To identify the contribution of soil organic and inorganic fractions to soil water retention, we compiled the data of soil water retention curves and soil physicochemical properties. Data includes site information (Table 1), soil pH, soil C, cation exchange capacity (CEC), exchangeable basic cations, particle size distribution, soil texture class, contents of short-range-order minerals (Table 2), and soil water retention at different pressures, saturated hydraulic conductivity, and termite nest percent in soil profile (Table 3).

Data compilation

Data was compiled from our study and the published data sources by searching “soil moisture characteristics curve” or “soil water retention curves” or “pF curves” in Google Scholar. The papers were selected when soil physicochemical properties related especially to soil organic component (organic matter or carbon content) and inorganic component (clay, silt, sand, and short-range-order (SRO) minerals) were provided.

Physicochemical properties of soil samples

Soil samples were air-dried to pass through a 2 mm sieve. Soil pH was measured using a soil-to-solution (H₂O or 1 M KCl) ratio of 1:5 (w/v) after shaking for 1 h. Total C concentrations in soils were determined using a CN analyzer (Vario Max CN, Elementar Analysensystem GmbH). The exchangeable basic cation concentrations and cation exchange capacity (CEC) were determined using the ammonium acetate (1 M and pH 7.0) method. Exchangeable Ca and Mg were measured using atomic absorption spectroscopy (A-A-640-01, Shimadzu). Exchangeable K was measured by flame photometry.

Particle size distribution [clay (<0.002 mm); silt (0.002―0.05 mm); sand (0.050―2 mm)] was determined by the pipette method. The contents of short-range-ordered (SRO) Fe and Al (hydr) substances (Fe_o, Al_o, Si_o) in soils were estimated by extraction in the dark with acidic (pH 3) 0.2 M ammonium oxalate (McKeague and Day, 1966). The contents of crystalline and SRO Fe and Al oxides and organic Fe and Al compounds (Fe_d, Al_d) were estimated by extraction with a citrate-bicarbonate mixed solution buffered at pH 7.3 with the addition of sodium dithionite (DCB) at 80°C (Mehra and Jackson, 1960). The contents of Fe and Al in organo-mineral complexes (Fe_p, Al_p) were estimated by extraction with 0.1 M pyrophosphate at pH 10 for 16 h (Schuppli et al., 1983). The Fe and Al concentrations in soil extracts were determined using an inductively coupled plasma atomic emission spectrometer (ICP-AES, SPS1500, Seiko Instruments Inc.).

The contents of allophane in the soil samples were estimated by multiplying Si_o with a factor (7.14), while those of ferrihydrite were estimated by multiplying Fe_o with a factor (1.7) (Parfitt and Henmi, 1982; Parfitt and Child, 1988), written as follows:

Allophane content (%) = Si_o content (%) × 7.14                                        (1)

Ferrihydrite content (%) = Fe_o content (%) × 1.7                                       (2)

The contents of crystalline clay minerals were estimated by subtracting the contents of allophane and ferrihydrite from the clay content, which was recalculated on the basis of bulk soil (Parfitt and Henmi, 1982; Parfitt and Child, 1988).

Crystalline mineral content (%) = Clay content (%) × {100 – Total soil C (%)×1.8}/100 – Allophane content (%) – Ferrihydrite content (%)                                                (3)

Soil water retention curves and saturated hydraulic conductivity

We measured bulk density by oven drying (105°C, 24 h) of 0.1 L core soil samples and saturated hydraulic conductivity [K_s (m day^-1)] using constant head or falling-head methods (Klute, 1965). The soil water retention curves were experimentally obtained for undisturbed soils sampled by 0.1 L cores using a sand column method (DIK-3521; Daiki Rika Kogyo Co., Ltd., Saitama, Japan) for 0 (saturated) to –3.1 kPa and pressure plate (DIK-3404, Daiki Rika Kogyo Co., Ltd.) for –9.8 to –98.0 kPa, centrifugation method for pF 4.2 (Kokusan or Hitachi Himac centrifuge), air-drying for pF 5.5, and oven drying (105°C, 24 h) for pF 7.0.

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