Ottinger Sarah L, Miniat Chelcy Ford, Wurzburger Nina
Odum School of Ecology, University of Georgia, Athens, GA, USA.
USDA Forest Service, Rocky Mountain Research Station, Albuquerque, NM, USA.
Oecologia. 2023 Feb;201(2):565-574. doi: 10.1007/s00442-023-05313-0. Epub 2023 Jan 13.
Symbiotic nitrogen fixation (SNF) is a critical mechanism of ecosystem recovery, and in forests of the eastern United States, the most common tree species that supports SNF is black locust (Robinia pseudoacacia L.). Despite its prevalence, black locust's fixation strategy-whether it maintains fixation at a constant rate (obligate fixation) or reduces its fixation rate (facultative fixation)-is unknown. Here, we examined how nitrogen and light control SNF by black locust, by growing seedlings under two nitrogen levels and across four levels of light. Seedlings were harvested after 12 weeks to determine biomass changes, nodule activity, and photosynthetic rates. Black locust seedlings increased biomass growth with increasing light, but only in the absence of nitrogen addition, while seedling root:shoot (biomass) modestly declined with increasing light regardless of nitrogen level. We found that black locust behaved like a facultative fixer, and regulated fixation by excising or maintaining nodules, and by controlling nodule biomass and activity. Specifically, nitrogen addition reduced seedling investment in nodule biomass (g g) by 63%, and reduced seedling allocation to nitrogen fixation (µmol CH g h) by 66%. In contrast, light affected nitrogen fixation through two indirect pathways. First, light increased plant growth, and hence nitrogen demands, which caused an increase in nitrogen fixation proportional to biomass. Second, light increasd photosynthetic activity, which was positively associated with nodule activity, but only in the absence of nitrogen addition. Our findings for how black locust regulates SNF can improve predictions of ecosystem SNF under the changing environmental conditions.
共生固氮（SNF）是生态系统恢复的关键机制，在美国东部的森林中，支持SNF的最常见树种是刺槐（Robinia pseudoacacia L.）。尽管刺槐很常见，但其固氮策略——是保持固定速率不变（专性固氮）还是降低固定速率（兼性固氮）——尚不清楚。在这里，我们通过在两种氮水平和四种光照水平下培育刺槐幼苗，研究了氮和光照如何控制刺槐的共生固氮。12周后收获幼苗，以确定生物量变化、根瘤活性和光合速率。刺槐幼苗的生物量增长随着光照增加而增加，但仅在不添加氮的情况下，而无论氮水平如何，幼苗的根：茎（生物量）随着光照增加而略有下降。我们发现刺槐表现为兼性固氮者，通过切除或维持根瘤以及控制根瘤生物量和活性来调节固氮。具体而言，添加氮使幼苗在根瘤生物量（克/克）上的投入减少了63%，并使幼苗分配到固氮的量（微摩尔碳/克/小时）减少了66%。相比之下，光照通过两条间接途径影响固氮。首先，光照促进植物生长，从而增加氮需求，这导致固氮量与生物量成比例增加。其次，光照增加光合活性，这与根瘤活性呈正相关，但仅在不添加氮的情况下。我们关于刺槐如何调节共生固氮的研究结果可以改善对不断变化的环境条件下生态系统共生固氮的预测。
