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
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.,
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
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
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).
||Amberger, A., and Schmidt, H. L. (1987). "Naturliche Isotopengehalte
von Nitat als Indikatoren fur dessen Herkunft", Geochim. et Cosmochim.
Acta, 51: 2699-2705.
||Bohlke, J.K. and Denver, J.M., 1995. "Combined use of ground- water
dating, chemical, and isotopic analyses to resolve the history and fate
of nitrate contamination in two agricultural watersheds, atlantic coastal
Plain, Maryland." Water Resources Research, 31(9): 2319-2339.
||Bottcher, J., Strebel, O., Voerkelius, S. and Schmidt, H. L. (1990).
"Using isotope fractionation of nitrate-nitrogen and nitrate-oxygen
for evaluation of microbial denitrification in a sandy aquifer", J.
Hydrol., 114: 413-424.
||Durka, W., Schulze, E.-D., Gebauer, G., & Voerkelius, S., (1994).
"Effects of forest decline on uptake and leaching of deposited nitrate
determined from 15N and 18O measurements". Nature,
||Junk, G., and Svec, H. (1958). "The absolute abundance of the nitrogen
isotopes in the atmosphere and compressed gas from various sources",
Geochim. et Cosmochim. Acta, 14: 234-243.
||Karamanos, E. E., Voroney, R. P., and Rennie, D. A. (1981). "Variation
in natural 15N abundance of central Saskatchewan soils",
Soil, Sci. Soc. Am. J., 45, 826-828.
||Kendall, C. (1998). "Tracing nitrogen sources and cycling in catchments",
In: C. Kendall and J.J. McDonnell (Eds.), Isotope
Tracers in Catchment Hydrology, Elsevier, Amsterdam, pp. 519-576.
||Kendall, C., Sklash, M. G., Bullen, T. D. (1995). "Isotope Tracers
of Water and Solute Sources in Catchments", in Solute Modelling
in Catchment Systems, John Wiley and Sons, New York, pp. 261- 303.
||Kendall, C., Campbell, D.H., Burns, D.A., Shanley, J.B., Silva, S.R.,
and Chang, C.C.Y., (1995)." Tracing sources of nitrate in snowmelt
runoff using the oxygen and nitrogen isotopic compositions of nitrate:
pilot studies at three catchments", In: K. Tonnessen, M. Williams,
and M. Tranter (Eds.), Biogeochemistry of Seasonally Snow-covered Catchments,
Intern. Assoc. of Hydrol. Sci. Pub., July 3-14, 1995, Boulder CO, IAHS
Publ. # 228, p. 329-338.
||Kendall, C., Silva, S.R., Chang, C.C.Y., Burns, D.A., Campbell, D.H.,
and Shanley, J.B., (1996). "Use of the d18O
and d15N of nitrate to determine
sources of nitrate in early spring runoff in forested catchments."
IAEA, Symposium on Isotopes in Water Resources Management, Vienna, Austria,
20-24 March, 1995, 1: 167-176.
||McMahon, P.B., and Bohlke, J.K., (1996). "Denitrification and mixing
in a stream-aquifer system: effects on nitrate loading to surface water",
J. Hydrol., 186: 105-128.
||Revesz, K., Bohlke, J. K., and Yoshinari, T. (1997) Anal. Chem,
||Silva, S.R., Kendall, C., Wilkison, D.H., Ziegler, A.C., Chang, C.C.Y.
and Avanzino, A.J., 1998. "Collection and analysis of nitrate from
fresh water for nitrogen and oxygen isotopes." (submitted).
||Shearer, G. and Kohl, D. H. (1988). "d-15N
method of estimating N2 fixation". In: P. W. Rundel, J.
R. Ehleringer and K. A. Nagy (Eds.), Stable Isotopes in Ecological Research,
Springer-Verlag, New York, pp. 342-374.
||Wassenaar, L., (1995). "Evaluation of the origin and fate of nitratein
the Abbotsford Aquifer using the isotopes of 15N and 18O
in NO3-", Appl. Geochem., 10:
For a very detailed compilation of nitrogen-related references,
see the references
from Kendall (1998).