Humus composition and the structural characteristics of humic substances in soils under different land uses in Leyte, Philippines

Figures & data

Table 1 Amounts of humic and fulvic acids and the spectrophotometric properties of humic acids under different land uses

Site	Depth (cm)	Corg	HA	FA	HA/FA	Degree of humification		Type of HA
		(g C kg^−1)				log(A 400/A 600)	A 600/C
Primary forest (PF)	0–20	23.5	0.97	6.24	0.16	0.79	1.70	Rp+
	20–40	13.5	0.20	3.33	0.06	0.82	1.03	Rp+
	40–60	12.4	0.10	1.78	0.05	0.81	0.29	Rp+
Mahogany plantation (MP)	0–20	22.1	0.98	4.82	0.20	0.94	1.00	Rp
	20–40	11.6	0.62	2.63	0.24	1.00	0.95	Rp
	40–60	9.0	0.26	2.07	0.12	0.97	0.87	Rp
Rainforestation farming (RF)	0–20	16.3	0.90	4.66	0.19	0.80	1.88	Rp++
	20–40	10.9	0.38	3.10	0.12	0.77	1.88	Rp++
	40–60	8.9	0.21	2.40	0.09	0.87	1.13	Rp++
Coffee plantation (CP)	0–20	21.5	0.92	5.58	0.16	0.77	2.26	Rp+
	20–40	16.8	0.21	4.60	0.05	0.58	2.17	P+++
	40–60	15.0	0.14	2.73	0.05	0.54	2.20	P+++
Grassland (GR)	0–20	13.5	0.55	3.41	0.16	0.87	1.23	Rp+
	20–40	11.8	0.26	3.28	0.08	0.79	1.08	Rp++
	40–60	11.9	0.14	3.00	0.05	0.80	0.84	Rp++
Corg, organic C; FAs, fulvic acids; HAs, humic acids; log(A 400/A 600), the ratio of absorbance at 400 nm to that at 600 nm on the logarithmic scale; A 600/C, the absorbance at 600 nm per mg C mL^−1 in 0.1 mol NaOH.

Figure 1 Amounts of C recovered in the polyvinylpyrrolidone (PVP)-adsorbed and PVP-non-adsorbed fulvic acid (FA) fractions in soils under different land uses. (a) 0–20 cm soil, (b) 20–40 cm soil and (c) 40–60 cm soil. PF, primary forest; MP, mahogany plantation; RF, rainforestation farming; CP, coffee plantation; GR, grassland.

Table 2 Elemental composition of the humic acids and polyvinylpyrrolidone-adsorbed fulvic acids in the topsoil (0–20 cm) under different land uses

Site	C	H	N	S	O	Ash (%)	H/C	O/C	O/H	C/N
	(weight %)^†						atomic ratios
Humic acids
Primary forest (PF)	45.6	5.9	6.3	2.2	40.0	6.62	1.54	0.66	0.43	8.46
Mahogany plantation (MP)	48.2	6.1	5.7	1.0	39.0	3.59	1.50	0.61	0.41	9.87
Rainforestation farming (RF)	44.1	5.7	6.3	1.6	42.2	5.30	1.54	0.72	0.46	8.18
Coffee plantation (CP)	46.5	5.7	5.7	1.3	40.9	4.10	1.45	0.66	0.46	9.59
Grassland (GR)	46.1	5.7	5.1	1.3	41.7	3.18	1.47	0.68	0.46	10.47
PVP-adsorbed fulvic acids
Primary forest (PF)	45.8	4.3	2.4	1.1	46.4	2.50	1.13	0.76	0.67	22.44
Mahogany plantation (MP)	49.9	4.9	3.5	0.7	40.9	2.35	1.19	0.61	0.52	16.22
Rainforestation farming (RF)	44.7	4.3	2.3	0.4	48.4	1.89	1.14	0.81	0.71	23.03
Coffee plantation (CP)	44.5	4.3	2.0	1.1	48.1	2.08	1.16	0.81	0.70	26.08
Grassland (GR)	46.0	3.9	1.3	0.3	48.5	1.45	1.02	0.79	0.77	40.04
^†Moisture and ash-free basis. PVP, polyvinylpyrrolidone.

Figure 2 Infrared spectra of (a) humic acids (HAs) and (b) polyvinylpyrrolidone (PVP)-adsorbed fulvic acids (FAs) in soils under different land uses. PF, primary forest; MP, mahogany plantation; RF, rainforestation farming; CP, coffee plantation; GR, grassland.

Figure 3 Elution curves of humic acids (HAs) and (b) polyvinylpyrrolidone (PVP)-adsorbed fulvic acids (FAs) in soils under different land uses. The void volume (V0) and void volume + inner volume (V0 + Vi) were 39.5 and 163.6 mL, respectively. Mw and d refer to the weight average molecular weight and polydispersity, respectively. PF, primary forest; MP, mahogany plantation; RF, rainforestation farming; CP, coffee plantation; GR, grassland.