The studied site, located at Tianbaoshan national forest farm, Pingyi County, Linyi, China (35° 18'N, 117° 44'E). The mean annual precipitation of the study area is 800 mm and the annual mean temperature is 13.7 ℃. Robinia pseudoacacia forest has been planted on northern slope both of brown soil and cinnamon soil for more than 45 years by now.

The surface and subsurface soils were sampled by genetic horizon in October 2015. Three replicates samples were taken at each soil type. Choose three sites under two soil types at mesoslope with 25° of slope. Using spade to make four soil profile each site, and collect surface (O horizon) and subsurface soils (A horizon). Then composite four soil samples to one sample. The fresh samples were sieved (< 2 mm) and stored at 4 ℃. Subsamples were air dried at ambient temperature (22 ℃) for at least one week and stored prior to chemical analysis.

Soil pH was measured in ultra pure water (soil∶ water ratio, 1∶ 2.5) with a glass electrode. Total C (TC) and total N (TN) were determined with an automatic elemental analyzer (Vario Macro Cube, Elementar, Germany). Soil N H4+— N and N O3-— N were extracted with 2 mol· L^-1 KCl and determined by indophenol blue method and dual wavelength ultraviolet spectrometry method. All results were calculated on an oven-dry soil weight basis (105 ℃, 24 h). The soil texture was analyzed by laser diffraction (Malvern Mastersizer MS2000, S/N 34355— 181, 5.1 version software) with wet analysis using the Hydro S dispersion unit (capacity 100~150 mL).

Humic substances were extracted followed classic method described as follows. Shaking 5.0 g air-dried soil (< 100 mesh) with 100 mL 0.1 mol· L^-1 Na₄P₂O₇— NaOH in conical flask for 1h on boiling water bath. Centrifuge the solution at 10 000 r· min^-1 to separate the supernatant, which contains humic acid and fulvic acid. The supernatant was acidified to pH of 2.0~3.0 with drops of 0.5 mol· L^-1 H₂SO₄ and kept at 80 ℃ water bath for 30 min and then stayed overnight. The mixture was filtrated to separate FA from HA precipitate. Dissolve the precipitate with 0.05 mol· L^-1 hot NaOH solution to obtain HA. The soil residues after the Na₄P₂O₇— NaOH extraction was treated with 0.5 mol· L^-1 H₂SO₄ to remove Fe, Al oxides and 1 mol· L^-1 HF— HCl solution to destroy the connections between clay and humic substances. Then the soil residue was washed by ultra pure water and extracted with 1 mol· L^-1 NaOH to obtain HM solution. Take equally FA, HA and HM extracts from three replicates to make a composite solution. Neutralize those solutions to pH 7.0 and then freeze dry FA, HA and HM samples.

Freeze dried FA, HA and HM samples were ground to < 2 μ m and mixed with KBr at 1∶ 100 ratio before grinding and finally to make a slide. All infrared spectra were measured from 4 000 to 400 cm^-1 at 4 cm^-1 resolution.

The differences between soil horizons were analyzed with a t-test using SPSS 16.0 for Windows. Each set of variables was log transformed when failed normality tests. Nonparametric-test were used when none transformation could pass normality tests. The FT-IR spectra were analyzed by MainFTOS software, to show the differences between spectrum, followed spectra were presented after vertical move.

Basic soil properties were shown in table 1, soil pH of cinnamon soil is significantly higher than that of brown soil but no difference was found between soil depths; Compared with brown soil, cinnamon soil contains more soil TC, TN, N O3-— N and sand content both O and A horizon; Soil N H4+— N and silt contents are high in brown soil in both hori-zons than those of cinnamon soil. Clay content showed no difference in O horizon between two soil types but was higher in cinnamon A horizon than brown soil.

Table 1

Table 1

Table 1 Surface and subsurface soil properties of brown and cinnamon Robinia pseudoacacia forests Soil BO BA CO CA pHa 5.24(4.84, 6.01)B 5.32(5.14, 6.32)b 6.94(6.93, 7.31)A 7.11(7.04, 7.13)a TC/(g· kg-1) 25.9± 2.58B* 9.08± 0.54b 47.4± 2.32A* 15.1± 1.26a TN/(g· kg-1) 2.39± 0.39B* 1.16± 0.05b 4.43± 0.46A* 1.88± 0.08a N H4+— N/(mg· kg-1) 11.7± 0.49A* 5.30± 0.66a 8.52± 0.65B* 3.11± 0.26b N O3-— N/(mg· kg-1) 13.1± 0.30B* 6.07± 0.44b 27.4± 0.82A* 9.21± 0.44a Clay/% 0.68A 0.48b 1.39A 1.11a Silt/% 71.1A 77.5a 47.2B 41.7b Sand/% 28.2B 22.0b 51.4A 57.1a Notes: CA, subsurface cinnamon soil; CO, surface cinnamon soil; BA, subsurface brown soil; BO surface brown soil. Different upper case letters indicate significant differences between BO and CO (p< 0.05); Lower case letters indicate significant differences between BA and CA(p< 0.05). Asterisks (* ) are significant differences between surface and subsurface soil (p< 0.05). Values of soil pH are presented as mean (first quartile, the third quartile) due to no data transformation could fit for normality test, significant differences among sites were analyzed by non-parametric test.

Table 1 Surface and subsurface soil properties of brown and cinnamon Robinia pseudoacacia forests

The FT-IR spectra of FA, HA and HM are displayed in Fig.1, 2 and 3, respectively. Fig.1 showed that FA in CO, CA, BO and BA has similar absorption bands only differed in the intensity. A broad band around 3 400 cm^-1 can be assigned to stretching vibration of carbohydrate H-bonded OH^[9]. Peaks at 1 655 cm^-1 were due to aromatic C＝C stretching^[10]. The sharp and strong peaks centered at 1 110 cm^-1 was caused by C— OH stretching of aliphatic OH. Weak bands around 1 030 cm^-1 in several spectra were attributed to C— O stretching of polysaccharide or polysaccharide-like compounds^[11]. Peaks are also be found around 620 and 532 cm^-1 as organic halide C— Br stretching and vibration of soil minerals^[10]. The intensities of FA peaks were highest in soil O horizon than those in A horizon and dominated in carbohydrate and aliphatic compounds. Compared between soil types, peaks of cinnamon soil are more board, which indicated more complicated structures than those under brown soil. The results also showed that FA aromatization in CA was weaker than CO, BA and BO soils.

Fig.1

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	Fig.1 FT-IR spectra of soim fulvic acid under brown and cinnamon Robinia pseudoacacia forest CA, subsurface cinnamon soil; CO, surface cinnamon soil; BA, subsurface brown soil; BO, surface brown soil

HA from two soils are illustrated in Fig.2. The spectra of HA showed similar peaks but the intensity was weaker than that of FA like peaks at 3 400, 1 655 and 1 110 cm^-1. Except for BA, other treatments all have weak peaks at 2 923 cm^-1 as aliphatic C— H stretching^[12]. BO soil has one more peak at 1 380 cm^-1 as CH₃ symmetric stretching^[13]. All treatments have bands at 1 110 cm^-1 as C— OH stretching of aliphatic OH, which has strong intensity under BO soil. Similar bands around 620 and 532 cm^-1 like FA were also found in HA. The intensity of the bands of HA was weaker than those of FA and only BO soil had surong peaks. In conclusion, the soil HA under Robinia pseudoacacia forest contained less proportion of carbohydrate and the relative degree of aromatization was weaker in CO, CA and BA soils. Surface soil under brown soil has more carbohydrate and aliphatic compounds than other treatments.

Fig.2

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	Fig.2 FT-IR spectra of soil humic acid under brown and cinnamon Robinia pseudoacacia forest CA, subsurface cinnamon soil; CO, surface cinnamon soil; BA, subsurface brown soil; BO, surface brown soil

As shown in Fig.3 the stretching vibration of carbohydrate H-bonded OH at 3 400 cm^-1 were typically strong in CO and BO than those in subsurface soil. Similar like HA spectra, weak peaks of aliphatic C— H stretching at 2 923 cm^-1 were found except for BA soil. The peaks of aromatic C=C stretching at 1 655 cm^-1 were also weaker compared with FA. Strong bands at 1 110 and 1 030 cm^-1 were found and the intensity were sequenced as CO> CA> BO> BA. Thus, cinnamon soil contained more aliphatic and carbohydrate groups than brown soil. Weak peaks of organic halide C— Br stretching were found around 620 cm^-1 under CO, CA and BO soils. Spectra of HM showed that HM contained more carbohydrate and aliphatic compounds than HA but less than FA. The relative degree of aromatization was weaker in HM than that in FA and HA.

Fig.3

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	Fig.3 FT-IR spectra of soil humin under brown and cinnamon Robinia pseudoacacia forest CA, subsurface cinnamon soil; CO, surface cinnamon soil; BA, subsurface brown soil; BO, surface brown soil