The lower maximum mass addition to DNA and RNA through 15N-labelling means a smaller shift of labelled nucleic acids away from unlabelled nucleic acids in an isopycnic gradient compared to 13C-labelling. Still, this smaller shift in density is nevertheless sufficient to detect labelling in DNA originating from a single organism, as was shown already in the classical work of Meselson and Stahl \cite{Meselson_1958}. However, for DNA-based SIP this creates a major challenge since double-stranded DNA migrates in a density gradient not only as a function of its mass but also as a function of its hydration state. The latter is ultimately determined by the G+C content of the DNA and causes an undesired migration of unlabelled high-GC DNA towards the denser regions of the gradient \cite{Rolfe_1959}. Already in the first attempts to develop 15N-SIP, it was noticed that due to the relatively small migration of 15N-labelled DNA, unlabeled DNA with high-G+C content could overlap with even fully-labelled DNA containing a lower G+C content, and obscure the ability to differentiate labelled from unlabelled taxa \cite{Cupples_2007,Youngblut_2014}. This is further intensified by the fact that A-T base pairs contain only seven nitrogen atoms compared to eight in a G-C base-pair, resulting in a lower, albeit minor labelling of the A-T base pair \cite{Cadisch_2005a}. XXX  However, more recent works employing 15N-DNA-SIP tended to avoid this two-step protocol and instead rely on the ability of high-throughput sequencing coupled with statistical modelling to detect labelled taxa and avoid false-positives via the use of parallel no-label controls \cite{Pepe_Ranney_2015} (see Section \ref{316470}). The first published attempt at 15N-RNA SIP is attributed to Addison and colleagues in 2010 \citep{Addison2010}, although the authors finally concluded that 15N-labelled RNA could not be definitely resolved from unlabelled RNA. However, it should be noted that the protocol used in that work deviated somewhat from the standard RNA-SIP protocol in several aspects, using much higher amounts of RNA, higher centrifugation speed, but lower temperature and shorter centrifugation time. Finally, a successful demonstration of a 15N-RNA-SIP protocol was published in 2018 and using a standard RNA-SIP protocol in combination with amplicon sequencing and statistical modelling \cite{Angel_2017}
Considering these, it is easy to understand why carbon is the most widely used isotope in SIP. Carbon is abundant enough in biomolecules to allow for easy labeling. In many cases carbon-based substrates are used for both assimilatory and dissimilatory process so biomass labelling is easily achieved while focusing on a particular process, or processes converting a single substrate. In contrast,  many N-transforming processes are dissimilatory, while at the same time many N-assimilation processes are common amongst many organisms and therefore provides relatively little differentiating power. Similarly, oxygen is also found abundantly in many redox couples used for aerobic or anaerobic respiration, or alternatively in water, which is assimilated in the biomass by all known organisms.