Exploring cycad foliage as an archive of the isotopic composition of atmospheric nitrogen

Abstract

Molecular nitrogen (N2) constitutes the majority of Earth's modern atmosphere, contributing ~0.79 bar of partial pressure (pN2). However, fluctuations in pN2 may have occurred on 107–109 year timescales in Earth's past, perhaps altering the isotopic composition of atmospheric nitrogen. Here, we explore an archive that may record the isotopic composition of atmospheric N2 in deep time: the foliage of cycads. Cycads are ancient gymnosperms that host symbiotic N2‐fixing cyanobacteria in modified root structures known as coralloid roots. All extant species of cycads are known to host symbionts, suggesting that this N2‐fixing capacity is perhaps ancestral, reaching back to the early history of cycads in the late Paleozoic. Therefore, if the process of microbial N2 fixation records the δ15N value of atmospheric N2 in cycad foliage, the fossil record of cycads may provide an archive of atmospheric δ15N values. To explore this potential proxy, we conducted a survey of wild cycads growing in a range of modern environments to determine whether cycad foliage reliably records the isotopic composition of atmospheric N2. We find that neither biological nor environmental factors significantly influence the δ15N values of cycad foliage, suggesting that they provide a reasonably robust record of the δ15N of atmospheric N2. Application of this proxy to the record of carbonaceous cycad fossils may not only help to constrain changes in atmospheric nitrogen isotope ratios since the late Paleozoic, but also could shed light on the antiquity of the N2‐fixing symbiosis between cycads and cyanobacteria.

Encephalartos villosus relies on atmospheric nitrogen than soil derived nitrogen to maintain growth in nutrient-deficient and acidic soils

“…Nitrogen supply depends on environmental conditions such as nutrient availability (Kipp et al, 2019). According to Shay et al (2015), nutrient de ciencies favour BNF, which may be why atmospheric N was the preferred N source in E. villosus growing in nutrient-de cient soils in Rhebu and Oceanview.…”

“…Vessey et al (2004) reported that although the N source preference of plants associated with N-xing bacteria has been extensively studied in plants such as legumes, the proportion of N derived from the atmosphere and soils in cycads is not studied. Also, studies such as those conducted by Kipp et al (2019) determine the composition of the 15 N isotope in cycad foliage to show that cycads partake in BNF, but the percentage N derived from the atmosphere is not quanti ed. In this study, we investigated the effects of soil characteristics (pH, nutrition, total cation, and exchange acidity) on cycad-microbe symbiosis and N preference of E. villosus growing in Rhebu and Oceanview.…”

Phylometagenomics of cycad coralloid roots reveals shared symbiotic signals

“…Biological nitrogen fixation (BNF) is believed to be the main function of the coralloid root microbiome, in exchange for carbon sources from the host [9,10]. Indeed, in nitrogen-poor environments, cycad leaves carry the same nitrogen fractionation signal as their diazotrophic cyanobionts, confirming the plant's reliance on symbiotic BNF [11]. Other less studied functions for cycad cyanobionts and associated microbes might also be relevant for the symbiosis, such as diverse biological and ecological functions supported by natural products or specialized metabolites [12][13][14].…”

Exploring the Influence of Ecological Niches and Hologenome Dynamics on the Growth of Encephalartos villosus in Scarp Forests

“…Also, many studies reported on the N-fixing bacteria associated with cycad species [20,24,25]; however, the influence of the bacterial isolates on plant nutrition and N source reliance is poorly understood. Moreover, studies that report on the composition of the 15 N isotope in cycad foliage, such as [26], do not report on the N reliance and the percentage of N derived from the atmosphere in cycads.…”