The nitrogen stable isotope composition in sedimentary rocks (δ¹⁵N) is increasingly used in deep-time studies for reconstructing changes in the N-biogeochemical cycle (e.g., reviews in Algeo et al., 2014; Ader et al., 2016; Stüeken et al., 2016). Analytical advances in EA-IRMS systems have greatly decreased the δ¹⁵N measurement time, complexity, and detection limit, while allowing simultaneous δ¹³C measurements, making the δ¹⁵N proxy a standard procedure in paleo-environmental studies (Mulvaney, 2012). Hence, it has already provided some insight into the Precambrian evolution of the marine nitrogen cycle (e.g., Thomazo et al., 2011; Ader et al., 2014; Stüeken et al., 2015, 2016, 2021a, Stüeken et al.; Michiels et al., 2017; Zerkle et al., 2017; Kipp et al., 2018; Luo et al., 2018) and its linkage to the history of Earth-surface oxygenation (Lyons et al., 2014) and biotic evolution (Stüeken, 2013). For the more recent Phanerozoic times, it has yielded detailed reconstructions of nitrogen cycling at the regional scale (e.g., Sachs and Repeta, 1999; Fulton et al., 2012), and documented wholesale reorganization of the marine nitrogen cycle in conjunction with major Phanerozoic biocrises (e.g., Xie et al., 2010, 2017; Luo et al., 2011; Schoepfer et al., 2016) as well as systematic changes related to major climate events (e.g., Algeo et al., 2008, 2014; Yao et al., 2015; Luo et al., 2016).

However, many uncertainties remain in the application of this proxy. The goals of the contributions to this Research Topic are thus to further refine the applicability of the δ¹⁵N proxy in sedimentary rocks and develop more nuanced interpretations and research questions for paleo-environmental reconstructions.

As reviewed in several studies (Ader et al., 2006; Robinson et al., 2012; Thomazo and Papineau, 2013; Ader et al., 2016; Stüeken et al.), including two of the present Research Topic (Stüeken et al.; Quan and Adeboye), one of the key uncertainties is the poor knowledge of the impact of diagenesis, catagenesis, metamorphism and fluid migration on primary δ¹⁵N signatures. Yet, it is of utmost importance to be able to precisely reconstruct primary δ¹⁵N values in order to make reliable paleo-environmental reconstructions, these aspects will therefore need to be better charaterized.

Another important uncertainty is the fact that, although the nitrogen fluxes operating at any given time in an ecosystem depend on the presence, absence or abundance of oxygen, several isotopic fractionations associated with N-cycling are not diagnostic of a particular redox state and hence of nitrogen speciation as nitrate or ammonium (Brandes et al., 2007; Thamdrup, 2012; Quan and Adeboye).

In addition, most of the current underlying assumptions supporting δ¹⁵N interpretation in the sedimentary rock record rely heavily on analogies with the present-day marine nitrogen cycle (e.g., Galbraith et al., 2008; Sigman et al., 2009; Robinson et al., 2012; Tesdal et al., 2013), which may not be the best analogue for deep-time lakes, restricted basins and oceans.

Two papers address the goal of refining the applicability of the δ¹⁵N proxy via the study of modern systems: the Dziani Dzaha, a saline and alkaline lake (Cadeau et al.), and the Coorong lagoon showing a strong salinity gradient (Priestley et al.). The results show that, while in the Coorong lagoon sediment δ¹⁵N records as expected the δ¹⁵N of primary producers (Priestley et al.), in the Dziani Dzaha lake the sediment δ¹⁵N values are more positive by three‰ compared to primary producers, possibly as a result of ¹⁵N-enriched ammonium assimilation in alkaline bottom waters (Cadeau et al.). Taken together, these two studies highlight that not only redox conditions but also salinity and pH may influence the nitrogen cycle. These aspects will need to be better integrated into future studies of deep-time nitrogen cycling.

The remaining papers address the goal of developing more nuanced interpretation of the δ¹⁵N proxy in deep-time by coupling it to other proxies, such as redox indicators, C/N ratio, δ¹³C_org and δ¹³C_carb and biomarker δ¹⁵N. Two papers characterize nitrogen speciation (ammonium or nitrate) in the water column by coupling δ¹⁵N to other aqueous redox proxies. Johnson et al. use concentrations of U, V and Mo, along with Fe-speciation, in sedimentary successions of the Yangtze Platform during the Neoproterozoic Cryogenian Period, allowing them to show that nitrate was stable in the water column and to identify temporal variations in denitrification rates. Riquier et al. in a detailed multi-proxy study (δ¹³C_org, δ¹⁵N_bulk, Rock-Eval and trace metals) of the oceanic anoxic event (OAE-2) of the DSDP 367 succession from Cape Verde, show that δ¹⁵N values can be interpreted as evidence of ammonium assimilation. This implies that ammonium (and hence anoxia) reached the photic zone, corroborating molecular studies indicating photic zone anoxia at that time (Higgins et al., 2012).

Three studies identify feedbacks between the C- and N-cycles by coupling δ¹⁵N to δ¹³C_carb and/or δ¹³C_org at high stratigraphic resolution. Xu et al. identify subtle changes in δ¹⁵N in the Yangtze platform that coincided with the Shuram Excursion, i.e., the largest negative δ¹³C_carb excursion in Earth history (Grotzinger et al., 2011; Cao et al., 2020). This work is novel in that it highlights the variability in fractionation factors as a possible driver of subtle δ¹⁵N shifts in the paleoenvironment. Fraga-Ferreira et al. this volume also identify subtle changes in the δ¹⁵N values that coincided with the onset of an extreme δ¹³C_carb positive isotope excursion and with variations in Sr isotope ratios and redox tracers in several sections of the Ediacarian/Cambrian Bambui Group (Brazil) (Caetano-Filho et al., 2021), further highlighting the close linkage between nitrogen and carbon cycling. In the same vein, Mercuzot et al. explore a combination of several proxies (δ¹⁵N_bulk to C/N, δ¹³C_org, Rock-Eval and palynofacies analyses) to deconvolute terrestrial from autochthonous organic matter in several sections of Late Carboniferous to Early Permian continental basins, providing insights into nitrogen cycling during times of enhanced autochthonous organic matter accumulation.

Finally, Hallmann et al., following the lead of a handful of previous studies (e.g., Sachs and Repeta, 1999; Higgins et al., 2012) reaffirmed the power of coupling of δ¹⁵N in bulk rocks and kerogens to that of N-containing biomarkers. In the case of Ediacaran sedimentary successions from Oman, the authors were also able to show that primary producers assimilated ammonium at a shallow redoxcline during times of enhanced water-column stratification. Furthermore, they identify and tentatively quantify the biomass produced by eukaryotic and cyanobacterial oxygenic primary producers, as well as anoxygenic primary producers.

The findings reported in these contributions show that providing context using other proxies allows the interpretation of even subtle changes in δ¹⁵N. Collectively, the studies in this Research Topic serve to further our understanding of the deep-time marine nitrogen cycle, providing a baseline for evaluating present-day anthropogenic changes in nitrogen cycling and their impacts on the environment and biosphere (Fowler et al., 2013).