Periodic Table--Nitrogen

There are two stable isotopes of N: ¹⁴N and ¹⁵N. Because the average abundance of ¹⁵N in air is a very constant 0.366% (Junk and Svec, 1958), air (AIR) is used as the standard for reporting d ¹⁵N values. Most terrestrial materials have d¹⁵N compositions between -20‰ and +30‰. The dominant source of nitrogen in most forested ecosystems is the atmosphere (d¹⁵N = 0‰); many plants fix nitrogen and organisms cycle this nitrogen into the soil. Other sources of nitrogen include fertilizers produced from atmospheric nitrogen with compositions of 0 ± 3‰ and animal manure with nitrate d¹⁵N values generally in the range of +10 to +25‰; rock sources of N are generally considered negligible.

Biologically-mediated reactions (e.g., assimilation, nitrification, and denitrification) strongly control nitrogen dynamics in the soil, as briefly described below. These reactions almost always result in ¹⁵N enrichment of the substrate and depletion of the product. Although precipitation often contains subequal quantities of ammonium and nitrate, because ammonium is preferentially retained by the canopy relative to atmospheric nitrate (Garten and Hanson, 1990), most of the atmospheric nitrogen that reaches the soil surface is in the form of nitrate. Soil nitrate is preferentially assimilated by tree roots relative to soil ammonium (Nadelhoffer et al., 1988).

The d¹⁵N of total soil N is affected by many factors including soil depth, vegetation, climate, particle size, cultural history, etc.; however, two factors, drainage and influence of litter, have a consistent and major influence on the ¹⁵N values (Shearer and Kohl, 1988). Soils on lower slopes and near saline seeps have higher d¹⁵N values than well-drained soils (Karamanos et al., 1981), perhaps because the greater denitrification in more boggy areas results in heavy residual nitrate. The d¹⁵N values of the soils from valley bottoms at the Walker Branch Watershed in Tennessee (USA) are heavier than for soils from ridges and slopes, consistent with a theoretical model that explains the increase in the d¹⁵N of inorganic N in soil as a function of the higher relative rates of immobilization and nitrification in these bottom soils (Shearer et al., 1974). Surface soils beneath bushes and trees often have lighter d¹⁵N values than those in open areas, presumably as the result of litter deposition (Shearer and Kohl, 1988). Fractionations during litter decomposition in forests result in surface soils with lighter d¹⁵N values than deeper soils (Nadelhoffer and Fry, 1988). Gormly and Spalding (1979) attributed the inverse correlation of nitrate- d¹⁵N and nitrate concentration beneath agricultural fields to increasing denitrification with depth.

Although enriched ¹⁵N tracer studies have been commonly used in agricultural investigations for decades (Bremmer, 1965), natural abundance studies to trace natural and anthropogenic sources of nitrate started with Kohl et al. (1971) but were rare until the last decade. Such studies take advantage of the observation that atmospherically-derived nitrogen and fertilizer nitrogen typically have light d¹⁵N values whereas animal-derived nitrogen (such as manure or septic-tank effluents) is typically considerably heavier; hence, under favorable conditions, the end members are distinguishable and the relative contributions of these two sources to ground water can be estimated. Numerous studies have shown that the d¹⁵N values of nitrate in ground water can be used to indicate the dominant source of the nitrate. Such applications are most successful in well-drained soils and oxygenated ground waters where nitrification is rapid and denitrification is minimal; under these conditions, d¹⁵N values are relatively conservative. There have been fewer successful studies in surface waters because of the higher likelihood that competing biological processes such as uptake, nitrification of ammonium, and denitrification will cause significant isotopic fractionations. Soil-derived nitrate and fertilizer nitrate usually have overlapping compositions, preventing their separation using ¹⁵N alone. Applications of ¹⁵N to trace relative contributions of fertilizer and animal waste to ground water (Kreitler, 1975; Kreitler and Jones, 1975; Kreitler et al., 1978; Gormly and Spalding, 1979) are complicated by a number of reactions including ammonia volatilization, nitrification, denitrification, ion exchange, and plant uptake. These processes can modify the d¹⁵N values of N sources prior to mixing and the resultant mixtures, causing estimations of the relative contributions of the sources of nitrate to be inaccurate.

Denitrification causes the d ¹⁵N of the residual (unreacted) nitrate to increase exponentially as nitrate concentrations decrease. For example, denitrification of fertilizer nitrate that originally had a distinctive d¹⁵N value of +1‰ can yield residual nitrate with a d¹⁵N value of +15‰; this value is within the range of compositions expected for nitrate from a manure or septic-tank source. Measured fractionation factors associated with denitrification range from 10 to 40‰. The N₂ produced by denitrification results in "excess N₂" contents in ground water. The total N₂ (which consists of air entrained during recharge plus N₂ produced by denitrification) can be collected, analyzed for d¹⁵N, and used to estimate the extent of denitrification, initial composition of the nitrate, or the mixing history of the water (Vogel et al., 1981; Bohlke and Denver, 1995; McMahon and Bohlke, 1996).

The recent development of methods to analyze nitrate for d¹⁸O (Amberger and Schmidt, 1987; Silva et al., in review; Revesz et al., 1997) should expand the use of isotopic techniques to trace nitrate sources and transformations. Nitrate d¹⁸O values show a wide range of values, ranging from about -10 to +80 ‰. Many nitrate sources have distinctive d¹⁸O and d¹⁵N values; in particular, nitrate derived from atmospheric or nitrate fertilizer sources is easily distinguished from nitrified ammonium and urea. A figure showing the approximate ranges of compositions of many compounds is available. Amberger and Schmidt (1987) determined that fertilizer nitrate and nitrate formed from nitrification of organic material have distinctive d¹⁸O and d¹⁵N values. All three oxygens in fertilizer nitrate are derived from atmospheric O₂ (+22 to +24 ‰), and hence the d¹⁸O of the nitrate is in this range. Two of the oxygens in soil nitrate derive from the oxygen in water molecules (and presumably reflect the d¹⁸O of the water) and one oxygen comes from atmospheric O₂. For waters with d ¹⁸O values in the range of -5 to -20 ‰, the d¹⁸O of soil nitrate formed from in situ nitrification of ammonium, should be in the range of +5 to -5 ‰, respectively.

Another useful application of the "dual isotope approach" is for determination of the relative contributions of atmospheric and soil- derived sources of nitrate in shallow ground water. This problem is intractable using just d¹⁵N because of overlapping compositions of soil and atmospherically derived nitrate. Recent data from several German forests (Durka, 1994; Durka et al., 1994) and three catchments in the USA (Kendall et al., 1995; Kendall et al., 1996) indicate that the d¹⁸O of nitrate in precipitation is in the range of +25 to +75 ‰. If such heavy d¹⁸O values are typical of atmospherically derived nitrate elsewhere, or at least in places where there are significant anthropogenic sources of atmospheric nitrogen, contributions from precipitation should easily be distinguished from soil nitrate. Durka (1994) analyzed nitrate in precipitation and spring samples from several German forests for both isotopes, and found that the d¹⁸O of nitrate in springs was correlated with the general health of the forest, with more healthy or limed forests showing lower d¹⁸O values closer to the composition of microbially produced nitrate; severely damaged forests show higher d¹⁸O values, indicative of major contributions of atmospheric nitrate to the system. Hence, acid-induced forest decline appears to inhibit nitrate consumption by soil micro-organisms (Durka et al., 1994).

A similar application is for determination of the source of nitrate in early spring runoff. In three pilot studies conducted in catchments in New York, Colorado, and Vermont during the 1994 snowmelt season, Kendall et al. (1995, 1996) found that almost all the stream samples had nitrate d¹⁸O and d¹⁵N values within the range of pre-melt and soil waters. Stream samples were significantly different from the composition of almost all the snow samples, indicating that atmospheric nitrate from the 1994 snowpack was not a significant source of nitrate in early runoff in these catchments. Therefore, the nitrate eluted from the snowpack appeared to go into storage, and most of the nitrate in streamflow during the period of potential acidification was apparently derived from pre-melt sources.

Amberger and Schmidt (1987) showed that denitrification results in enrichment in d¹⁸O of the residual nitrate, as well as enrichment in d¹⁵N. Therefore, analysis of both d¹⁵N and d¹⁸O of nitrate should allow denitrification effects to be distinguished from mixing of sources. This dual isotope approach takes advantage of the observation that the ratio of the enrichment in ¹⁵N to the enrichment in ¹⁸O in residual nitrate during denitrification appears to be about 2:1 (Amberger and Schmidt, 1987; Bottcher et al., 1990). Hence, the nitrate from systems that have experienced significant denitrification plot along a very distinctive line on d¹⁵N vs. d¹⁸O plots. If this ratio is constant, and the d¹⁵N and d¹⁸O compositions of the two potential sources of nitrate contributing to ground water are known, are distinguishable, and do not show much scatter in composition, the "original" relative contributions of these two sources to the nitrate in any sample of ground water can be estimated from the d¹⁵N and d¹⁸O values of the nitrate.

Further information can be found in the section: "Isotopic Composition of Nitrate" in Clark and Fritz (1997), Environmental Isotopes in Hydrology , CRC Press.

Additional information can be found in the chapter: "Tracing Nitrogen Sources and Cycling in Catchments" by Kendall (1998), In: C. Kendall and J.J. McDonnell (Eds.), Isotope Tracers in Catchment Hydrology, Elsevier, Amsterdam, pp. 519-576.

Source of text: This review was assembled by Carol Kendall, primarily from Kendall et al. (1995) and Kendall (1998).

For a very detailed compilation of nitrogen-related references, see the references from Kendall (1998).