1 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 Deposition 23 April 2015, Göttingen, Germany

2 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)

3 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 = NO NO NH DON = NO NO N_Kjeldahl N_Kjeldahl determined by digestion (Gerhardt) followed by titration (Vapodest) Detection limit 0.5 mg N L –1 NO 3 - and NO 2 - were determined by ion chromatography (Dionex, DCS3000), which required filtration (0.45 µm) 1) Kjeldahl method

4 Methods Formacs HT (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

5 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)

6 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 fieldthroughfallstemflowO horizonA horizonB horizonC horizonTotal wet-onlybulk WIJ RAV BRA GON HOE Total Samples collected open fieldthroughfallstemflowO horizonA horizonB horizonC horizonTotal wet-onlybulk WIJ RAV BRA GON HOE Total Samples retained

7 Results 1) Comparison of Kjeldahl method with C/N analyzer For all fractions the difference in mean value between the two methods is limited

8 Results Difference is limited More positive than negative outliers Difference: TN_analyzer – TN_Kjeldahl

9 Results For soil solution (high NO 3 – concentrations at certain plots) absolute difference sometimes up to >3 mg N L –1 Absolute value of difference: abs(TN_analyzer – TN_Kjeldahl)

10 Results For bulk open field, throughfall and stemflow percentage differences up to >40% (low TN concentration) Difference (%): 100 (TN_analyzer – TN_Kjeldahl)/TN_Kjeldahl

11 Results Larger difference (in absolute value and in %) for samples with high TN concentration n=320 n=69 n=52

12 Results y = 1.020x R² A = *** y = 1.035x R² A = *** y = 1.025x R² A = *** y = 1.066x R² A = *** y = 1.032x R² A = *** y = 1.060x R² A = *** y = 1.077x R² A = *** y = 1.073x R² A = *** Linear regression Slope >0: 1.020–1.035 for low TN conc. (deposition excl. stemflow) 1.032–1.077 for higher TN conc. (stemflow and soil solution)

13 Results 2) 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

14 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 NO 3 - ) 2) TN with and without filtration (C/N analyzer) y = 1.038x – R² A = ***

15 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

16 Any questions?