Evaluation of stacking nanostructure in soil humic acids by analysis of the 002 band of their X-ray diffraction profiles

Figures & data

Table 1 Characteristics of soil humic acids used in this study.

Sample name	Soil order	Land use	Horizon	Location	Atomic ratios of humic acids			Degree of darkness
					H/C	O/C	O/H	A600/C ^†
ND	Andisols	Forest	A2	Kanazawa, Ishikawa Pref.	0.67	0.51	0.70	12.0
HZ ^‡	Andisols	Grassland	A1	Hiruzen, Tottori Pref.	0.75	0.48	0.64	11.3
MZ	Andisols	Grassland	2A1	Miyakonojou, Miyazaki Pref.	0.85	0.49	0.57	9.5
SA ^‡	Andisols	Forest	A	Aburahi, Shiga Pref.	0.81	0.49	0.61	9.4
KGY	Andisols	Grassland	Bw	Kanazawa, Ishikawa Pref.	0.84	0.58	0.68	8.9
MK ^‡	Andisols	Grassland	A1	Myoko, Niigata Pref.	1.03	0.48	0.46	5.6
H12	Entisols	Paddy field	Ap	Nayoro, Hokkaido	1.18	0.48	0.41	3.8
GF	Entisols	Paddy field	Ap	Gifu, Gifu Pref.	1.22	0.50	0.41	2.8
OKY	Entisols	Paddy field	Ap	Okayama, Okayama Pref.	1.26	0.48	0.38	2.6
KT ^‡	Inceptisols	Forest	A	Kuta, Kyoto Pref.	1.27	0.52	0.41	2.8
ASU	Inceptisols	Forest	A	Asu, Kyoto Pref.	1.22	0.47	0.38	2.7
TRG	Inceptisols	Forest	A	Hakusan, Ishikawa Pref.	1.22	0.56	0.46	2.3

H, hydrogen; C, carbon; O, oxygen.

^†: A₆₀₀ is the absorbance at 600 nm and C is the organic C concentration (mg C mL^–1).

^‡: These samples were also used in the study reported in Katsumi et al. (2015).

Figure 1 Carbon-13 (¹³C) nuclear magnetic resonance (NMR) spectra of humic acids (HAs).

Table 2 Distribution of carbon (C) species and aromaticity estimated by carbon-13 (¹³C) nuclear magnetic resonance (NMR).

	Alkyl C	O-alkyl C	Aromatic C	Carboxyl C	fa-NMR^†
Sample name	(0–48 ppm)	(48–110 ppm)	(110–165 ppm)	(165–190 ppm)
ND	10.1	10.6	63.9	13.0	0.76
HZ	10.3	14.9	58.3	13.2	0.70
MZ	18.0	21.2	43.0	13.7	0.52
SA	13.5	17.6	54.6	13.0	0.64
KGY	14.4	15.5	50.2	15.0	0.63
MK	19.4	26.5	37.1	13.9	0.45
H12	33.5	25.1	27.7	11.7	0.32
GF	37.6	22.6	25.3	12.2	0.30
OKY	39.2	20.8	26.1	10.7	0.30
KT	37.6	23.9	23.3	11.2	0.27
ASU	43.1	23.0	21.1	10.7	0.24
TRG	35.9	28.9	24.6	10.0	0.28
Mean value
Andisol HAs (n = 6)	14.3 ± 1.6 b	17.7 ± 2.2	51.2 ± 4.0 a	13.6 ± 0.3	0.61 ± 0.05 a
Entisol HAs (n = 3)	36.7 ± 1.7 a	22.8 ± 1.2	26.4 ± 0.7 b	12.9 ± 0.5	0.31 ± 0.01 b
Inceptisol HAs (n = 3)	38.9 ± 2.2 a	25.3 ± 1.8	23.0 ± 1.0 b	11.5 ± 0.3	0.26 ± 0.01 b
ANOVA	**	NS	**	NS	**

Values are mean ± standard error.

HAs: humic acids.

ANOVA: analysis of variance. Asterisks (**) indicate statistical significance at P < 0.01. NS indicates no significant difference.

Different lowercase letters indicate statistically significant differences at P < 0.01 (Tukey honestly significant difference test).

^†: f_a-NMR = aromatic C/(alkyl C + O-alkyl C+aromatic C).

Figure 2 X-ray diffraction profiles of humic acids (HAs) and related substances.

Figure 3 X-ray diffraction profiles of (a) HZ (Andisols), (b) OKY (Entisols), and (c) ASU (Inceptisols) humic acids after waveform separation.

Table 3 Stacking structure properties of soil humic acids (HAs) estimated from the 002 band of the X-ray diffraction profile.

	Aγ1/A002^†	Lc^‡	d002^§	Nc^¶	fa-XRD
Sample name		nm
ND	0.24	1.84	0.340	5.40	0.81
HZ	0.61	1.79	0.344	5.21	0.62
MZ	0.52	1.72	0.341	5.05	0.66
SA	0.32	1.70	0.343	4.95	0.76
KGY	0.62	1.67	0.345	4.85	0.62
MK	1.38	1.55	0.346	4.48	0.42
H12	3.15	1.40	0.342	4.10	0.24
GF	2.66	1.30	0.348	3.72	0.27
OKY	2.57	1.20	0.344	3.50	0.28
KT	5.95	1.17	0.343	3.40	0.14
ASU	3.37	1.14	0.349	3.28	0.23
TRG	4.34	1.04	0.345	3.01	0.19
Mean value
Andisol HAs (n = 6)	0.61 ± 0.17 b	1.71 ± 0.04 a	0.343 ± 0.001	5.00 ± 0.13 a	0.65 ± 0.06 a
Entisol HAs (n = 3)	2.79 ± 0.18 a	1.30 ± 0.06 b	0.345 ± 0.002	3.77 ± 0.18 b	0.26 ± 0.01 b
Inceptisol HAs (n = 3)	4.55 ± 0.75 a	1.12 ± 0.04 b	0.346 ± 0.002	3.23 ± 0.11 b	0.19 ± 0.02 b
ANOVA	**	**	NS	**	**

Values are mean ± standard error.

ANOVA: analysis of variance. Asterisks (**) indicate statistical significance at P < 0.01. NS indicates no significant difference.

Different lowercase letters indicate statistically significant differences at P < 0.01 (Tukey honestly significant difference test).

^†:A_γ1/A₀₀₂ = area ratio of 002 band/area ratio of γ₁ band.

^‡: mean stacking structure length along the c axis.

^§: spacing between carbon planes in humic acid.

^¶: Number of stacked carbon planes.

f_a-XRD = area ratio of 002 band/(area ratio of 002 band + area ratio of γ₁ band).

Figure 4 Relationships between mean stacked structure length along c axis (L_c) and proportion of aromatic carbon (C) (%), (alkyl C + O-alkyl C) (%), and degree of darkness (A₆₀₀/C, where A₆₀₀ is the absorbance at 600 nm and C is the organic C concentration in milligrams of C per milliliter).

Figure 5 Proposed average stacking nanostructure models for humic acids (HAs) analyzed in the present study. These nanostructure models used theoretical values for the size of the condensed aromatic rings along the a axis L_a) and approximate value for the average number of C planes in the stacking nanostructure (N_c).

Katsumi N, Yonebayashi K, Fujitake N, Okazaki M 2015: Relationship between stable carbon and nitrogen isotope ratios of humic acids extracted from Andisols and non-Andisols. Catema, 127, 214–221. doi:10.1016/j.catena.2015.01.005

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