation—the degree to which rates of SNF change in response to limitation by N versus other resources. SNF that is obligate or exhibits incomplete downregulation can result in excess N fixation, whereas a facultative SNF strategy does not. We hypothesized that tree-based SNF strategies differed by latitude (tropical vs. temperate) and symbiotic type (actinorhizal vs. rhizobial). Specifically, we expected tropical rhizobial symbioses to display strongly facultative SNF as an explanation of their success in low-latitude forests. In this study we used ¹⁵N isotope dilution field experiments in New York, Oregon, and Hawaii to determine SNF strategies in six N-fixing tree symbioses. Nitrogen fertilization with +10 and +15 g N m⁻² year⁻¹ for 4–5 years alleviated N limitation in all taxa, paving the way to determine SNF strategies. Contrary to our hypothesis, all six of the symbioses we studied sustained SNF even at high N. Robinia pseudoacacia (temperate rhizobial) fixed 91% of its N (%N_dfa) in controls, compared to 64% and 59% in the +10 and +15 g N m⁻² year⁻¹ treatments. For Alnus rubra (temperate actinorhizal), %N_dfa was 95%, 70%, and 60%. For the tropical species, %N_dfa was 86%, 80%, and 82% for Gliricidia sepium (rhizobial); 79%, 69%, and 67% for Casuarina equisetifolia (actinorhizal); 91%, 42%, and 67% for Acacia koa (rhizobial); and 60%, 51%, and 19% for Morella faya (actinorhizal). Fertilization with phosphorus did not stimulate tree growth or SNF. These results suggest that the latitudinal abundance distribution of N-fixing trees is not caused by a shift in SNF strategy. They also help explain the excess N in many forests where N fixers are common.

中文翻译：

---

尽管氮供应过剩，但树木共生维持固氮作用

共生固氮 (SNF) 是一个关键的生态过程，其影响取决于 SNF 调控策略——SNF 速率响应 N 限制与其他资源限制的变化程度。专性或表现出不完全下调的 SNF 可导致过量的 N 固定，而兼性 SNF 策略则不会。我们假设基于树的 SNF 策略因纬度（热带与温带）和共生类型（放线菌与根瘤菌）而异。具体来说，我们预计热带根瘤菌共生体会表现出强烈的兼性 SNF，以解释它们在低纬度森林中的成功。在这项研究中，我们使用了¹⁵在纽约、俄勒冈和夏威夷进行的 N 同位素稀释野外实验，以确定六种固氮树共生体中的 SNF 策略。^{+10 和 +15 g N m −2}  year ⁻¹的氮肥施肥4-5 年减轻了所有类群的 N 限制，为确定 SNF 策略铺平了道路。与我们的假设相反，我们研究的所有六种共生体即使在高 N 情况下也能维持 SNF。刺槐（温带根瘤菌）在对照组中固定了 91% 的 N (%N _dfa )，而在 +10 组中分别为 64% 和 59% +15 g N m ^-2 年^-1处理。对于赤杨（温带放线菌），%N _dfa为 95%、70% 和 60%。对于热带物种，%NGliricidia sepium（根瘤菌）的_dfa为 86%、80% 和 82% ；木麻黄(放线菌) 79%、69% 和 67% ；91%、42% 和 67%相思树（根瘤菌）；Morella faya (actinorhizal)分别为 60%、51% 和 19% 。用磷施肥不会刺激树木生长或 SNF。这些结果表明，固氮树的纬度丰度分布不是由 SNF 策略的转变引起的。它们还有助于解释许多 N 固定剂很常见的森林中 N 过多的原因。