Changes in elemental composition, molecular weight and ¹H NMR spectra of the water-extractable hydrophobic acid fraction in Cambisol with season and soil depth

Figures & data

Table 1 Selected properties of the soil horizons investigated

Horizon	Depth (cm)	pH^‡		Total C^§ (g kg^−1)	Total N^§ (g kg^−1)	Clay^¶ (g kg^−1)	Fed ^†† (g kg^−1)	Ald ^†† (g kg^−1)	Feo ^‡‡ (g kg^−1)	Alo ^‡‡ (g kg^−1)
		(H2O)	(KCl)
Oi^†	+3.1–+1.7	–	–	472	13.4	–	–	–	–	–
Oe/Oa^†	+1.7–0	–	–	392	19.2	–	–	–	–	–
A	0–15	5.19	3.74	25.7	1.7	90	8.56	1.08	1.12	0.680
B	15–80	5.61	3.89	2.7	0.15	87	8.33	0.885	2.63	0.649
^†Mean values from samples collected in winter, spring, summer and autumn 2004.
^‡Measured potentiometrically at a soil-to-solution ratio of 1:2.5.
^§Total carbon and total nitrogen determined with a CHN/S analyzer.
^¶Clay content estimated using the sieve-pipette method.
^††Fed and Ald, dithionite citrate bicarbonate extractable Fe and Al, respectively.
^‡‡Feo and Alo, oxalate extractable Fe and Al, respectively. The properties of the A and B horizons refer to the fine earth fraction (< 2 mm).

Figure 1  Carbon contents in the hydrophobic acid fraction (HoA-C) and in the non-hydrophobic acid fraction (non-HoA-C) of water-extractable organic matter derived from (a) organic and (b) mineral horizons. Error bars represent standard deviation (n = 3).

Table 2 Elemental composition of the hydrophobic acid fraction obtained from different horizons

Hydrophobic acid	Season	Elemental content (weight %)
		C	H	N	O
Oi horizon	Winter	51.3	5.06	1.75	41.9
	Spring	52.6	5.05	1.78	40.6
	Summer	50.6	5.14	1.91	42.4
	Autumn	54.1	5.08	2.31	38.5
Oe/Oa horizon	Winter	50.9	5.12	3.03	41.0
	Spring	51.9	4.97	2.92	40.2
	Summer	49.9	5.06	2.25	42.8
	Autumn	51.9	4.49	2.23	41.4
A horizon	Winter	56.0	5.22	3.39	35.4
	Spring	51.5	4.41	2.01	42.1
	Summer	52.4	4.90	2.24	40.5
	Autumn	53.1	4.46	2.00	40.4
B horizon	Winter^†	–	–	–	–
	Spring	51.6	4.28	2.10	42.0
	Summer	55.1	4.69	2.12	38.1
	Autumn	50.9	5.14	2.45	41.5
^†Not determined.

Figure 2  Changes in the atomic ratio of (a) H/C, (b) O/C and (c) N/C of the hydrophobic acid fraction obtained from different horizons. •, winter; ○, spring; ▾, summer; ▿, autumn.

Table 3 Molecular weight at peak maximum of the hydrophobic acid fraction obtained from different horizons

Horizon	Molecular weight at peak maximum (Da)
	Winter	Spring	Summer	Autumn	Mean^†
Oi horizon	1665	1577	1564	1511	1579 ± 64
Oe/Oa horizon	1444	1363	1319	1405	1383 ± 54
A horizon	1234	1135	1173	1086	1157 ± 63
B horizon	1136	924	1178	1093	1083 ± 111
^†Mean value for molecular weight at peak maximum of winter, spring, summer and autumn samples.

Figure 3  Size exclusion chromatograms of the hydrophobic acid fraction obtained from different horizons collected in winter.

Figure 4  1H nuclear magnetic resonance spectra of the hydrophobic acid fraction obtained from different horizons. Residual protons of water (HOD), TMSP, trimethylsilylpropionate-2,2,3,3.

Table 4 Distribution of proton species in the hydrophobic acid fraction obtained from different horizons

Hydrophobic acid	% of proton species (δ, p.p.m.)
	Hγ (0–1.0)	Hβ (1.0–1.6)	Hα (1.6–3.0)	HC–O (3.0–4.3)	Har (6.0–9.0)	Hal ^† (0–3.0)
Oi horizon
Winter	11.4	18.5	22.0	36.7	11.5	51.8
Spring	7.7	21.3	27.0	33.4	10.5	56.0
Summer	4.6	23.8	31.3	29.0	11.3	59.7
Autumn	8.0	19.8	25.2	33.9	13.1	53.0
Oe/Oa horizon
Winter	7.6	17.5	25.0	35.5	14.4	50.1
Spring	7.5	20.0	26.4	33.7	12.3	54.0
Summer	8.1	22.4	28.1	30.0	11.4	58.6
Autumn	6.3	19.4	29.6	31.3	13.2	55.4
A horizon
Winter	8.6	22.4	27.2	29.6	12.2	58.2
Spring	6.4	21.8	32.9	25.2	13.7	61.1
Summer	7.4	25.9	31.0	23.5	12.3	64.2
Autumn	6.5	24.7	32.4	22.8	13.5	63.6
B horizon
Winter	9.5	30.4	32.7	17.8	9.6	72.6
Spring	9.2	30.9	33.9	15.6	10.4	74.0
Summer	8.3	23.2	32.8	20.7	14.9	64.4
Autumn	7.8	23.4	33.3	23.1	12.4	64.5
†Hal = Hγ+ Hβ+ Hα.

Figure 5  Changes in the proportion of (a) carbohydrate and (b) aliphatic protons in the hydrophobic acid fraction from different soil horizons. •, winter; ○, spring; ▾, summer; ▿, autumn.