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Comparison of total nitrogen (TN) concentrations in deposition and soil solution samples determined by

two different methods

Arne Verstraeten and Gerrit Genouw

Meeting of the ICP Forests Expert Panel on Deposition23 April 2015, Göttingen, Germany

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Objectives of the experiment• To compare analytical methods for total nitrogen (TN)

concentrations in water samples: - Kjeldahl method <-> C/N analyzer- C/N analyzer with and without filtration

• Research questions:- Is there any difference in TN between the Kjeldahl method and C/N analyzer and, if yes, how much?- How much of TN is retained by filtration (0.45 µm)?

• Conducted at the lab of the INBO (F03)

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Methods

• Was the standard method used at INBO (2008–2013) and the lab of Ghent University (2005–2007) for TN in Level II samples

• TN = DIN + DON = NO3

- + NO2- + NH4

+ + DON = NO3

- + NO2- + N_Kjeldahl

• N_Kjeldahl determined by digestion (Gerhardt) followed by titration (Vapodest)

• Detection limit 0.5 mg N L–1

• NO3- and NO2

- were determined by ion chromatography (Dionex, DCS3000), which required filtration (0.45 µm)

1) Kjeldahl method

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Methods

• FormacsHT (Skalar)• Catalytic oxidation (750–950°C) to nitric oxide (NO)

N detector (chemiluminescence) measures TN directly• Filtration is not necessary• Calibration range 0.5–25.0 mg N L–1

2) C/N analyzer

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Samples• Were collected at five ICP Forests Level II plots in Flanders, Northern

Belgium (January till December 2014, 24 sampling periods)• Were subsamples of standard Level II samples from:

- deposition (wet-only and bulk open field precipitation, throughfall, stemflow)

- soil solution (O, A, B and C horizon)---> gradient in N concentrations

Coniferous plots Ravels (RAV) Brasschaat (BRA)

Deciduous plots Wijnendale (WIJ) Gontrode (GON) Hoeilaart (HOE)

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Samples• Multiple samples per fraction available• Each sample was analyzed by both methods• 5 missing values (analytical errors)• 1 outlier was removed (bulk deposition sample presumably

contamination by bird droppings)

open field throughfall stemflow O horizon A horizon B horizon C horizon Total

wet-only bulk

WIJ - 16 16 16 14 12 16 16 106

RAV - 15 15 - 11 11 12 14 78

BRA 14 15 15 - 14 9 9 11 87

GON - 15 15 15 3 11 11 11 81

HOE - 16 15 15 7 14 13 10 90

Total 14 77 76 46 49 57 61 62 442

Samples collected

open field throughfall stemflow O horizon A horizon B horizon C horizon Total

wet-only bulk

WIJ - 16 15 16 13 12 16 16 104

RAV - 15 15 - 11 11 12 14 78

BRA 14 15 15 - 14 9 9 11 87

GON - 15 15 15 3 9 11 11 79

HOE - 15 15 14 7 14 13 10 88

Total 14 76 75 45 48 55 61 62 436

Samples retained

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Results1) Comparison of Kjeldahl method with C/N analyzer

For all fractions the difference in mean value between the two methods is limited

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Results

Difference is limitedMore positive than negative outliers

Difference: TN_analyzer – TN_Kjeldahl

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Results

For soil solution (high NO3– concentrations at certain plots)

absolute difference sometimes up to >3 mg N L–1

Absolute value of difference: abs(TN_analyzer – TN_Kjeldahl)

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Results

For bulk open field, throughfall and stemflow percentage differences up to >40% (low TN concentration)

Difference (%): 100 (TN_analyzer – TN_Kjeldahl)/TN_Kjeldahl

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Results

Larger difference (in absolute value and in %) for samples with high TN concentration

n=320 n=69 n=52

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Results y = 1.020x - 0.009R²A = 0.9509***

y = 1.035x + 0.084R²A = 0.9744***

y = 1.025x + 0.079R²A = 0.9685***

y = 1.066x + 0.03R²A = 0.9846***

y = 1.032x - 0.165R²A = 0.9888***

y = 1.060x - 0.175R²A = 0.9963***

y = 1.077x - 0.228R²A = 0.9923***

y = 1.073x - 0.151R²A = 0.9922***

Linear regression

• Slope >0:• 1.020–1.035 for

low TN conc.(deposition excl.stemflow)

• 1.032–1.077 forhigher TN conc.(stemflow and soil solution)

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Results2) TN with and without filtration (C/N analyzer)

• 35 different samples from 1 single sampling period• Divided in 2 subsamples:

- not filtered- filtered (0.45 µm)

• Analyzed on C/N analyzer

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Results

• TN concentrations about 3.8% higher if not filtered prior to analysis• For oxidized N (applied to deposition data 2013):

- open field: 10.3–12.5%- throughfall + stemflow: 12.1–15.2%of oxidized N removed by filtration (particulate NO3

-)

2) TN with and without filtration (C/N analyzer)

y = 1.038x – 0.088R²A = 0.9963***

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Conclusions

• Differences in TN between the Kjeldahl method and C/N analyzer were overall limited

• Difference:- 2.0–3.5% for fractions with low TN concentration- 3.2–7.7% for fractions with high TN concentration

• On average 3.8% of TN was retained by the 0.45 µm filter, more tests are needed to get a better view

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Any questions?