Analyzing biases of nitrogen contents and δ<sup>15</sup>N values arising from acidified marine sediments with different CaCO<sub>3</sub> concentrations

Analyzing biases of nitrogen contents and δ¹⁵N values arising from acidified marine sediments with different CaCO₃ concentrations

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Yajun PENG¹^,², Dongyan LIU¹^,³^,^*, Yujue WANG¹, Pierre RICHARD⁴, John K. KEESING⁵

Acta Oceanologica Sinica | 2018, 37(8) : 1 - 5

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Acta Oceanologica Sinica | 2018, 37(8): 1-5

• Marine Chemistry •

Analyzing biases of nitrogen contents and δ¹⁵N values arising from acidified marine sediments with different CaCO₃ concentrations

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Yajun PENG¹^,², Dongyan LIU¹^,³^,^*, Yujue WANG¹, Pierre RICHARD⁴, John K. KEESING⁵

Affiliations

¹ Key Laboratory of Coastal Zone Environmental Processes and Ecological Remediation, Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai 264003, China

² School of Management Science, Guizhou University of Finance and Economics, Guiyang 550000, China

³ State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China

⁴ Littoral, Environment et Sociétés, UMR 7266 CNRS-Université de La Rochelle, 2 rue Olympe de Gouges, La Rochelle 17000, France

⁵ CSIRO Oceans and Atmosphere and University of Western Australia Oceans Institute, Indian Ocean Marine Research Centre, Crawley 6009, Australia

Published: 2018-08-25 doi: 10.1007/s13131-018-1188-2

Outline

Abstract

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In some studies, the researchers pretreated and measured organic carbon, nitrogen and their isotopes (δ¹³C and δ¹⁵N) of marine sediment together, to save costs and resources of analysis. However, the procedure of acidification to remove inorganic carbon for analysing δ¹³C can affect the values of nitrogen and δ¹⁵N, and the biases vary a lot depending on the CaCO₃ contents of sediments. In this study, the biases of total nitrogen (TN) and δ¹⁵N values arising from acidified sediments were compared between the CaCO₃-poor (1%–16%) and CaCO₃-rich (20%–40%) samples. TN and δ¹⁵N values were altered during acid treatment (without centrifugation) that possibly led to N-containing compounds volatilization. For CaCO₃-poor samples, acidification led to a range of 0%–40% TN losses and 0‰–2‰ shift in δ¹⁵N values; and 10%–60% TN losses and 1‰–14‰ shift in δ¹⁵N values for CaCO₃-rich samples. The biases from most samples exceeded the precision of the instrument (0.002% for TN and 0.08‰ for δ¹⁵N), and high biases could mislead our judgment for the environmental implication of the data. Thus, avoiding co-analysis of organic carbon, nitrogen and their isotopes (δ¹³C and δ¹⁵N) in sediments, even for CaCO₃-poor marine sediments, is necessary.

Key words

stable isotope nitrogen / nitrogen / CaCO₃ / acidification / marine sediment

Cite this Article

Yajun PENG, Dongyan LIU, Yujue WANG, Pierre RICHARD, John K. KEESING. Analyzing biases of nitrogen contents and δ¹⁵N values arising from acidified marine sediments with different CaCO₃ concentrations[J]. Acta Oceanologica Sinica, 2018 , 37 (8) : 1 -5 . DOI: 10.1007/s13131-018-1188-2

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1 Introduction

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Organic matter contents (e.g., total carbon and nitrogen) and their isotope fingerprints (δ¹³C and δ¹⁵N) in the sediments are important environmental proxies to evaluate the sediment quality and trace the material sources in coastal and estuarine ecosystems (Tucker et al., 1999; Voss et al., 2000; Savage, 2005). For example, anthropogenically enriched organic nitrogen (e.g., human sewage, livestock runoff) can significantly elevate the values of δ¹⁵N to 10‰–25‰ (Sweeney and Kaplan, 1980; Savage, 2005). In some studies, the researchers pretreated and measured organic carbon, nitrogen and their isotopes (δ¹³C and δ¹⁵N) in marine sediment together, to save costs and resources of analysis (e.g., Middelburg and Nieuwenhuize, 1998; Kennedy et al., 2005; Hu et al., 2006; Wang et al., 2013). Acidification is a necessary procedure to remove inorganic carbon for meeting the requirement of total organic carbon (TOC) and δ¹³C measurements. However, when samples were acidified, the loss of total nitrogen (TN) and even the shift of δ¹⁵N values occurred (Bunn et al., 1995; Jacob et al., 2005; Jaschinski et al., 2008; Serrano et al., 2008; Schlacher and Connolly, 2014). This biases caused by acidification varied greatly depending on the species of acid reagent (e.g., HCl, H₂SO₃, H₃PO₄ or HF) and carbonate contents in the sediments (Schubert and Nielsen, 2000; Lorrain et al., 2003; Kennedy et al., 2005; Schmidt and Gleixner, 2005; Fernandes and Krull, 2008). For examples, Harris et al. (2001) only detected a slight increase in δ¹⁵N values (0.04‰–0.14‰) after acidification using HCl fumigation; Fernandes and Krull (2008) also found 1‰–2‰ deviation of δ¹⁵N values between the acidified (using HCl washing) and non-acidified samples. For the biases of nitrogen content, Lohse et al. (2000) reported there were approximately 28%–47% losses in nitrogen compounds for CaCO₃-rich (55%–61%) samples after acidification using H₂SO₃; Kennedy et al. (2005) however only found about 0.3% nitrogen losses in the acidified samples (30% CaCO₃) using HCl. Therefore, there is uncertainty on the biases in TN and δ¹⁵N values after acidification, and it is necessary to evaluate the biases according to the CaCO₃ contents of the sediments.

Global mean CaCO₃ content in marine surface sediments is approximately 34.8%, with a wide range of approximately 0%–80% (Ridgwell and Hargreaves, 2007). For examples, the samples from north and west Pacific Ocean contain approximately 0%–20% CaCO₃, but CaCO₃ contents in the samples from Atlantic Ocean can reach to 80% (Ridgwell and Hargreaves, 2007). In this study, TN and δ¹⁵N values in 236 marine sediment samples with higher CaCO₃ contents (20%–40%) from the coastal waters in the western Australia and 273 samples with lower CaCO₃ contents (0%–16%) from the coastal waters in China’s seas were measured, respectively, under acidified and non-acidified conditions. The biases of TN and δ¹⁵N values were evaluated and the correlation between the biases and CaCO₃ contents in the sediments were examined to understand the impact of CaCO₃ contents.

2 Materials and methods

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2.1 Sample collection

Two sediment cores were collected from the coast of King George River (KGR) in the northwestern Australia, and 236 sediment samples were obtained by slicing at 1-cm interval. Two and one sediment cores were collected from the Bohai Sea and Yellow Sea in China, respectively, and 221 sediment samples were obtained by slicing at 1-cm interval. In addition, 52 surface marine sediment samples were collected from the South Yellow Sea, China (Table 1). Totally, 509 marine sediment samples were stored in a freezer at –20°C preparing for the analysis of TOC, TN, δ¹⁵N values and CaCO₃ contents.

2.2 Analytical procedures

TOC, TN and δ¹⁵N values were measured using a continuous-flow isotope-ratio mass spectrometer (Delta V Advantage, Thermo Scientific, Germany) coupled to an elemental analyzer (Flash EA 1112, Thermo Scientific, Italy) in Littoral Environment et Sociétés (LIENSs)-UMR7266, France. Firstly, all samples were freeze-dried and homogenized by grinding. Each sample was divided into two shares, using for the pretreatment with and without acidification, respectively. One share was acidified by adding 1 mL of 0.5 mol/L HCl (or 1 mol/L HCl) to every 100 mg of samples. The acidified samples were dried at <60°C, then mixed with 1 mL Milli-Q waters and freeze-dried again. The freeze-dried samples were weighed into 5 mm×8 mm tin capsules, and the prepared capsules were used for the measurement of total organic carbon (TOC), total nitrogen (TN_acidified) and stable nitrogen isotope (δ¹⁵N_acidified). The corrections for TOC and TN_acidified values were performed because the weight of samples had increased after decarbonation (formation of CaCl₂). The other sample share, using for non-acidification treatment, was weighed directly and placed into 5 mm×8 mm tin capsules for the measurement of total carbon (TC), total nitrogen (TN_unacidified) and stable nitrogen isotope (δ¹⁵N_unacidified). Based on the results of TOC and TC, the CaCO₃ contents were estimated using CaCO₃=(TC– TOC)×8.33 (Stein et al., 1994; Schubert and Calvert, 2001).

The isotopic composition results are expressed relative to atmospheric N₂ for δ¹⁵N. Replicate measurements of an acetanilide standard (Thermo Scientific) indicated that the δ¹⁵N analytical errors were less than 0.1‰. The total organic matter and total nitrogen weight data obtained as part of the stable isotope analyses. Replicate measurements of a certified reference material (Low Organic Content Soil, Elemental MicroAnalysis, UK) indicated relative errors of 0.025% and 0.002% for total organic carbon and total nitrogen, respectively.

2.3 Statistical analysis

We tested the differences of TN and δ¹⁵N values between non-acidified and acidified samples, using software SPSS 16.0 (t-test). The correlation between these differences and CaCO₃ contents were analyzed respectively using SigmaPlot 12.0. The biases in TN and δ¹⁵N values after acidification were calculated, respectively, using the following equations:

(1)$\Delta {\rm{N}}\left({{\rm{of}}\;{\rm{the}}\;{\rm{TN}}} \right) = \frac{{{\rm{T}}{{\rm{N}}_{{\rm{unacidified}}}} - {\rm{T}}{{\rm{N}}_{{\rm{acidified}}}}}}{{{\rm{T}}{{\rm{N}}_{{\rm{unacidifed}}}}}} \times {\rm{100, }}$

(2)${\Delta ^{15}}{\rm{N}} = {\rm{\delta ^{15}}}{{\rm{N}}_{{\rm{unacidified}}}} - {\rm{\delta ^{15}}}{{\rm{N}}_{{\rm{acidified}}}}.$

3 Results and discussion

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3.1 CaCO₃ contents of all samples

The CaCO₃ contents were approximately 20%–40% and 1%–16% for the sediments collected from the northwestern Australia and China’s seas, respectively (Table 1). Based on the CaCO₃ contents of global marine sediments (Ridgwell and Hargreaves, 2007), the samples were divided into CaCO₃-poor samples (CaCO₃ contents <20%) and CaCO₃-rich samples (CaCO₃ contents >20%), respectively, for a comparison.

3.2 Effects of acid treatment on TN

The results of paired-sample t-test indicated a significant difference in TN contents (P=0.000<0.01, 2-tailed) between the samples measured with and without acid treatment. For the non-acidified samples, TN contents ranged from 0.02%–0.19%, while this range reduced to 0.01%–0.16% for the acidified samples. About 99% non-acidified vs. acidified TN values are located below the ideal 1:1 relation, indicating that acidification resulted in the losses of nitrogen contents from the samples (Fig. 1a). In addition, losses of nitrogen contents were on average higher in CaCO₃-rich samples compared to CaCO₃-poor samples after acidification. About 0%–40% and 10%–60% of nitrogen contents were lost from non-acidified CaCO₃-poor samples and CaCO₃-rich samples, respectively (Fig. 2a). Moreover, our data suggest a statistically significant positive correlation between the losses of nitrogen contents and the CaCO₃ contents (Fig. 2a). These findings partially agree with Lohse et al. (2000) and Fernandes and Krull (2008), who found a particularly high losses of nitrogen contents in CaCO₃-rich (>30%) samples, up to 64% after acid treatment (Table 2), but no significant correlation between the loss of nitrogen contents and the CaCO₃ contents.

The loss of nitrogen contents from acidified samples was observed in the present and previous studies, although the acidification methods were different (Table 2). In our study, we used an acid treatment with 0.5 mol/L or 1 mol/L HCl in glass tubes (Peng et al., 2015), which avoided the loss of acid-soluble organic matter and/or fine particles by rinsing process (rinse method) and intensive effervescence (in situ acidification). This indicates that volatilisation of nitrogen compounds seems to be the most reasonable explanation for our results, and further investigation is still need for the explanation of the mechanism. Previous studies have demonstrated that in the CaCO₃-rich samples the fraction of proteinaceous compounds absorbed on carbonate surface layers became unstable with the increasing of acid addition, and this acid-soluble component was lost during carbonate dissolution (Fernandes and Krull, 2008). Meanwhile, this fraction of acid-soluble organic matter is greater in more calcareous sediments (Froelich, 1980). This may explain the higher losses observed in the CaCO₃-rich samples compared to the CaCO₃-poor samples.

3.3 Effects of acid treatment on δ¹⁵N

Nitrogen isotope (δ¹⁵N) values of samples measured with and without acidification are shown in Fig. 1b. There have significant shifts in δ¹⁵N values between acidified and non-acidified samples, and these shifts have statistically significant positive correlation with the CaCO₃ contents, indicating that losses of acid-soluble nitrogen compounds with different δ¹⁵N values (Fig. 2b). For CaCO₃-poor sediments the measured δ¹⁵N values after acid treatment varied in the range of –1‰ to 6‰ (Fig. 1b). The shift of δ¹⁵N values of these samples after acidification (up to 3‰) is similar to the deviation (0‰–2‰) observed by Fernandes and Krull (2008) (Table 2). For CaCO₃-rich samples about 40% of the measured δ¹⁵N values were significant negative after acidification (down to –9‰, Fig. 1b), and bias in δ¹⁵N values (up to 14‰) were remarkable compared to non-acidified samples (Fig. 2b). Schmidt and Gleixner (2005) also found some samples with unusual negative δ¹⁵N values (e.g., –4.24‰ for a podzol Sample 6) after acid treatment due to the accumulation of coal-derived products. However, it is really difficult to understand what cause such depletion for part of CaCO₃-rich samples, since there is no rinse of the samples, which could trigger a loss of ¹⁵N-enriched compounds. Volatilisation of nitrogenous compounds like chloramine should lead to an enrichment of δ¹⁵N values not depletion.

In addition, our data suggest that isotopic shifts were not consistent for all samples (Fig. 2b). The depletion of δ¹⁵N values was observed in 95% samples after acid treatment reflecting the loss of ¹⁵N-enriched compounds such as protein and ammonium (Brodie et al., 2011a, b) (Fig. 2b). The loss of heavy isotope during acid treatment had previously been reported by other studies (Table 2). However, about 5% acidified samples (originating in CaCO₃-poor sediments collected from the Bohai Sea) detected a 0‰–1‰ increase signature in δ¹⁵N values, suggesting the loss of ¹⁴N-enriched compounds. Harris et al. (2001) observed a small increase (0.04‰–0.11‰) in δ¹⁵N values of acidified soil samples following HCl fumigation. This contrasting shifts (depletion or enrichment) in δ¹⁵N values probably influenced by the organic matter composition and clay mineralogy of samples (Baldock and Skjemstad, 2000; Schubert and Calvert, 2001; Fernandes and Krull, 2008), but more work need to do to identify differences in acid-soluble compounds, inorganic nitrogen and mineralogy of samples containing different CaCO₃ contents during acid treatment.

4 Conclusions

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Our results suggest that the analysis of nitrogen contents and its isotopic composition should be measured on non-acidified whole sample, because acidification to remove carbonates in marine sediments leads to significant nitrogen losses through volatilisation and isotopic shifts, particularly for samples with high CaCO₃ contents (>20%). Meanwhile, there was a statistically significant positive correlation between the CaCO₃ contents and the biases in nitrogen contents and its isotopic composition after acid treatment (Fig. 2), indicating that the CaCO₃ contents in the sample play an important role in determining what extent the nitrogen fraction may have been affected by acid treatment.

Funding

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The National Natural Science Foundation of China under contract No. 41376121; the Strategic Priority Research Program of the Chinese Academy of Sciences under contract No. XDA11020405; the Natural Science Foundation of Shandong Province under contract No. JQ201414; the Program of Advanced Talents of The Guizhou University of Finance and Economics, 2016.

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Brodie C R, Leng M J, Casford J S L, et al. 2011b. Evidence for bias in C and N concentrations and δ¹³C composition of terrestrial and aquatic organic materials due to pre-analysis acid preparation methods. Chemical Geology, 282(3–4): 67–83

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Appendix

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Year 2018 volume 37 Issue 8

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Article Info

doi: 10.1007/s13131-018-1188-2

Receive Date：2017-05-26
Online Date：2026-04-14
Published：2018-08-25

Article Data

Affiliations

History

Received：2017-05-26
Accepted：2017-08-30

Funding

The National Natural Science Foundation of China under contract No. 41376121; the Strategic Priority Research Program of the Chinese Academy of Sciences under contract No. XDA11020405; the Natural Science Foundation of Shandong Province under contract No. JQ201414; the Program of Advanced Talents of The Guizhou University of Finance and Economics, 2016.

Affiliations

¹ Key Laboratory of Coastal Zone Environmental Processes and Ecological Remediation, Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai 264003, China

² School of Management Science, Guizhou University of Finance and Economics, Guiyang 550000, China

³ State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China

⁴ Littoral, Environment et Sociétés, UMR 7266 CNRS-Université de La Rochelle, 2 rue Olympe de Gouges, La Rochelle 17000, France

⁵ CSIRO Oceans and Atmosphere and University of Western Australia Oceans Institute, Indian Ocean Marine Research Centre, Crawley 6009, Australia

Corresponding:

*E-mail: dyliu@sklec.ecnu.edu.cn

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表12种不同金属材料的力学参数

科 Family	属数 Number of genus	种数 Number of species	占总种数比例 Percentage of total species (%)	属 Genus	种数 Number of species	占总种数比例 Percentage of total species (%)
鹅膏菌科Amanitaceae	2	11	5.26	鹅膏菌属 Amanita	10	4.78
小菇科 Mycenaceae	2	12	5.74	丝盖伞属 Inocybe	5	2.39
多孔菌科 Polyporaceae	8	14	6.70	蜡蘑属 Laccaria	5	2.39
红菇科 Russulaceae	3	23	11.00	小皮伞属 Marasmius	6	2.87
				小菇属 Mycena	11	5.26
				光柄菇属 Pluteus	5	2.39
				红菇属 Russula	17	8.13
				栓菌属 Trametes	5	2.39

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Table 1. Description of sample property

Samples identifier	Location	Number of samples	Description	TOC¹⁾/%	TN²⁾%	C/N²⁾	δ¹⁵N²⁾/‰	CaCO₃/%
Note: ¹⁾ Measurement on acidified sample; ²⁾ measurement on unacidified whole sample.
KGR6	13°55′13.44″S, 127°19′13.8″E	114	Down-core marine sediment from King George River, northwestern Australia (11.8 m depth)	0.40–0.75	0.03–0.11	8–19	3.6–5.7	20–31
KGR10	13°53′08.88″S, 127°08′22.56″E	122	Down-core marine sediment from King George River, northwestern Australia (11.2 m depth)	0.38–0.83	0.04–0.12	7–22	4.0–7.8	23–40
B62-2	38°52′12″N, 119°40′12″E	76	Down-core marine sediment from Bohai Sea, China (24 m depth)	0.46–0.92	0.06–0.12	6–12	4.9–6.4	1–7
B24-1	38°09′27.6″N, 122°29′0.24″E	63	Down-core marine sediment from North Yellow Sea, China (45 m depth)	0.68–1.14	0.09–0.18	7–12	4.5–5.6	1–5
H10-1	34°59′24″N, 123°29′56.04″E	82	Down-core marine sediment from South Yellow Sea, China (78 m depth)	0.20–1.09	0.03–0.14	7–14	3.8–6.2	1–7
SMS	–	52	Surface marine sediment from South Yellow Sea, China	0.14–1.18	0.02–0.19	6–21	3.7–6.0	1–16

Table 2. Comparison of differences caused by acidification in sedimentary TN and δ¹⁵N values with references

Sample type	CaCO₃/%	ΔN/%	Δ¹⁵N/‰	Acidification method	Possible derives	Analysis mode	Reference
Note: Single or dual indicate separate or simultaneous C and N analyses; ¹⁾ deep-sea sediment; ²⁾ surface marine sediment; ³⁾ marine suspended particulate organic matter; ⁴⁾ surface estuarine sediment; ⁵⁾ international soil standard; ⁶⁾ measurement on unacidified whole sample for TN and δ¹⁵N; ⁷⁾ measurement on acidified sample for TN and δ¹⁵N.
DSS¹⁾	55–61	28–47	–	in situ H₂SO₃	H₂SO₃volatilises nitrogenous organic matter	single⁶⁾	Lohse et al. (2000)
DSS¹⁾	7–17	2–4	–	in situ H₂SO₃	H₂SO₃volatilises nitrogenous organic matter	single⁶⁾	Lohse et al. (2000)
SMS²⁾	–	11–29	–	direct HCl acidification	nitrogen appears to be liberated during direct acidification	single	Ryba and Burgess (2002)
MPOM³⁾	<5	0	0	HCl fumes for 4 h	HCl fume treatment did not alter δ¹⁵N and N values for low carbonate contents	dual⁷⁾	Lorrain et al. (2003)
DSS¹⁾	30	0.3	0.005	in situ 2 mol/L HCl	weak HCl (2 mol/L) did not significantly affect δ¹⁵N and TN values	dual	Kennedy et al. (2005)
SMS²⁾	>30	30–64	1–2	HCl washing	losses through volatilisation or solubilisation (acid-soluble compounds)	single	Fernandes and Krull (2008)
SES⁴⁾	<10	19–40	0–1	HCl washing		single	Fernandes and Krull (2008)
SOILB⁵⁾	–	15	0.7	in situ 5% HCl	loss of soluble organic and/or inorganic N species, e.g., ¹⁵N-rich species (e.g., protein, ammonium)	single	Brodie et al. (2011a, b)

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