Direct N2 Quantitation

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Direct N2 Quantitation Opportunities and
challenges of using direct N2 measurements of
denitrification Klaus Butterbach-Bahl Institute
for Meteorology and Climate Research Atmospheric
Environmental Research (IMK-IFU) Forschungszentrum
Karlsruhe Todd Kana Horn Point
Laboratory University of Maryland Cambridge,
Maryland
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Natural variation of dissolved N2 and N2Ar
caused by pressure-solubility effects.
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N2based denitrification studies fall in two
categories
Open systems DNF determined from spatial and/or
temporal gradients of N2 in natural
environments. Advantage Integrates large spatial
and long temporal scales. Disadvantage Often
difficult to constrain physical processes that
affect N2 Examples Ocean Basins Sediment
profiles Rivers Groundwater
Contained systems DNF determined in isolated
(small) systems. Advantage Fluxes measured under
controlled conditions important parameters well
defined. Disadvantages Small spatial and short
temporal scales. Semi-natural conditions. Example
Sediment cores
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Deviations from solubility equilibrium in open
systems or Places where we can find for an effect
of DNF on ambient N2
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Open Systems
Sediment profile
Ocean (Sea) Basins
Measured by capillary probe MIMS Flux determined
from diffusion relationship DNF rate 2-4 mmol N2
m-2 day-1
N2/Ar measured by gas extraction MS DNF rates
0.3 nmol l-1 h-1 (15N technique)
(Codispodi et al. 2001)
(Hartnett and Seitzinger, 2003)
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Open Systems
River Reaches Length gradient approach (Laursen
Seitzinger) Time series approach (McCutchan
et al.)
Groundwater
From Blicher-Mathiesen et al. 1998
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Contained systems Continuous flow stirred
reactor
Flux determined from difference in inlet and
outlet concentrations
Ci
Co
Vf
A
Sediment
Kana et al. 1998
Advantages Steady-state conditions Optimal
method for measuring DN2 Disadvantages
Elaborate setup Requires control of gases in
inlet water Often a many-day experiment
(Fb flux in blank chamber)
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Contained systems Batch reactor
Flux determined from time-course measurements
Courtesy of J. Cornwell and M. Owens
Advantages Relative simplicity Relatively
short incubations (24 hrs) Disadvantages
Bubble formation/entrapment artifacts
Solubility-driven fluxes O2 concentration
changes Control is not completely adequate
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Measurement of N2 from water samples
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MIMS description (modified from Kana et al. 1994)

• Features of the Dissolved Gas Analyzer
• Analyze raw water samples
• Measurement time of 1-2 min.
• Precision lt0.03 for ratios, lt0.2 for gases
• Minimum volume 5ml
• Easily modified for additional gases or on-line
  measurements.

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Operational issues regarding MIMS precision and
accuracy

• Precision can be limited at the sample
  collection/storage step.
• Calibration relies on solubility tables and
  determinations of instrument drift.
• Sample storage requires temperature control and
  air-tight containers to avoid bubble formation
  and/or air contamination.
• Provisions for handling low temperature samples
  (high gas concentrations) are necessary if
  analyzing at room temperature.
• Ultra-high precision depends on evaluating
  effects of chemical reactions in the ion source.

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Future directions Analytical enhancement of
MIMS Development of routine procedures for
measuring other gases including N2O, H2S, and
CO2. Development and application of new MIMS
inlets or methods for sampling small volumes and
on-line sampling systems. Experimental and
Methodological enhancements Better assessment of
potential artifacts in incubations including
improved controls. Advance applications of
isotope methodologies using MIMS.
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• Motivation and Background
• Since 1989 our group is involved in the
  characterization of N cycling at forest sites
  affected by high rates of atmospheric
  N-deposition (gt20 kg N ha-1 yr-1).
• Major results are
• High losses of N by NO3-leaching (under spruce
  approx. 20 kg N ha-1 yr-1)
• Insignificant changes of N stored in the soil
• Significant losses of N-trace gases (NO, N2O)
• Hugh differences in NO3-leaching and N-trace gas
  emissions (NO, N2O) between beech and spruce
• Imbalance between N-input and N-losses of approx.
  10 kg N ha-1 yr-1
• Question
• Can N2 losses via denitrification explain the
  N-imbalance?

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N-input-output balance for the Höglwald Forest,
South-Germany (all fluxes in kg N ha-1 yr-1)
Spruce Imbalance approx. 10 kg N ha-1 yr-1
Beech Imbalance approx. 11 kg N ha-1 yr-1
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• Challenge Direct quantification of N2 losses
  from forest soils
• Problem
• C2H2-technique has several problems (e.g.
  inhibition of nitrification, co-reaction of NO
  with O2)
• Introduction of label is necessary if Isotopes
  are used, also sensitivity may still be a problem
• Direct measurements -
• Background of N2 in the atmosphere
• Relatively low sensitivity of TCDs for N2
• Solution Gas-Flow-Core-Method
• Exchange of soil atmosphere versus an artificial
  N2-free atmosphere, i.e. replacement of N2 with
  He or Ar, but with same concentrations of O2, CO2
  .
• Increase of N2 in a gas-tight chamber should
  equal N2-production via denitrification

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Gas-Flow-Core-Method
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• Exchange time depends on
• Texture
• compaction
• Moisture content
• Can be shortened by pressure changes !

Increase of N2-concentration in the headspace
atmosphere with time of purging an autoclaved
soil column (loamy texture) with an N2-free
artificial gas mixture at a rate of 200 ml min-1.
To measure the increase of N2, purging was
stopped for at least 4 h. The dotted line
represents the system inherent leakage rate of
0.20 ppmv N2 h-1.
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Increase of N2 and N2O concentrations (?SE) in
the headspace of the incubation vessel before and
after autoclaving an intact soil core.
(Intercalibration with 15N method showed
comparable results (R. Brumme, Göttingen), but
detection limit was better for Gas-Flow-Core-Metho
d)
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Pine forests on sandy soil, East Germany
High N site (N-deposition gt 20 kg N yr-1)
Low N site (N-deposition approx. 10 kg N yr-1)
Medium N site (N-deposition approx. 10 kg N yr-1)
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Pine forests on sandy soil, East Germany Moisture
dependency (high N site)
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Pine forests on sandy soil, East Germany

• Annual N2-losses
• 3.5 5.1 kg N
• No significant differences between the different
  sites
• Annual N2O-losses
• 0.3 1.4 kg N
• Low N lt medium N lt high N
• N2 N2O ratio
• Low N gt medium N gt high N
• Ratio slightly decreased with increasing
  temperature
• At 14C ratio significantly decreased with
  increasing soil moisture

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N-cycling and gaseous N-losses at the Höglwald
spruce site
NPlant Biomass retention 10 Cycling 115 1537
-10
NHumus 9010
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Fluxes of N2O and N2 from nitrate (50 kg N ha-1
and glucose (363 kg C ha-1) amended soil cores.
Samples were taken from a grassland site at
Devon, UK (Cardenas et al. 2003).
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• Advantages
• Simultaneous detection of N2 and N2O (CO2, CH4,
  NO)
• High precision of measurements
• detection limit for N2 lt 10 µg N m-2 h-1 or lt 1
  kg N ha-1 yr-1 (Helium-ionisation detector)
• Detection limit for N2O lt 3 µg N m-2 h-1 or lt
  0.3 kg N ha-1 yr-1 (Electron capture detector)
• Parameterization of N2- and N2O-production can
  easily be done
• Applicable to a wide range of terrestrial
  ecosystems
• Disadvantages
• Variability of fluxes from core to core
  (transferability)
• Incubation
• time needed to remove N2 in original atmosphere
• Possible changes of soil moisture with time
  (however, no change in N2O emissions was found
  after change of soil atmosphere and changes in
  soil moisture were lt5)

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Recent applications of the Gas-Flow-Core
Method Butterbach-Bahl K, Willibald G, Papen H,
2002, Soil core method for direct simultaneous
determination of N2 and N2O emissions from forest
soils. Plant and Soil, 240, 105-116. Cárdenas
LM, Hawkins JMB, Chadwick D, Scholefield D, 2003,
Biogenic gas emissions from soils measured using
a new automated laboratory incubation system.
Soil Biol. Biochem. 35, 867-870. Scholefield D,
Hawkins J M B, Jackson S M 1997a, Development of
a Helium atmosphere soil incubation technique for
direct measurement of nitrous oxide and
dinitrogen fluxes during denitrification. Soil
Biol. Biochem., 29, 1345-1352. Scholefield D,
Hawkins J M B, Jackson S M 1997b, Use of a
flowing helium atmosphere incubation technique to
measure the effects of denitrification controls
applied to intact cores of a clay soil. Soil
Biol. Biochem., 29, 1337-1344. Stefanson RC,
Greenland DJ, 1970, Measurement of nitrogen and
nitrous oxide evolution from soilplant systems
using sealed growth chambers. Soil Science 109,
203206. Swerts M, Uytterhoeven G, Merckx R,
Vlassak K 1995 Semicontinuous measurement of soil
atmosphere gases with gas-flow soil core method.
Soil Sci. Soc. Am. J., 59, 1336-1342.
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• Future directions
• Inclusion of NO measurements in addition to N2O
  and N2
• Simultaneous measurements of gross nitrification
  (denitrification) and mineralization rates via
  the Barometric Process Separation technique
  BaPS
• Additional sensors for pressure, CO2 and O2
  needed

• Problems
• Sensor sensitivity (especially for 02)
• Respiration coefficient

CO2-Sensor
Pressure sensor
Septum
24-pol- cable
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