Marine Chemistry 88 (2004) 1 – 20
www.elsevier.com/locate/marchem
Coupling the
15
N/14N and 18O/16O of nitrate as a constraint on
benthic nitrogen cycling
Moritz F. Lehmann a,*, Daniel M. Sigman a, William M. Berelson b
a
Department of Geosciences, Princeton University, Guyot Hall, Princeton, NJ 08544, USA
b
Department of Earth Sciences, U.S.C., Los Angeles, CA 90089-0740, USA
Received 19 September 2003; received in revised form 19 February 2004; accepted 19 February 2004
Abstract
We report 15N/14N and 18O/16O ratios of nitrate in benthic chamber incubations in the continental shelf sediments of the
Santa Monica Bay (SMB) to deconvolve the effects of nitrification and denitrification. Estimates of denitrification rate from
benthic flux stoichiometry range from 0.9 to 2.5 mmol N m 2 d 1. Between 46% and 100% of the total denitrification can be
explained by nitrate or nitrite from nitrification. In general and independent of the denitrification rate and the fraction of
remineralized N being denitrified, nitrate 15N/14N and 18O/16O ratios do not change significantly with progressive nitrate
depletion in the chambers. A lack of change in nitrate 15N/14N could be due to either the lack of effective N isotope fractionation
associated with sedimentary denitrification or the balancing of a denitrification isotope effect by the addition of low-15N/14N
nitrate from nitrification. However, the lack of an increase in nitrate 18O/16O indicates that the isotopic fractionation specifically
associated with sedimentary denitrification is, in fact, negligible. The coupled N and O isotope measurements also indicate that
there is no significant gross efflux of 15N-depleted nitrate from nitrification, leading to the conclusion that nitrification is closely
coupled to denitrification, even in the bioturbated sediments of the SMB.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Nitrate isotopes; Sedimentary denitrification; Nitrification; Isotope effects; Benthic flux chamber; Santa Monica Bay
1. Introduction
Denitrification, the bacterially mediated reduction
of NO3 and NO2 to N2O and N2, is the major
mechanism of fixed nitrogen loss from the ocean
(Codispoti and Christensen, 1985; Seitzinger, 1988).
It occurs in both the water column and sediments
* Corresponding author. Tel.: +1-609-258-7544; fax: +1-609258-0796.
E-mail address: [email protected] (M.F. Lehmann).
0304-4203/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.marchem.2004.02.001
where O2 concentration is f 5 Amol/l and lower
(Codispoti et al., in press). Denitrification in continental margin sediments probably accounts for more
than half of global ocean denitrification (Christensen,
1994; Brandes and Devol, 2002). Yet, the absolute
rate of sedimentary denitrification is poorly constrained, with recent estimates ranging from 7.1 to
32.1 Tmol N yr 1 (100 to 450 Tg N yr 1) (Middelburg et al., 1996; Devol et al., 1997; Codispoti et al.,
2001; Brandes and Devol, 2002).
Denitrification, like other microbially mediated
biogeochemical reactions, exhibits a significant bi-
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M.F. Lehmann et al. / Marine Chemistry 88 (2004) 1–20
ological N and O isotope fractionation; that is, 14Nand 16O-bearing nitrate is preferentially consumed
by the denitrifying organism, leaving the residual
nitrate pool enriched in 15N (Cline and Kaplan,
1975; Liu and Kaplan, 1989) and 18O (Lehmann et
al., 2003 and references therein). It appears that the
N isotope effect, e, for denitrification in the ocean
water column, where isotope fractionation should be
nearly fully expressed, is more or less invariant in
the global ocean, close to 25x(Brandes et al.,
1998; Altabet et al., 1999; Voss et al., 2001; Sigman et al., 2003) (e={15k/14k 1}1000, where 15k
and 14k are reaction rates heavier and for the lighter
nitrate molecules, respectively). However, Bender
(1990) predicted that the ‘‘effective’’ isotope fractionation (community fractionation) during microbial reactions in sediments should be markedly
reduced with respect to the ‘‘intrinsic’’ (biological)
fractionation, due to the fact that the oxidant (e.g.,
nitrate) is not consumed from a mixed pool, that is,
partial substrate consumption occurs in deeper parts
of the sediments where it is already enriched in the
heavier isotope. Indeed, it was subsequently observed that sedimentary denitrification does not
produce any significant apparent N isotope fractionation during nitrate loss (Brandes and Devol, 1997,
2002; Sebilo et al., 2003). Based on previous data
from Puget Sound (Brandes and Devol, 1995),
Brandes and Devol (1997) hypothesized that the
lack of isotope effect is due to the limitation of
denitrification by the diffusion of nitrate into reactive microsites.
If both the N isotope effects associated with
water-column and sedimentary denitrification are
known, the 15N/14N of water-column nitrate can
potentially be used to infer the relative importance
of benthic versus pelagic denitrification, globally
(Brandes and Devol, 2002) or in a specific environment (Sigman et al., 2003). These estimates are
highly sensitive to the overall isotope effect of
Fig. 1. Schematic of the benthic nitrogen cycle with sources and sinks of DIN, and intrinsic isotope effects (e) associated with N-cycling
reactions (large arrows). Mineralization ( FMin) usually causes only a small fractionation between degrading organic N and NH+4 (Kendall, 1998),
whereas nitrification and denitrification occur with significant intrinsic isotope effects. Yet, relative to intrinsic isotope effects, apparent isotope
effects (as determined in the overlying water) may be highly reduced as a result of diffusion limitation. Large letters refer to selected fluxes of
DIN and are used in the text. The ammonium – efflux ratio QNH4 defines the partitioning of remineralized ammonium (efflux versus
nitrification). GCND is the fraction of newly produced nitrate being denitrified within the sediments, and hence represents a constraint on the
degree of coupling between nitrification and denitrification in the sediments. The simple box model described in the text is based on a similar
benthic DIN flux scheme.
M.F. Lehmann et al. / Marine Chemistry 88 (2004) 1–20
benthic nitrogen cycling on the marine dissolved
inorganic nitrogen (DIN) pool. Yet, there has been
only limited study of the typical effective e for
sedimentary denitrification and the isotopic impact
of other benthic N cycle reactions on the overlying
water column.
Fig. 1 compiles the most important sources and
sinks of nitrate in shallow marine sediments, which
include the microbial oxidation (nitrification) of ammonium (Flux B) that is generated from organic matter
mineralization (Flux FMin), the net diffusive flux of
nitrate across the sediment – water interface (Flux
Db Flux C), and the loss of nitrate due to microbial
denitrification ( FDenit = Fluxes Dn + Db). Very often,
nitrification rather than the diffusive flux from overlying waters is the major source of nitrate for denitrification within the sediments (Christensen et al., 1987;
Seitzinger, 1988; Devol and Christensen, 1993; Middelburg et al., 1996; Jahnke and Jahnke, 2000), and
nitrification can produce a net nitrate flux out of the
sediments, even in actively denitrifying sedimentary
environments. Because partial nitrification occurs with
a marked isotope effect (e.g., Casciotti et al., 2002),
newly produced nitrate can be significantly depleted in
15
N with respect to substrate ammonium. Indeed,
Hartnett (1998) reported that, although nitrate was
being consumed by denitrification in Washington and
Mexican continental margin sediments, nitrate in overlying water became strongly depleted during incubation experiments, due to the efflux of low-15N/14N
nitrate from nitrification within the sediments.
Here, we report nitrate-isotope data from in situ
incubation experiments conducted in the Santa Monica Bay (SMB). The main objective of this study is to
use the combined measurement of 15N/ 14N and
18 16
O/ O (dual isotope approach) in benthic-chamber
nitrate as a constraint on the role of microbial nitrate
production within sediments, an uncertainty that has
been recognized but not directly addressed in the
estimation of the N isotope effect of sedimentary
denitrification. Using a simple model, we explore
the effect of variations in the relative importance of
nitrification and denitrification on the benthic nitrate
N and O isotope dynamics. Using the dual-isotope
approach, we find that SMB sediments, despite active
bio-irrigation, are ‘‘tight’’ with respect to nitrate; that
is, a given nitrate ion that is transported in from
bottom water or produced by nitrification within the
3
sediment will most likely be consumed before it has
the opportunity to exit the sediment.
2. Sampling and methods
Benthic chamber devices were deployed in the
Santa Monica Bay in August 2000 and March 2001
at four sites, all of which are shallower than 65 m
water depth (Fig. 2). The sediments on the SMB shelf
are characterized by relatively high total organic
carbon contents (5% to 8%). In general, bioturbation
plays an important role in the SMB sediments and the
solute transport by bio-irrigation may be a widespread
phenomenon (Fraser and Berelson, unpublished data).
Oxygen (O2) concentration within sediments were not
determined, but modeled bioirrigation depths between
2 and 15 cm (Fraser and Berelson, unpublished data)
may indicate O2 penetration depths of several centimeters. The O2 concentration in the bottom water
ranged between 110 and 150 AM. Benthic fluxes were
measured using techniques described in Berelson and
Hammond (1986). Each chamber was sampled six
times during deployment periods of 12 to 21 h. The
O2 concentration within the chamber was monitored
by a pulsed electrode with readings made every 6 min.
In all experiments, chamber water remained oxic
during the incubation period. At each site, bottomwater samples were collected with a 5 l Niskin bottle.
Fig. 2. Benthic chamber deployment sites (Malibu, Topanga,
Marina del Rey, Hyperion) in the Santa Monica Bay (California).
4
M.F. Lehmann et al. / Marine Chemistry 88 (2004) 1–20
Water samples were filtered (0.45 Am) and analyzed
within 24 h or stored frozen for later analyses.
Ammonium, nitrate, nitrite (in 2001 only), phosphate,
and silicic acid were analyzed by colorimetric autoanalyzer techniques with replicate precision of 0.2,
0.1, 0.01, 0.05, and 1 Amol/l, respectively. Alkalinity
(TA) was determined by titration using 5 ml sample
aliquots, with a precision of 10 Amol/l. Total dissolved
inorganic carbon (TCO2) was calculated from TA and
pH. In addition, it was measured directly using a
coulometer (UIC 5012). Most of the TCO2 fluxes
reported here represent mean values combining the
two methods, and the uncertainty in the mean was
calculated two ways: (a) as the standard error of the
mean and (b) as the uncertainty expected in the mean
of a group considering the uncertainty of each value.
The larger of these two uncertainties is reported.
Oxygen electrode measurements were calibrated using
Winkler titration. All chamber concentrations were
corrected for dilution due to sample withdrawal from
the chamber and subsequent volume replenishment
with ambient water. Benthic fluxes were estimated
from the derivative of the linear fit to the concentrations through time and the chamber volume, as
determined from dilution of a Br-tracer injected early
in the incubation. Solute concentration change
through time was not always linear. Flux calculations
were made using the initial linear portions of the
concentration versus time plots. The uncertainty in
the flux includes analytical precision of concentration
measurements, the uncertainty associated with the
linear regression, and the uncertainty in the determination of the benthic chamber volume.
The N and O isotopic composition of dissolved
nitrate + nitrite was determined with the ‘‘denitrifier’’
method of Sigman et al. (2001) and Casciotti et al.
(2002) (henceforth, when reporting isotope ratios, the
term ‘‘nitrate’’ includes the very small contribution,
generally < 2%, from nitrite). In brief, sample nitrate
was converted to nitrous oxide (N2O) by denitrifying
bacteria that lack N2O-reductase activity, Pseudomonas chlororaphis (ATCC# 43928) or Pseudomonas
aureofaciens (ATCC# 13985). N2O was stripped from
the sample vial using helium as carrier gas, purified,
and analyzed for its N and O isotopic composition.
Oxygen atom exchange with H2O and blank size were
measured for each sample run and corrected for following the procedure described in Casciotti et al.
(2002). Oxygen exchange was always < 3% (for
P. aureofaciens) and the blank was generally less than
0.3 nmol N2O (as compared to 10 –20 nmol in the
sample). The use of P. aureofaciens allowed for the
simultaneous measurement of nitrate 15N/14N and
18 16
O/ O ratios, whereas P. chlororaphis was only used
for N isotope analysis (Sigman et al., 2001; Casciotti
et al., 2002). There was good agreement between d15N
measurements using P. aureofaciens and those using P.
chlororaphis. Nitrogen and oxygen isotope ratios are
reported in the conventional d-notation with respect to
atmospheric N2 (AIR) and V-SMOW, respectively:
Rsample
dsampleðxÞ ¼
1 1000
ð1Þ
Rstandard
where R represents the 15N/14N or 18O/16O ratios,
respectively. Isotope values were calibrated using
IAEA-N3, an international KNO3 reference material
with an assigned d15N of + 4.7x(Gonfiantini et al.,
1995) and reported d18O of + 22.7xto + 25.6x
(Revesz et al., 1997; Silva et al., 2000; Lehmann
et al., 2003; Böhlke et al., 2003). In this study, we
adopt a d18O of + 22.7x
. Based on replicate measurements of standards and samples, the analytical
precision for d15N and d18O was generally better
than F 0.2xand F 0.3x(1 S.D.), respectively.
3. Results and discussion
3.1. Benthic flux stoichiometry and denitrification
The overall trends in concentration –time data are
consistent with metabolic oxidation of sedimentary
organic matter during early diagenesis and the associated remineralization of organic carbon, nitrogen, and
phosphorous (Fig. 3). The sediments represent a sink
for dissolved O2 and a source of TCO2, ammonium,
and phosphate (Table 1). Measured nitrate fluxes
suggest that, in general, the SMB sediments are a net
nitrate sink (negative fluxes in Table 1). In some cases,
however, the nitrate concentration within the benthic
chamber increased slightly during benthic chamber
incubation or did not change significantly. Whether
nitrate is released from or consumed within the sediments indicates the relative importance of N-producing and N-consuming processes. Nitrite fluxes out of
M.F. Lehmann et al. / Marine Chemistry 88 (2004) 1–20
5
Fig. 3. Examples of flux-chamber time-series from Station 1 at the Hyperion site in March 2001. Bottom-water solute concentrations are shown
on the vertical axes (t = 0). The different symbols represent different chambers deployed at the same station. Shown TCO2 data were determined
coulometrically.
the sediment generally contribute less than 3% to the
overall DIN flux; hence, they are not included in the
subsequent discussion of the benthic N budget.
The rate at which organic N is remineralized during
organic matter decomposition in the sediments ( FMin)
is not measured directly. We use diagenetic stoichiometry to estimate this flux, with the relationship between
TCO2, PO43 , and ammonium fluxes being set by
assuming ‘Redfield’ behavior (C:N:P = 106:16:1; Redfield, 1958). As outlined below, we found that using
TCO2 fluxes is generally the most viable approach to
predict the flux of N from organic matter decomposition and subsequently derive denitrification rates. To
calculate the latter, we adapted the concept of N*
(Gruber and Sarmiento, 1997; Deutsch et al., 2001).
Here, N* quantifies the deviation of the [DIN]:[TCO2]
6
M.F. Lehmann et al. / Marine Chemistry 88 (2004) 1–20
Table 1
Summary of benthic flux measurements in the Santa Monica Bay
Station/chamber
August 2000
Hyperion
1-B
1-R
Malibu
2-B
2-R
7-B
7-R
Marina del Rey
8-B
March 2001
Hyperion
1-B
1-R
1-Y
5-Y
Topanga
3-B
3-R
3-Y
Marina del Rey
4-B
O2 (mmol
m 2 d 1)
TCO2 (mmol
m 2 d 1)
ALK (mmol
m 2 d 1)
PO34 (mmol
m 2 d 1)
NO
3 (mmol
m 2 d 1)
NH+4 (mmol
m 2 d 1)
Si(OH)4 (mmol
m 2 d 1)
6.6 F 1.4
7.0 F 1.4
14.8 F 6.3*
13.3 F 5.8*
11.0 F 5.2
10.5 F 5.7
0.07 F 0.048
0.05 F 0.04
0.10 F 0.21
0.06 F 0.06
0.21 F 0.17
0.31 F 0.19
2.3 F 0.3
2.4 F 0.2
10.3 F 2.1
11.4 F 2.4
18.7 F 3.8
8.9 F 1.9
9.5 F 3.7
12.1 F 3.0
15.3 F 2.7
9.7 F 1.7
4.1 F 3.8
4.7 F 3.7
6.5 F 3.2
2.4 F 2.1
0.04 F 0.05
0.07 F 0.03
0.08 F 0.02
0.13 F 0.02
0.14 F 0.06
0.06 F 0.05
0.11 F 0.05
0.94 F 0.32
0.15 F 0.12
0.26 F 0.12
0.59 F 0.15
0.26 F 0.07
3.5 F 0.4
3.8 F 0.5
4.1 F 0.6
3.8 F 0.4
9.9 F 2.3
10.3 F 2.4
5.2 F 2.5
0.065 F 0.18
0.01 F 0.1
0.08 F 0.08
0.51 F 0.09
6.0 F 1.2
4.9 F 1.1
6.8 F 1.5
7.8 F 2.0
13.5 F 2.4
13.0 F 4.1
11.2 F 2.5
17.9 F 6.8
8.9 F 2.4
8.5 F 2.3
6.3 F 3.3
13.8 F 9.1
0.164 F 0.036
0.155 F 0.036
0.118 F 0.035
0.306 F 0.09
0.81 F 0.08
0.72 F 0.20
0.63 F 0.10
1.09 F 0.22
0.86 F 0.09
1.15 F 0.16
0.76 F 0.13
1.75 F 0.35
2.30 F 0.20
1.97 F 0.32
2.26 F 0.30
3.78 F 0.68
2.3 F 2.3
3.6 F 2.6
3.1 F 2.7
0.164 F 0.035
0.09 F 0.031
0.143 F 0.039
0.36 F 0.07
0.33 F 0.10
0.37 F 0.07
0.52 F 14
0.15 F 0.05
1.10 F 0.27
2.30 F 0.14
1.90 F 0.27
2.48 F 0.30
12.6 F 5.8
0.152 F 0.051
0.4 F 0.13
0.58 F 0.09
2.49 F 0.38
5.3 F 1.0
5.9 F 1.3
6 F 1.3
6.0 F 1.2
2.7 F 3.0*
5.2 F 3.4
4.4 F 6.1
12.4 F 5.7
Negative fluxes are from the overlying water into the sediments. Unless indicated by an asterisk, TCO2 fluxes represent mean values combining
measured and calculated (based on TA and pH) TCO2 concentrations. TCO2 fluxes with an asterisk were derived using only calculated TCO2
concentrations.
ratio from ‘Redfield’ N:C stoichiometry. The flux of
N*TCO2 (FNc*) is defined as:
FNc* ¼ FNO3 þNO2 þ FNH4 ð16=106 FTCO2 Þ
ð2Þ
Negative N*TCO2 fluxes (Table 2) correspond to
lower than expected DIN:TCO2 ratios and suggest
that the SMB sediments are a net sink of N. FNc* can
be directly translated into the total denitrification rate
FDenit (assuming that denitrification is the dominant
N2-producing process) and (16/106) FTCO2 equals
the total remineralization of organic N ( FMin). Accordingly, the nitrification rate ( FNit) equals the difference between FMin and FNH4. TCO2 flux would
overestimate organic C oxidation in the case of
carbonate dissolution. A relatively high TA flux with
respect to TCO2 flux could be taken to indicate
carbonate dissolution in the SMB. However, from
similar depositional systems, it has been reported that
carbonate dissolution plays a relatively unimportant
role (less than 5% of the TCO2 flux; McNichol et al.,
1988; Hammond et al., 1999; Hopkinson et al., 2001),
suggesting that the TA flux in the SMB is primarily
driven by net sulfate reduction rather than carbonate
dissolution (ammonium production and nitrate consumption also contribute to the total TA flux but only
at a low level).
In an alternative approach to calculate total denitrification rates, we use phosphate fluxes according to
Eq. (3):
FNp* ¼ FNO3 þNO2 þ FNH4 ð16 FPO4 Þ
ð3Þ
If the PO43 flux is a conservative tracer of benthic
organic matter decomposition (that is, P displays
Redfield-stoichiometric behavior during mineralization and is not produced or consumed by additional
processes), FNp* should, again, be a direct measure of
the total denitrification rate FDenit. In some cases, the
denitrification rate based upon phosphate flux agrees
M.F. Lehmann et al. / Marine Chemistry 88 (2004) 1–20
Table 2
N* fluxes
Station/chamber
August 2000
Hyperion
1-B
1-R
Malibu
2-B
2-R
7-B
7-R
Marina del Rey
8-B
March 2001
Hyperion
1-B
1-R
1-Y
5-Y
Topanga
3-B
3-R
3-Y
Marina del Rey
4-B
FNc* (mmol
m 2 d 1)
FNp* (mmol
m 2 d 1)
2.12 F 1.03
1.76 F 0.94
1.01 F 0.83
1.05 F 0.68
1.15 F 0.61
1.50 F 0.53
1.83 F 0.54
2.14 F 0.46
0.93 F 0.82
0.80 F 0.52
0.80 F 0.41
2.76 F 0.55
1.46 F 0.44
0.95 F 2.89
1.93 F 0.47
1.52 F 0.72
1.52 F 0.47
2.01 F 1.16
2.56 F 0.72
2.04 F 0.75
1.75 F 0.66
4.23 F 1.70
0.24 F 0.73
0.95 F 0.53
0.07 F 0.97
2.47 F 0.72
1.62 F 0.56
1.56 F 0.80
1.64 F 0.91
2.25 F 0.91
N* fluxes represent a direct measure of the total denitrification rate.
Discrepancies between TCO2-based ( FNc* ) and PO34 -based ( FNp* )
N* fluxes are in most cases most likely due to the nonconservative
behavior of phosphate during early sedimentary diagenesis (PO34 retention in sediments).
well with that calculated using TCO2 flux (Table 2),
supporting our assumption that marine phytoplankton
is the primary organic matter source in the SMB,
undergoing diagenesis according to near-Redfield
stoichiometry. However, for some locations, and particularly in August 2000, when DIN and phosphate
fluxes were generally lower and afflicted with a high
relative uncertainty, FNp* values do not correspond
well to FNc* values. This imbalance is probably
primarily due to the fact that phosphate is not always
a conservative indicator of benthic metabolism (that
is, P remineralization also involves authigenic and Fesorbed P). Hence, we conclude that using phosphate
fluxes is not a viable alternative approach to derive
total denitrification rates in the SMB.
The consumption of O2 and the efflux of Si(OH)4
can give additional constraints on organic matter
diagenesis. Rates of O2 consumption were often
7
significantly lower than the TCO2 flux out of the
sediment (Table 1), suggesting that anaerobic organic
matter decomposition (denitrification, sulfate reduction) accounts for a large portion of total organic
carbon oxidation at these sites, and that reduced end
products of anaerobic respiration remained stored in
the sediments or were released to the overlying water
prior to being reoxidized. Thus, O2-based calculations
of N and P fluxes in the SMB would lead to underestimates. The fact that ‘‘for about half the chamber
experiments’’ Si(OH)4:TCO2 flux ratios are relatively
close to an elemental ratio typical for diatoms
(0.13 F 0.5; Brzezinski, 1985) suggests that fresh pelagic diatoms represent the major biogenic component
in SMB sediments. In turn, the Si(OH)4 data support
the TCO2 flux as generally a good measure of benthic
metabolic rates in this environment. Hence, we favor
the use of TCO2 flux as a basis for calculating ammonium production and total denitrification rates.
Calculated total denitrification rates are mostly
between 1.0 and 2.0 mmol N m 2 d 1 (Table 3).
The overall mean of 1.74 F 0.33 mmol N m 2 d 1
falls into the range of denitrification rates reported for
other coastal marine environments (compilation in
Seitzinger, 1988; Hopkinson et al., 2001; Laursen
and Seitzinger, 2002 and references therein). There
was not a consistent variation in total denitrification
rate among the different locations or between seasonal
regimes. The CO2 flux that is due to denitrification
can be estimated according to the equation:
ðCH2 OÞ106 ðNH3 Þ16 ðH3 PO4 Þ þ 84:8 HNO3
! 106 CO2 þ 42:4 N2 þ 16 NH3 þ H3 PO4
þ 148:4 H2 O
ð4Þ
(Froelich et al., 1979). In the SMB sediments, 12% to
22% (on average 15%) of the total carbon mineralization can be attributed to microbial denitrification.
3.2. Benthic nitrogen pathways
The fraction of FMin that is denitrified in the
sediments depends on two branching points: (1) the
fraction of remineralized N that is nitrified within the
sediments (B versus A in Fig. 1) and (2) the fraction of
nitrified N that is denitrified within the sediments (Dn
versus C in Fig. 1). The ratio of the ammonium flux
8
M.F. Lehmann et al. / Marine Chemistry 88 (2004) 1–20
Table 3
Total organic N remineralization rates FMin, nitrification rates FNit, denitrification rates FDenit, and ammonium – efflux ratios QNH4 in the SMB
Station/chamber
August 2000
Hyperion
1-B
1-R
Malibu
2-B
2-R
7-B
7-R
Marina del Rey
8-B
March 2001
Hyperion
1-B
1-R
1-Y
5-Y
Topanga
3-B
3-R
3-Y
Marina del Rey
4-B
FMin (mmol N
m 2 d 1)
FNit (mmol N
m 2 d 1)
FDenit (mmol N
m 2 d 1)
QNH4
(%)
2.23
2.01
2.02 F 1.03
1.70 F 0.94
2.12 F 1.03
1.76 F 0.94
9.4
15.4
95
97
1.43
1.83
2.31
1.46
1.29 F 0.61
1.56 F 0.53
1.83 F 0.54
1.20 F 0.46
1.15 F 0.61
1.50 F 0.53
1.83 F 0.54
2.14 F 0.46
20.1
17.6
25.5
17.8
112
104
100
56
1.55
1.47 F 0.44
1.46 F 0.44
5.8
101
2.04
1.96
1.69
2.70
1.12 F 0.46
0.78 F 0.69
0.89 F 0.46
0.92 F 1.14
1.93 F 0.47
1.52 F 0.72
1.52 F 0.47
2.01 F 1.16
44.9
59.9
47.3
65.9
58
52
59
46
2.62
1.44
2.29
2.10 F 0.72
1.28 F 0.56
1.18 F 0.80
2.47 F 0.72
1.62 F 0.56
1.56 F 0.80
19.9
11.6
48.5
85
79
76
1.87
1.24 F 0.91
1.64 F 0.91
33.5
76
FNit/FDenit
(%)
FMin is predicted from ‘Redfield’ stoichiometry. The nitrification rate FNit is, in essence, the difference between the predicted FMin and the
observed ammonium flux out of the sediments. The denitrification rate FDenit is the difference between FNit and the observed nitrate flux (where
nitrate fluxes into the sediments have a negative sign). The ammonium – efflux ratio QNH4 is calculated by dividing the net flux of ammonium
out of the sediments by FMin. (1 QNH4) can be translated into an upper limit of the percentage of remineralized N that is eventually denitrified
(i.e., ‘‘denitrification efficiency’’, Berelson et al., 1998). The ratio FNit/FDenit defines the maximum portion of total denitrification that may be
fueled by nitrification within the sediments. FMin and dependent parameters are mostly calculated using TCO2 fluxes. Only at Station 3
(Topanga) in March 2001, phosphate fluxes are used to predict total organic N remineralization. The validity of thus derived values for FNit,
FDenit, and QNH4 at this station is corroborated by P:Si flux ratios close to the expected value of 1:15.
out of the sediments (Flux A) over the total amount of
remineralized nitrogen ( FMin), from now on referred
to as the ammonium – efflux ratio QNH4, characterizes
the first branching point: the lower QNH4 is, the higher
is the fraction of FMin that is available for denitrification. This branching point can be estimated from the
benthic flux data alone ( FNH4/[(16/106)FTCO2]).
The ‘‘coupling of nitrification to denitrification’’
describes the second branching point. GCND, which
denotes the degree of this coupling, is the ratio of the
fraction of newly produced nitrate that is denitrified in
the sediment over the total amount of nitrified nitrate
(Flux Dn/(Flux C + Flux Dn) in Fig. 1). The closer the
coupling between nitrification and denitrification (or
the higher GCND), the lower is the portion of newly
produced nitrate fluxing out of the sediments. As
described next, GCND is not constrained by the benthic
flux data alone but does influence isotope data.
The net nitrate flux into the sediments determined
from the benthic chamber data is the net inward flux
of nitrate that is consumed by denitrification (Db in
Fig. 1) minus the net outward flux of nitrate driven by
nitrification that is not coupled to denitrification (C in
Fig. 1). Total denitrification ( FDenit) is, in a sense,
composed of two components: the denitrification fed
by the inward flux of bottom-water nitrate (Db) and
that fed directly by nitrification in the sediments (Dn).
In addition, a fraction of the inward flux of bottomwater nitrate driven by Db may be countered by an
outward flux of nitrate from nitrification in the sediments. For both of these reasons, we expect and
observe that the denitrification rate estimated from
M.F. Lehmann et al. / Marine Chemistry 88 (2004) 1–20
benthic flux stoichiometry is significantly higher than
the rate of nitrate loss from benthic chamber water
(Tables 1 and 2). The two alternative explanations for
the difference between FDenit and bottom-water nitrate
disappearance (the denitrification term coupled to
nitrification and the efflux of newly produced nitrate
into the water column) cannot be distinguished based
on the benthic flux data alone. That is, for a given
total denitrification rate FDenit, the benthic flux data
cannot discern a case of higher C, higher Db, and
lower Dn from a case of lower C, lower Db, and higher
Dn. We will see that the nitrate isotopes can distinguish between these two alternatives. In a similar
vein, the isotope data also provide an indication of
how much exchange of bottom water-derived nitrate
occurs across the sediment –water interface (E in Fig.
1), a process that, like the interfering terms C and Db,
cannot be extracted from the benthic flux data alone.
Like denitrification rates FDenit, ammonium – efflux
ratios QNH4 and nitrification rates (listed as FNit and
equivalent to flux B in Fig. 1 are estimated from
benthic flux data at each of the chamber sites (Table
3). In essence, FNit is the difference between denitrification FDenit and the net change in chamber water
[NO3]. FNit has often been referred to as ‘‘the coupled
nitrification –denitrification rate’’, neglecting the uncertainty of gross fluxes of newly produced nitrate out
of the sediment (e.g., Berelson et al., 1998; Laursen
and Seitzinger, 2002). However, in net denitrifying
systems, FNit is equivalent to Dn in Fig. 1 only in the
case where the efflux of newly nitrified nitrate (Flux C
in Fig. 1) is zero. Because we cannot neglect a
possible efflux of newly nitrified nitrate, we restrict
the term ‘‘coupled nitrification – denitrification’’ for
flux Dn in Fig. 1. For the same reason, the nitrification:denitrification ratio in Table 3 defines the maximum portion of total denitrification that may be fueled
by nitrification within the sediments, but does not
necessarily indicate the actual degree of coupling
between nitrification and denitrification.
The highest ammonium –efflux ratio QNH4 (i.e.,
the highest net outward NH4+ flux relative to FMin)
was observed for Station 5-Y (Table 3). Elevated
radon fluxes (data not shown) measured in the same
chamber suggest the enhanced exchange of solutes
across the sediment– water interface due to nondiffusive flux, most likely as a result of bioirrigation. It is very likely that irrigation rates have a
9
strong effect on benthic N cycling (e.g., Blackburn
and Henriksen, 1983; Mayer et al., 1995; Berelson
et al., 1998). Because they represent an important
control on the exchange between NH4+-rich sediment
pore waters and NH4+-poor bottom waters, irrigation
rates are likely to have a significant impact on
QNH4 , and thus on the sediment nitrification-denitrification rate as well. For example, at Station 5-Y,
nondiffusive processes enhanced the escape of
remineralized N from the sediments as NH4+ prior
to its oxidation to nitrate, resulting in a comparatively high value for QNH4 (66%). The bay-wide
ammonium – efflux is lower, with generally less than
50% of the remineralized NH4+ escaping from the
sediments. Bioirrigation will also influence the gross
flux of nitrate into the sediment. Thus, the effect of
bioirrigation on total denitrification rates will depend
on the relative potential of bottom-water nitrate and
pore water ammonium to serve as nitrate sources for
denitrification.
In the SMB, as observed in other coastal marine
systems (Seitzinger, 1988; Laursen and Seitzinger,
2002), the rate of nitrification is >50% of the total
denitrification rate. If the sediment efflux of newly
nitrified nitrate is not significant, then this would
imply that coupled nitrification – denitrification within the sediments is greater than that of ‘‘direct’’
denitrification (where nitrate in sediments is supplied by diffusive flux from overlying water). For
example, in March 2001, although near-bottom
water nitrate concentrations were relatively high
(20 F 2 Amol/l), nitrate from nitrification within
the sediment had the potential to contribute more
than 50% of the nitrate for denitrification. In August
2000, when near-bottom nitrate concentrations were
significantly lower, nitrification may have played an
even more important role, potentially accounting for
90% to 100% (mean 91.7%) of the total denitrification. On average, nitrification could account for
78 F 18% of the nitrate required by the total denitrification rate, attesting to the potential importance
of this process on the role of continental shelf
sediments as effective sinks for fixed nitrogen
(Devol, 1991; Christensen, 1994). However, it must
again be pointed out that, by looking at the benthic
flux stoichiometry alone, it is not possible to
address how much newly nitrified nitrate may
actually be evading to the bottom water.
10
M.F. Lehmann et al. / Marine Chemistry 88 (2004) 1–20
3.3. Effects of benthic N cycling on nitrate d15N and
d18O
The N and O isotopic composition of ‘initial’
nitrate in the SMB chambers as determined in Niskin
bottom-water samples was more or less constant for
all incubation experiments and averaged + 7.8 F
0.45xand + 1.4 F 0.7x
, respectively. Only in
August 2000 at Station 8 (Marina del Rey), where
near-bottom water was strongly nitrate-depleted
([NO3] < 2 Amol/l), d15N and d18O values were significantly higher ( + 9.3xand + 5.0x
, respectively).
As generally lower nitrate concentrations in August
( f 10 Amol/l compared to f 20 Amol/l in March)
are due to phytoplankton assimilation, the same explanation presumably applies to the elevation of
nitrate d15N and d18O. Even under high-[NO3] conditions in March 2001, the nitrate d15N and d18O in
the SMB are high relative to the global ocean mean
d15N of 5x(Sigman et al., 2000) and d18O of f 0x
(a global database for marine nitrate d18O does not yet
exist but deep North Pacific nitrate has a d18O close to
that of seawater ( f 0x); Casciotti et al., 2002; A.
Knapp, M.F. Lehmann, and D.M. Sigman, unpublished data). The elevated nitrate d15N in subsurface
SMB nitrate during winter fit well into the general N –
S trend along the coast of Mexico and California, with
a continuously increasing nitrate d15N toward the
eastern tropical North Pacific (ETNP; Altabet et al.,
1999; Sigman et al., 2003). Therefore, it indicates the
impact of a northward undercurrent of denitrificationinfluenced water from the ETNP (Altabet et al., 1999),
which has been recognized as an important region of
water-column denitrification and, hence, as a source
of 15N-enriched nitrate (Cline and Kaplan, 1975;
Brandes et al., 1998; Sigman et al., 2000; Voss et
al., 2001).
Bacterial denitrification is associated with a substantial intrinsic isotope fractionation of 25 F 5x
(e.g., Mariotti et al., 1981; Brandes et al., 1998;
Barford et al., 1999). Although a significant amount
of nitrate was lost from most of the chambers through
denitrification within the sediments, the nitrate N
isotope ratios at the end of the deployment period
were not markedly different from those in the beginning (Fig. 4). This basic observation is consistent with
observations made by Brandes and Devol (1997,
2002) during incubation experiments in the Puget
Sound and along the Washington and Mexican continental margins.
There are two possible explanations for the lack of
N isotope enrichment observed in the SMB. Firstly, as
proposed by Brandes and Devol (1997), the effective
N isotope fractionation is indeed close to zero because
denitrification is limited by diffusion through sediments. Secondly, a nonzero isotope effect of sedimentary denitrification on the N isotopic composition of
nitrate in overlying water may be effectively balanced
by a 15N-depleted nitrate source, i.e., nitrate from the
oxidation of ammonium. The latter option represents
an alternative that has not been addressed in detail by
previous work. In fact, in the chamber experiment 5Y, where the net DIN flux out of the sediment was
greatest, we indeed observe a subtle but continuous
decrease in d15N from + 7.8xto + 6.8xduring
deployment time (Fig. 4). This suggests that some
fraction of the 15N-depleted nitrate produced via
nitrification of ammonium in oxic sediments is not
consumed by denitrifying bacteria and ends up in the
benthic chamber, counteracting any N isotope enrichment of chamber nitrate due to denitrification. This
observation is consistent with results reported from
lander-incubation and whole-core incubation experiments on the Mexican and Washington continental
margins (Hartnett, 1998), where decreasing nitrate
d15N trends corresponded to net nitrate consumption
within the sediments, and nitrification occurring with
a significant isotope fractionation was suggested to be
the source of 15N-depleted nitrate.
The partitioning between ammonium and newly
produced nitrate fluxing out of the sediment and
ammonium being nitrified and then denitrified within
the sediments not only controls the amount of fixed
nitrogen escaping from the sediments but also affects
its 15N/14N. Thus, the ammonium – efflux ratio QNH4
and the degree of coupling between nitrification and
denitrification are important constraints on the total
isotope effect of benthic N cycling in the overlying
water.
Three specific scenarios will elucidate this point. In
all of these scenarios, NH4+ with a d15N of + 6xis
produced during mineralization of organic matter,
given that the d15N of surface-sedimentary organic
matter in SMB is f 6x(data not shown) and
assuming that eMineralization c 0x(Wada et al.,
1980; Kendall, 1998). The intrinsic N isotope effect
M.F. Lehmann et al. / Marine Chemistry 88 (2004) 1–20
11
Fig. 4. Nitrate d15N and d18O during benthic chamber incubations in March 2001. Different symbols represent chambers deployed at different
stations (1-B, 1-Y, 5-Y: Hyperion; 3-B, 3-R: Topanga; 4-B: Marina del Rey). Error bars represent standard deviations (r1) calculated from
duplicate or replicate isotope analyses. For samples that were analyzed only once, the length of the error bars are based upon estimates on
instrument precision ( F 0.2xfor d15N and F 0.3xfor d18O, respectively).
for nitrification can vary substantially with the molecular nitrification enzymes but is generally large
( 12xto 38x
; Mariotti et al., 1981; Horrigan
et al., 1990; Casciotti et al., 2003). Here, we adopt an
average enrichment factor of 17x
, derived from
experiments with two marine nitrifier isolates (Casciotti et al., 2003). Under steady-state conditions in
the SMB sediments, if the ammonium from organic
matter degradation is c ompletely nitrified
( QNH4 = 0%), then newly produced nitrite +nitrate
will, by mass balance, have the same d15N as the
source organic matter ( + 6x). At the other extreme,
if almost all of the remineralized ammonium fluxes
out of the sediments before being oxidized to nitrate
( QNH4 c 100%), its d15N would be + 6x
. In this
case, the small fraction of the ammonium that is
nitrified will yield nitrate that is extremely depleted
in 15N ( 11x
). In an intermediate scenario, where
50% of the ammonium is lost by diffusion out of the
sediments and the other 50% is oxidized by nitrifying
bacteria ( QNH4 = 50%; e.g., March 2001 Stations 1-B,
1-Y, 3-Y; Table 3), at steady state, the d15N of newly
produced nitrate will be 2.5xand that of effluxing
ammonium will be + 14.5x(similar d15N values for
effluxing ammonium from Puget Sound sediments
have been reported by Brandes and Devol, 1997). In
essence, the lower the fraction of NH4+ being nitrified
within the sediments (or the higher the QNH4, which
equals Flux B/(Flux A + Flux B) in Fig. 1), the lower is
the d15N of product nitrate that potentially contributes
to the nitrate d15N signal in the benthic chamber.
However, there is one additional consideration: nitrate
from nitrification can only be a ‘‘light’’ nitrate source
to overlying water if it is not readily utilized by
denitrifying bacteria before entering the benthic chamber, that is, if nitrification is not closely coupled to
denitrification within the sediments. The greater the
fraction of newly produced nitrate that is denitrified
within the sediments (i.e., high GCND and close
coupling of nitrification and denitrification), the
smaller is the potential flux of nitrate into the overlying water. Moreover, this denitrification would also
transform the pore water nitrate from 15N-depleted to
15
N-enriched.
From the nitrate d15N data alone, it is not possible
to infer a priori whether the apparent N isotope effect
associated with denitrification in SMB sediments is
indeed close to 0xor whether it is significantly
higher but effectively balanced by the gross flux of
low-15N nitrate from nitrification. The d15N of N2 gas
fluxing out of the sediments could provide additional
information on the N isotope balance (see Brandes
and Devol, 1997). However, large amounts of background N2 in marine waters limit the use of N2 isotope
12
M.F. Lehmann et al. / Marine Chemistry 88 (2004) 1–20
measurements to constrain the system. It can also be
shown that the d15N of N2 would not provide a
constraint on the net isotope effect of sedimentary
denitrification because it does not allow for the actual
DIN pathways to be distinguished. Ammonium isotope data should allow for characterization of QNH4
but not the degree of nitrification – denitrification
coupling. More generally, N-isotope ratios in nitrogen
species do not unequivocally allow for a deconvolution of coexisting nitrate production, consumption,
and gross diffusive transport terms.
Nitrate d18O provides the constraint needed to
disentangle these co-occurring N cycle reactions (Lehmann et al., 2003; Sigman et al., 2003). Nitrate
15 14
N/ N and 18O/16O are similarly affected by microbial denitrification. As a result, if the isotope effect is
expressed at all, both the d15N and d18O in residual
nitrate increase with the degree of nitrate consumption, and, at least in marine environments, the ratio of
15
N and 18O enrichment seems to be close to unity
(Casciotti et al., 2002; Sigman et al., 2003). In
contrast to denitrification, nitrate production by nitrification has a very different effect on the d15N and
d18O of the nitrate pool. It was traditionally assumed
that two-thirds of the oxygen atoms in nitrificationderived nitrate originated from ambient H2O and onethird from dissolved O2 (Kumar et al., 1983; Durka et
al., 1994; Bohlke et al., 1997). More recently, Casciotti et al. (2002) argued that less than one sixth of
the oxygen atoms in marine nitrate come from dissolved O2, which was supported by their observation
that the d18O of deep-sea nitrate is close to 0xversus
V-SMOW. Either way, given a d18O of + 1.6 F 0.8x
for bottom-water nitrate in the SMB, the d18O of
nitrate within the benthic chamber should change very
little due to microbial nitrification. This is in contrast
to the large 15N-depleting effect of nitrification (see
above). Thus, if there were a flux of remineralized
(i.e., nitrified) nitrate out of the sediments, the N and
O isotopes of chamber nitrate would evolve in different ways. This does indeed occur in chamber 5-Y,
which shows the greatest evidence for irrigationdriven efflux of solutes from the sediments: the
chamber nitrate in 5-Y decreases in d15N but not in
d18O (filled circles in Fig. 4). Thus, the possible
contribution of nitrification to nitrate in the chamber
can be discerned by the combined measurement of
nitrate d15N and d18O.
We use a simple one-box model (based on the
scheme depicted in Fig. 1) that simulates the change
of nitrate d15N and d18O in a benthic chamber as a
function of the effective isotope enrichment associated
with denitrification, and as a function of the N and O
isotopic composition of newly produced nitrate.
Based upon recent marine studies (Casciotti et al.,
2002; Sigman et al., 2003), we assume that the isotope
effect for O is the same as for N. We assume steadystate conditions within the sediments and an N isotope
effect for nitrification of 17x(the effective N
isotope fractionation may actually be lower because
ammonium is not nitrified from a homogenized ammonium pool). d18O of newly produced nitrate is
assumed to be 0x, independent of the ammonium –
efflux ratio. For each time step, we calculate the
amount and the isotopic composition of nitrate lost
due to denitrification and gained through nitrification,
respectively.
The isotopic composition of newly produced nitrate is controlled by the partitioning of DIN fluxes
which, in turn, is determined by the degree of coupling between nitrification and denitrification (given
as GCND = % of newly produced nitrate being denitrified within the sediments) and, in the case of d15N,
by QNH4 (45% in Fig. 5A and 10% in Fig. 5B). Again,
GCND is the ratio Flux Dn/(Flux C + Flux Dn), where
Flux C represents the fraction of newly produced
nitrate that escapes from the sediment to the overlying
water and Flux Dn represents that fraction of newly
produced nitrate that is denitrified within the sediments before it diffuses into the bottom water.
It can be seen that the trend in nitrate d15N that is
observed in the model benthic chamber depends on
the relative contribution of denitrification and nitrification as well as on the isotope fractionation associated with denitrification. Denitrification occurring
with even a small apparent isotope effect will drive
the d15N of nitrate in overlying water towards higher
values if ammonium oxidation does not play a significant role at all or if it is closely coupled to denitrification in the sediments. On the other hand, if
nitrification contributes to a gross flux of nitrate out
of the sediments and that nitrate results from incomplete nitrification, the d15N of benthic chamber nitrate
can decrease. The DIN pathways control the degree of
N isotope change. The simultaneous effects of the
apparent N isotope enrichment due to isotope frac-
M.F. Lehmann et al. / Marine Chemistry 88 (2004) 1–20
tionation during sedimentary denitrification and the
release of 15N-depleted microbial nitrate can potentially cancel each other (circles in the upper panels of
Fig. 5A and B), producing no N isotope change.
Similarly, no N isotope change is observed when both
e = 0xand GCND = 100% (crosses in the upper
panels of Fig. 5A and B). Hence, two completely
different scenarios can create the same N isotope
trends in benthic chamber nitrate, indicating that the
d15N values alone do not yield unambiguous information on GCND and e.
The nitrate d18O that is observed in the model
chamber is also dependent on the isotope effect
associated with sedimentary denitrification and the
coupling of nitrification and denitrification (Fig. 5A
and B, lower panels). However, the opposing effect
of microbial nitrate input to the chamber water on
the d18O of chamber nitrate is insignificant because,
in contrast to d15N, the initial nitrate d18O and that
from nitrification do not differ much. Different
scenarios (varying values for e and GCND) result in
characteristic d15N –d18O combinations; hence, these
signatures provide constraints on the apparent isotope effects and the degree of coupled nitrification–
denitrification. It is noteworthy that the exact sensitivity of this dual-isotope technique depends on the
boundary conditions (initial d15 N NO 3 , d18 O NO 3 ,
[NO 3], eNitrification , Q NH 4 , denitrification rates,
d15Norg). For example, in marine environments with
relatively high QNH4, the dissimilarity of coupled N
and O signatures for varying values of GCND will be
more pronounced because (more 15N-depleted) microbial nitrate added to the chamber nitrate will have
a higher potential to drive the N isotopic composition of chamber nitrate away from its initial value.
The fact that both nitrate d15N and d18O generally do
not change during the benthic chamber experiments
in the SMB is consistent only with a setting where
the ‘‘effective’’ N and O isotope effects associated
with denitrification are not greater than 1 F 1x,
and most of the nitrate from microbial nitrification is
completely consumed within the sediments; that is,
no significant gross efflux of nitrate occurs (Fig. 6).
As outlined above, the ammonium – efflux ratio QNH4
represents one control on the amount and isotopic
composition of nitrate potentially contributing to the
bottom-water nitrate pool. Accordingly, additional
evidence in support of our conclusion that the gross
13
flux of nitrate out of the sediments is generally close
to zero is provided by the fact that the SMB chamber
isotope trends show no sign of correlation with
variations in QNH4. As can be seen when comparing
Fig. 5A and B, different QNH4 values as observed in
the SMB would lead to significant changes in the
spectrum of possible effective isotope effects if a
significant flux of nitrate out of the sediments
occurred, yet this is not observed.
Although overall denitrification rates varied quite
significantly among benthic chamber incubations (Table 3), the N and O isotopic trends observed during
deployment were always similar, showing no significant change. We thus infer that, independent of the
overall denitrification rate, diffusion-limitation led to
complete nitrate removal within SMB sediments. In
the SMB, solute exchange between pore water and
bottom water is dominated by nondiffusive transport
(Fraser and Berelson, unpublished data), so that it
would seem to be an ideal environment to observe
some expression of the large ( f 25x
) intrinsic
isotope effect of denitrification. This suggests that the
lack of expression of the denitrification isotope effect
originally noted by Brandes and Devol (1997) is
likely to be a widespread phenomenon. Mechanistically, it yields two possible alternative causes for this
lack of expression. The impact of microsites, highlighted by Brandes and Devol as the cause for the lack
of isotope effect expression, may be important in a
broad range of environments. Alternatively, other
geometries of nitrate diffusion and uptake may be
equally as effective at driving diffusion-limitation,
such that the loss of nitrate by denitrification is not
associated with any net isotope fractionation. Generally, no apparent isotope fractionation occurs when
denitrification completely consumes the nitrate in
sediment pore waters. It needs to be stressed that
even if sedimentary denitrification is not associated
with any significant effective isotope effect (measured
in the overlying water), it is certainly associated with
an intrinsic/biologic isotope effect. Pore water nitrate
isotope data (Sigman et al., 2001; Lehmann et al.,
unpublished) clearly show that denitrification within
sediments is associated with a marked intrinsic isotope effect probably not much different from that
observed in open-water environments. Yet, whether
this isotope effect is communicated with nitrate in
overlying water is likely to depend on the extent of
14
M.F. Lehmann et al. / Marine Chemistry 88 (2004) 1–20
M.F. Lehmann et al. / Marine Chemistry 88 (2004) 1–20
diffusion limitation across the sediment – water interface or into microzones.
We argue above that the overall effect of benthic
nitrogen cycling on the isotopic composition of nitrate
in a benthic chamber is sensitive to the degree of
coupling between nitrification and denitrification. As
a consequence, one might expect that the redox
condition of the sediments, which controls the spatial
coexistence of O2, NH4+, and NO3, and thus regulates
nitrification and denitrification (Rysgaard et al.,
1994), has a strong influence on the nitrate N and O
isotope effects perceived in the water column. Based
on work by Bender (1990) and Brandes and Devol
(1997), one tends to expect a significantly higher
apparent N and O isotope fractionation associated
with denitrification in less actively denitrifying environments, where concentration gradients are weaker
and diffusion-limitation is less important. However,
particularly in environments where nitrate production
exceeds nitrate consumption (e.g., deep-sea sediments), it is very likely that nitrate that escapes to
the overlying water carries the depleted isotope signal
of nitrification rather than that of denitrification because nitrification occurs in shallower parts of the
sediment column than denitrification (Sigman et al.,
2001). As a result, we hypothesize that, in general and
probably independent of environmental conditions,
marine sediments are not a significant direct source
of 15N-enriched nitrate to the overlying water.
3.4. The effect of effluxing NH4+
In those cases where the isotope effect of nitrification produces low-d15N nitrate, it also results in a
high-d15N for the ammonium in pore water (Brandes
and Devol, 1997). In our simulation depicted in Fig. 5,
15
for example, the d15N of ammonium diffusing out of
the sediments would be + 15.4xand + 21.5xfor
QNH4 values of 45% and 10%, respectively. Apparently, this ammonium is not oxidized rapidly within
the chamber water (as outlined above, we would
expect to a see dual isotope signature typical for
nitrification if reoxidation rates were high). However,
it would eventually be oxidized to nitrate in the ocean
water column, so that 15N depletion of nitrate caused
by nitrification in the sediments may be completely
compensated by the oxidation of the 15N-enriched
ammonium in the water column. Accordingly, if
denitrification expresses an isotope effect, the nitrate
pool in an ocean region or the global ocean will
become enriched in 15N. In the same sense, nitrification does not represent a real (long-term) mechanism
for lowering the d15N of oceanic nitrate. Quite the
contrary, if high-d15N ammonium (from partial nitrification) escapes from the sediments and is eventually
oxidized in the open ocean, and nitrification is closely
coupled to denitrification (preventing efflux of the
complementary 15N-depleted nitrate), the marine nitrate pool will become enriched in 15N.
From this perspective, if the N isotopes are to be
used as a tool to study N loss from the ocean, it is
crucial not only to distinguish between denitrification
without an isotope effect and denitrification with an
isotope effect that is countered by nitrification in the
short term (as we have done here using chamber
experiments) but also to constrain actual ammonium
fluxes out of the sediments and their isotopic composition. In the case of the SMB, we have demonstrated
that (a) the sediment denitrification expresses no
isotope effect and (b) no significant gross flux of
newly produced nitrate out of the sediments occurs.
Hence, the SMB sediments represent a sink for fixed
Fig. 5. Simulation of nitrate d15N and d18O trends during benthic chamber incubation. Boundary conditions in (A) QNH4 = 45%, with NO
3
flux = 0.8 mmol m 2 d 1, NH+4 flux = 1.0 mmol m 2 d 1, and PO34 flux = 0.14 mmol m 2 d 1 and in (B) QNH4 = 10%, with NO
3
flux = 0.8 mmol m 2 d 1, NH+4 flux = 0.2 mmol m 2 d 1, and PO34 flux = 0.14 mmol m 2 d 1. In both simulations, the initial nutrient
concentrations were: 22.0 Amol/l for nitrate, 0.0 Amol/l for ammonium, and 1.4 Amol/l for phosphate (see small graphs), and the initial nitrate
d15N and d18O values were + 7.8xand + 1.2, respectively. Fluxes and initial conditions in (A) are similar to those observed at the Hyperion
site in March 2001. In (B), the net ammonium flux was lowered to create a lower QNH4. QNH4 has a direct effect on the d15N of newly produced
nitrate, with a lower d15N value at higher QNH4 (see text). In (A), the d15N of newly produced nitrate was 1.6x
; in (B), it was 4.5x(given
a sedimentary organic d15N of + 6xand assuming an N isotope effect for nitrification of 17x
). Simulated isotope trends are illustrated for
different effective isotope enrichment factors (e) and varying degrees of the coupling of nitrification and denitrification ( GCND, which is
inversely proportional to the efflux of microbial nitrate from the sediments). The combined nitrate d15N and d18O trends give unequivocal
constraints on the actual pathways of nitrate and the apparent isotope effects associated with sedimentary denitrification (see text for
discussion).
16
M.F. Lehmann et al. / Marine Chemistry 88 (2004) 1–20
Fig. 6. Measured versus predicted nitrate d15N and d18O at Station 1-Y (A) and Station 5-Y (B). Real nitrate isotope trends observed during
incubation experiments in the SMB are only reconcilable with a setting where the apparent N and O isotope effects are not greater than 1 F 1x
and GCND (the fraction of newly produced nitrate that is denitrified) is not lower than 85 F 5% (that is, nitrification is tightly coupled to
denitrification and very little of the newly produced nitrate escapes from the sediments). See also Fig. 5.
N from the ocean that has no short-term effect on the
d15N of the overlying nitrate pool. However, benthic
N-cycling reactions can still combine to produce a
flux of 15N-enriched ammonium that eventually contributes to the N isotope enrichment of water-column
nitrate in the SMB and, in turn, the global ocean. This
remains to be addressed.
3.5. Alternate dissimilatory N transformations
In coastal marine sediments, dissimilatory nitrate
reduction to ammonium (DNRA) has been reported to
be a nitrate-reduction pathway as important as denitrification (Rysgaard et al., 1996; An and Gardner,
2002). However, most of these high-DRNA environments (mostly lagoonal) were characterized by hypersulfidic conditions. High sulfide concentrations inhibit
nitrification and denitrification but may enhance
DRNA (Rysgaard et al., 1996). We do not have
sulfide data for the SMB sediments but the benthic
stoichiometry indicates that sulfate reduction is not the
dominant respiratory mechanism in the SMB, suggesting that DRNA is not prevalent. Even if DNRA
were an important nitrogen pathway within the SMB
benthic N cycle, the dual-isotope approach to constrain the pathway of newly produced microbial
nitrate would still be valid. To our knowledge, reports
on the N isotope fractionation associated with DRNA
do not exist. However, data by McCready et al. (1983)
have shown that ammonium produced during DRNA
is strongly depleted in 15N. Hence, DRNA would
presumably make the chamber isotopic measurements
more sensitive towards the efflux of microbial nitrate
because the partial nitrification of DNRA-derived
ammonium would lead to an even more distinct
(i.e., more negative) nitrate d15N signal. If the isotope
M.F. Lehmann et al. / Marine Chemistry 88 (2004) 1–20
fractionation pattern for DRNA and denitrification
were similar, the direct effect of DRNA on the pore
water nitrate pool would not be different from that of
denitrification. Thus, fractionation by DRNA would
be constrained by the dual isotopes just as denitrification is.
Another aspect of benthic N cycling that requires
further investigation are alternate modes of suboxic
N2 production, e.g., the anammox process, Mn(II)
oxidation by nitrate, or Mn(IV) reduction by ammonium (Luther et al., 1997; Dalsgaard et al., 2003;
Kuypers et al., 2003). We have no evidence to
suggest that these processes are important in the
SMB, but they may be important in other environments and on a global scale (Devol, 2003). At this
point, it is difficult to address the potential effect of
those particular N-cycling reactions on the benthic
nitrate isotope dynamics because we do not know
about the possible range of isotope effects associated
with these reactions. It is possible that, if they are
indeed important elements of the benthic N cycle,
the alternate modes of suboxic N2 production may
affect the sensitivity of the dual isotope approach, as
well as the overall impact of benthic N cycling on
the isotopic composition of marine nitrate. We are
only beginning to understand all the pathways that
can lead to N2 production in marine sediments.
However, using stable isotope ratios in DIN species
in a similar way as has been done in this study may
eventually help to gain insight into the complexity of
closely coupled nitrogen conversion processes in
benthic environments.
4. Summary and concluding remarks
The Santa Monica Bay shelf sediments represent
an effective sink for fixed nitrogen, indicating that
denitrification in these sediments is an important
pathway for organic matter mineralization. Independent of denitrification rates and of the partitioning of
ammonium fluxes, sedimentary denitrification did not
cause any significant N and O isotope enrichment in
residual nitrate in the benthic incubation chambers.
The ultimate N and O isotope impacts of benthic
nitrogen cycling provide a constraint on the interplay
between nitrification and denitrification and on actual
nitrogen pathways during microbial organic matter
17
decomposition. Comparing the benthic flux N and O
isotope data with a model of the relevant benthic
processes, we find that, in the SMB, microbial nitrate
is readily denitrified almost to completion within the
sediments; thus, coupled nitrification –denitrification
sensu stricto accounts for the major portion of total
denitrification (generally between 50% and 100%).
Moreover, both N and O isotope enrichment associated with denitrification was effectively f 0x, indicating that essentially all bottom-water nitrate
entering the sediment was denitrified before having
the opportunity to escape back into the overlying
chamber water. Given that bio-irrigation is an extremely active process in the SMB sediments (Fraser
and Berelson, unpublished data), one might have
expected the SMB to be an environment where
isotope enrichment in the sediment pore water nitrate
would be rapidly communicated into the overlying
water. Thus, the lack of such isotope enrichment in the
SMB suggests that the diffusion-limitation, the explanation previously posited for negligible or very small
N isotope fractionation in other environments
(Brandes and Devol, 1997, 2002), can still dominate
NO3 isotope behavior in sediments where solute
transport across the sediment – water interface and
within the sediments is facilitated by nondiffusive
processes. Thus, this study extends significantly the
range of sedimentary environments in which the
nitrate isotope effect of microbial denitrification is
essentially not expressed.
On the scale of an oceanic region, total DIN fluxes
(including ammonium) have to be considered when
addressing the N isotopic effects of benthic N cycling
on the marine nitrate pool. The low mean d15N
of f + 5xfor nitrate in the global ocean interior
has been explained as the result of very low degrees
of isotope enrichment associated with sedimentary
denitrification (Brandes and Devol, 2002). Brandes
and Devol (2002) proposed a ratio of global sedimentary to pelagic denitrification of up to 4:1, based
upon estimates on effective N isotope fractionation
for sedimentary and water-column denitrification
(e = 1.5xand 25x
, respectively), yielding a
rate of global sedimentary denitrification (21 Tmol N/
yr) that is significantly higher than most previously
published rates (Codispoti and Christensen, 1985;
Devol et al., 1997). In many actively denitrifying
marine sediments, the efflux of NH4+ represents an
18
M.F. Lehmann et al. / Marine Chemistry 88 (2004) 1–20
important component of the benthic N cycle (e.g.,
Seitzinger, 1988 and references therein; Hopkinson et
al., 2001; Laursen and Seitzinger, 2002). We argue
that this ammonium would ultimately contribute to
the marine nitrate pool. If, on average, the released
ammonium is indeed enriched in 15N relative to mean
oceanic nitrate (as a result of partial nitrification), the
overall N isotope effect of benthic N cycling on
oceanic nitrate may be greater than previously assumed. In turn, an even higher ratio of sedimentary to
pelagic denitrification would be needed to meet the
global N isotope balance according to Brandes and
Devol (2002). Addressing this issue further will
require isotope measurements of ammonium in pore
waters and benthic chamber waters.
Acknowledgements
This study was supported by DFG grant LE 1326/
1-1 to M.F.L., NSF grants OCE-0136449 and OCE9981479 to D.M.S., and USC Sea Grant to W.M.B.
We thank R. Archer, N. Fraser, G. Cane, and R. Ho
for their help and technical assistance. We thank J.
Brandes and R. Jahnke for valuable comments on an
earlier version of the manuscript. We also thank
associate editor M. Altabet who helped to improve the
manuscript significantly.
Associate editor: Dr. Mark Altabet
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