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ABSTRACT
ABSTRACT
The characteristics of humus composition are important for understanding the mechanism of carbon storage in the Qinghai–Tibet Plateau. The aim of this study was to characterize the quality of soil organic matter (SOM) in this region. Soil samples from four soil profiles in fenced study sites in the alpine grassland were collected at altitudes of 4200, 4000, 3800, and 3400 m, along the southwest facing slope in the Qilian Mountains. The humus composition and humification degree of the humic acid (HA) were determined by two methods: (1) extraction with 0.5% sodium hydroxide (NaOH) followed by 0.1 M sodium pyrophosphate (Na4P2O7) (OH-PP method); and (2) treating once with 0.1 M hydrochloric acid (HCl) followed by extracting with 0.5% NaOH (Cl-OH method). Physico-chemical analysis revealed higher exchangeable cation content and higher base saturation ratios could be related to slightly acidic to neutral soils, which could be regarded as calcium (Ca)-rich soils. The amounts of combined-form HAs obtained by HCl pretreatment (HACl – HAOH; ∆HACl) were remarkably higher than those extracted with Na4P2O7 (HAPP), indicating that the combined form of HAs is mainly Ca. In addition, the proportion of HAPP in the total HAs extracted with both NaOH and Na4P2O7 (HAOH + HAPP) obtained in the OH-PP method increased with soil depth and decreasing elevation, indicating that HAs associated with aluminum (Al) and iron (Fe) were distinguished in the subsoils of lower elevation. Therefore, the formation of the organo-mineral complex may contribute to stabilizing SOM in the Qinghai–Tibet Plateau. Moreover, Type A-HA with the highest degree of humification was obtained from the deeper horizons with the Cl-OH method and almost all horizons by extraction with Na4P2O7 in the OH-PP method. Further studies using various spectroscopic analyses are necessary to elucidate the chemical properties of SOM in this region.
1. Introduction
Soil organic matter (SOM) is a key soil component that performs a variety of functions and defines the physical and chemical properties of soil. The retention of nutrient elements such as potassium, the pH buffer function, and supply of air and water because of particle aggregation has long been recognized as the role of SOM (Stevenson 1994). Moreover, SOM plays an important role in influencing soil structure, microbial activity, and carbon (C) storage (Bronick and Lal, 2005; Leinweber et al. 2008; Dinakaran et al. 2014; Jobbágy and Jackson 2000; Stockmann et al. 2013). Since the function of SOM depends on its quantity and quality, understanding the amount and chemical composition of SOM is important to grasp the impacts of climate change and the fundamental functional mechanism of soil.
The Qinghai–Tibet Plateau is the largest elevated plateau in the Eurasian continent, and it plays an important role in the global terrestrial ecosystem because it is an important C pool and the highest plateau in the world (average 4000 m a.s.l.) (Gao et al. 2007; Wang et al. 2002). Alpine grasslands, covering approximately 35% of the Qinghai–Tibet Plateau area, are the major pastureland of the region (Zheng et al. 2000), and approximately 33.5 Pg of organic C is stocked in the 0–75 cm layer of soil (Cao et al. 2016). Therefore, many studies have estimated the SOM stocks in this region, indicating a considerable accumulation of SOM (Tan et al. 2010; Yang et al. 2008; Zhang et al. 2007). Moreover, there are sharp changes in the environmental conditions such as climate, vegetation, and soil mineral along an elevation gradient in the mountainous plateau (Wang et al. 2002; Yang et al. 2008). Therefore, study of the effects of environmental conditions on SOM along the elevation gradient is important for understanding the C stocks in alpine grasslands.
However, details of the qualitative characteristics of SOM have so far not been discussed in detail. SOM is heterogeneous and consists of various fractions with different stability, resolution, and rotation rates. Two dominant fractions of SOM are the labile soil organic C (SOC) fractions and the chemically recalcitrant SOC fractions, which are potential indicators of global climate change (Steinberg 2003; Cao et al. 2016; Hu et al. 2017; Wang et al. 2016). The latter (e.g., humic substances) is traditionally separated into humic acid (HA), fulvic acid (FA), and humins (HN) based on the solubility characteristics of each fraction, and the humus composition is an essential characteristic of humic substances in SOM. In Japan, humus composition analysis including the classification of soil HA, which is referred to as the Kobo-Oba’s (Oba 1964) or Nagoya method (Kumada, 1987) has been widely adapted (e.g., Kumada 1987; Kawasaki et al. 2015; Tani et al. 2012; Watanabe et al. 2001). These methods are able to provide an estimate of the relative proportion of HA, FA, and HN in SOM, and the chemical characteristics of HA such as the degree of humification. As these simple techniques do not require the purification of HA samples and expensive instruments, they are convenient, especially in cases in which many soils are compared and the amounts of samples or available instruments are limited (Ikeya and Watanabe 2003). These simple techniques are thus appropriate methods as first step to obtaining comprehensive information of the SOM characteristics of the alpine meadow of the Qinghai–Tibet Plateau and to explain why there are considerable C stocks in this region.
Thus, two different extractions based on the Nagoya method and the International Humic Substances Society (IHSS) method (Swift 1996) were selected in this study to determine humus composition and HA classification, namely, extraction with NaOH followed by Na4P2O7 (Kumada 1987), and treatment once with HCl followed by extraction with NaOH (IHSS method). Based on the differences in the extraction treatments, we expected that humus composition and humus characteristics of free forms and combined forms could be revealed.
The purpose of this study was to characterize the quality of SOM along an elevational gradient ranging from 4200 to 3400 m based on the humus composition and the humification degree of the HAs. The ultimate goal was to clarify the reasons for higher C stocks in this region, and to identify the continuous variations due to differences in SOM characteristics and elevation gradient.
2. Materials and methods
2.1. Soil sampling sites and soil samples
Soil sampling sites were located in the Qilian Mountains in Menyuan Hui Autonomous County, Haibei Tibetan Autonomous Prefecture, Qinghai province, China. It lies on the northeastern part of the Qinghai–Tibet Plateau (Fig. 1). The mean annual precipitation and temperature are 561 mm and −1.7°C, respectively. The mean altitude is 4000 m a.s.l. Along a southwest facing slope in the Qilian Mountains, four sites were selected based on four altitudes: site 4200 m, site 4000 m, site 3800 m, and site 3400 m (Table 1). To minimize interference by livestock grazing, these sites were protected by fence since 2006. Soil classification was performed based on the World Reference Base for soil resources (IUSS Working Group WRB 2014). The soil samples were taken from each horizon of all the soil profiles in September 2015. Soil samples were air-dried and passed through a 2-mm mesh sieve for analyses of soil physico-chemical properties. Visible plant debris and roots were manually removed using tweezers as far as possible. Furthermore, the sieved soil samples were pulverized to <0.5 mm with a vibrating sample mill (TI-100, Heiko Seisakusyo Ltd), and used for humus composition analysis.
Table 1. General physico-chemical properties of alpine grassland soils in the Qinghai–Tibet Plateau along the elevational gradient from 4200 to 3400 m.
Published online:
22 November 2018![]({"type":"image","src":"/na101/home/literatum/publisher/tandf/journals/content/tssp20/2019/tssp20.v065.i01/00380768.2018.1547098/20190210/images/medium/tssp_a_1547098_f0001_b.gif"})
Figure 1. Location of sampling sites in the Qinghai–Tibet Plateau.
2.2. Analyses of soil physico-chemical properties
Air-dried soils (≤2 mm) were used for physico-chemical analyses (Committee of soil Environmental Analysis, 1997). Particle size distribution was measured by the sedimentation method. Soil pH was measured using a glass electrode in a 1:2.5 soil-water suspension. Total carbon and nitrogen was analyzed by the dry combustion method using a CHNS/O analyzer (Perkin Elmer 2400II). The inorganic carbon content was measured by a weight-loss method (Blakemore et al. 1981). The organic carbon content was obtained by subtracting inorganic carbon content from total carbon content. The exchangeable cations were extracted with 1 M ammonium acetate solution (pH 7.0) and were determined using inductively coupled plasma atomic emission spectrometry (ICP-AES) (SPS3100, SII NanoTechnology Inc., Japan). Residues were leached after extracting exchangeable cations with 80% ethanol and extracting NH4+ with 10% NaCl solution. The cation exchange capacity (CEC) was determined by an ammonia electrode method (Toa Denpa IM-20B, Ammonia Electrode AE-235) (Muramoto et al. 1992). The air-dried soil samples were also used for selective dissolution analysis based on the sodium dithionite-citrate method, ammonium oxalate method, and sodium pyrophosphate method (Committee of soil Environmental Analysis, 1997). Sodium dithionite-citrate is assumed to selectively extract crystalline Fe and Al oxides, allophane, and Al-humus complexes (Parfitt and Childs 1988; Wagai and Mayer 2007); acid ammonium oxalate dissolves mainly the non-crystalline Fe and Al oxides, allophane, imogolite, and Fe- and Al-humus complexes (Committee of soil Environmental Analysis, 1997), Sodium pyrophosphate is used to target Fe and Al associated with SOM (Aran et al. 2001; García-Rodeja et al. 2007). Moreover, the extractants were determined by ICP-AES. Fe and Al extracted by sodium dithionite-citrate were named as Fed and Ald, those extracted by ammonium oxalate were named as Feo and Alo, and those extracted by sodium pyrophosphate were named as Fep and Alp.
2.3. Humus composition analysis
Pulverized soils (≤0.5 mm) were used for humus composition analysis. Two extraction methods, based on Nagoya method (Kumada 1987) and IHSS method (Swift 1996), were performed using the following procedure. (1) OH-PP method: 30 mL of 0.5% NaOH was added to the weighted soil samples containing less than 100 mg carbon, and heated at 100°C for 30 min. After heating, the soil suspension was cooled to room temperature and centrifuged at 10,000 × g for 20 min, which is referred to as the first extract. Next, the soil residue was resuspended with 0.1 M Na4P2O7 and shaken for 30 min at room temperature. After shaking, the residue suspension was centrifuged again at 10,000 × g for 20 min, which is referred to as the second extract. In this report, this method is named as the OH-PP method. The HA obtained from the first extract and the second extract using the OH-PP method are named HAOH and HAPP, respectively. (2) Cl-OH method: to remove calcium carbonate that might combine with humic substances, the soil samples were washed with 0.1 M HCl solution and shaken for 30 min before the soil samples were extracted with 0.5% NaOH in the same manner as in the first extraction of the OH-PP method. This extraction method is named as the Cl-OH method. The HA obtained with the Cl-OH method is named as HACl.
From each method, 20 mL of extract was taken in 45-mL plastic centrifuge tubes. The extracts were acidified with 0.2 mL of concentrated H2SO4 and incubated overnight. The supernatant (FA fraction) was obtained by centrifugation at 10,000 × g for 20 min. The precipitate was dissolved with 0.1 M NaOH in a 50-mL measuring flask (HA fraction). The organic carbon contents of the HA and FA solutions were determined using the total organic carbon analyzer (TOC-V CPH, Shimadzu).
Furthermore, a part of the HA solution was used to record the absorbance values at 400 nm (A400) and 600 nm (A600) using a spectrophotometer (UV-VIS, V-630, JASCO), within 2 h after dissolution. The degree of humification was estimated using the two indices as follows:
⊿log K = log A400−log A600,
and A600/C, where C is the organic carbon content of HA solution (Ikeya and Watanabe 2003).
3. Results and discussion
3.1. Physico-chemical properties
The general physico-chemical properties of each horizon from the four sites (4200, 4000, 3800, and 3400 m) are shown in Table 1. All soil samples in the Qinghai–Tibet Plateau had highest contents in silt fraction, and most of the soil textures were silty clay (SiC) or silty clay loam (SiCL) types. Previous studies have shown that silty soils in the Qinghai–Tibet Plateau were mainly derived from loess (Sun and Liu 2000;Wang et al. 2005; Li et al. 2008). Our study sites, positioned in the northeastern part of Qilian Mountains, were located between the west end of the Loess Plateau and the east of the Qaidam Basin Desert and the Taklamakan Desert. Therefore, loess may have been transported and deposited in this region located in the transfer line from these deserts to the Loess Plateau (Li et al. 2010). Vriend and Prins (2005) showed the loess-palaeosol records in the north‐eastern margin of the Tibetan Plateau including our study area. This suggests that the parent materials are derived from loess.
As shown in Table 1, SOC contents in the A horizons and the below horizons in all the soil profiles ranged from 34 to 117 and 14 to 36 g kg−1, respectively. This was similar from a previous study data which recognized as the higher SOC accumulation in the same region by Li et al. (2010). To evaluate the influence of elevation, SOC contents were compared along the elevational gradient. Li et al. (2017) showed that the SOC contents at 0–20 cm depth in a Tibetan alpine meadow increased gradually with elevation from 19.7 g kg−1 at 4400 m to 46.7 g kg−1 at 4950 m, presumably because overgrazing may have led to low above-ground biomass at the lower elevation (Ohtsuka et al. 2008; Yang et al. 2010). However, the SOC contents at 0–20 cm depth estimated by the horizon thickness and SOM content in each horizon decreased gradually with increasing elevation from 83.5 g kg−1 at 3400 m to 58.8 g kg−1 at 4200 m in this study. This disagreement may result in an abundant above-ground biomass at the lower elevation was well protected by the fence to avoid livestock grazing.
The soil pH (H2O) ranged from 6.0 to 7.7 and increased with soil depth in each of the soil profiles (Table 1), from being slightly acidic to neutral. Most base saturation ratios were over 100% due to the presence of carbonate. Moreover, exchangeable Ca content ranged from 18.2 to 53.8 cmolc kg−1 was similar to a calcaric soil ranged from 14.0 to 47.4 cmolc kg−1 (Li et al. 2010). Therefore, the soils in this study were characterized as Ca-enriched soil.
In Table 1, the amounts of Fed ranged from 13.8 to 18.7 g kg−1, and those of Feo ranged from 2.2 to 6.5 g kg−1, showing relatively higher Fed values and lower Feo values at an elevation of 3400 m compared to other elevations. Fed was supposed to represent the total free Fe oxides including crystalline and non-crystalline Fe oxides, and Feo was taken to represent the non-crystalline Fe oxides (Parfitt and Childs 1988). The values of Fed−Feo (8.7–16.1 g kg−1) were larger than those of Feo for each horizon, indicating that most of the free Fe oxides present were crystalline (Kleber et al. 2005), particularly, at an elevation of 3400 m. In contrast, the values of Ald were similar to those of Alo, indicating that Al-humus complexes and allophane were extracted from this soil (Parfitt and Childs 1988).
3.2. Humus composition
The humus composition of each horizon the 4200, 4000, 3800, and 3400 m sites determined by the Cl-OH and the OH-PP method is shown in Table 2. The proportion of HN fractions obtained by the Cl-OH method (HNCl) and by the OH-PP (HNOH-PP) method ranged from 34% to 65% and from 36% to 61% in SOC, respectively. The HN fractions always had the highest proportion of humus composition in all soil profiles. However, they had no clear tendency with soil depth or along the elevational gradient. There is a lack of knowledge about HN, despite the fact that it generally represents more than 50% of SOC (Stevenson 1994). Therefore, we cannot discuss the relationship between the quality of HN and the accumulation of SOC in more detail, and further research is necessary to study it. The proportions of FA fractions obtained by the Cl-OH method (FACl) and those by the OH-PP method (FAOH-PP) ranged from 13 to 29% and 18 to 35%, respectively, and both showed decreasing tendency with soil depth. In contrast, the proportions of HA fractions obtained by the Cl-OH method (HACl) and those by the OH-PP method (HAOH-PP) ranged from 22% to 44% and 19% to 37%, respectively, both showing an increasing tendency with soil depth. This suggests that the accumulation of SOC in the deeper soils was more related to HA fractions than FA fractions.
Table 2. Humus composition of each soil in the Qinghai–Tibet Plateau (carbon-based).
3.3. Forms of HAs present in the soil
In the Cl-OH method (Table 2), HACl obtained by HCl pretreatment was a mixture of free-form and combined-form HAs. Thus, the ∆HACl, corresponding to combined-form HAs, could be obtained by subtracting HAOH (free-form HAs) from HACl. The proportion of ∆HACl to the HACl (∆HACl/HACl) ranged from 5% to 65%, indicating that more free-form HAs were obtained by HCl pretreatment than combined-form HAs. However, the values of ∆HACl/HACl increased with soil depth in each of the soil profiles, while there was a tendency to increase with decreasing elevation.
In the OH-PP method, the sequential extraction with NaOH followed by Na4P2O7 was used to directly distinguish between the free and combined forms (Kumada et al. 1967). The amounts of NaOH-extracted HAs (HAOH) and FAs (FAOH) were remarkably larger than those of Na4P2O7-extracted HAs (HAPP) and FAs (FAPP), suggesting that free-form HAs were larger than combined-form HAs. Moreover, the amounts of HAOH, FAOH, and FAPP were the highest in the surface horizon decreased with soil depth in each soil profile. However, those of HAPP at 3400 m increased with soil depth, especially in the Bw2+Bw3 horizon, the HAPP value was higher than HAOH value. In this region, these results indicated that HAs mainly exist as a free form in surface soils, but that the contribution of the combined-form HAs increases in subsoils of lower elevation.
When the combined-form HAs were compared between the Cl-OH and OH-PP method, the amounts of ∆HACl were substantially larger than those of HAPP and increased with decreasing elevation (Table 2). This suggests that the presence of HAs associated with Ca was larger than with Fe and Al. HAs extracted with Na4P2O7 revealed a stronger association with Fe and Al than with Ca (e.g., Nakamura et al. 2007). As described in Section 3.1, the soils in this study were Ca-enriched. Muneer and Oades (Muneer and Oades 1989a, 1989b, 1989c) showed that Ca has an important effect on stabilizing the SOM in Ca-rich soils. These results suggest that HAs associated with Ca act as the main combined form of HAs, probably due to loess fallout and deposition from higher to lower sites.
The proportion of HAPP amounts in the total HAs extracted with both NaOH and Na4P2O7 (HAOH + HAPP) obtained by the OH-PP method (HAPP/(HAOH + HAPP)) increased with soil depth in each of the soil profile, and showed an increasing tendency with decreasing elevation (Table 2). This agrees with the distribution of ∆HACl and suggests that HAs associated with Fe and Al were remarkable in the deeper soils of lower elevation. In addition, the combined-form HAs are considered more stable than those of the free forms (Kumada 1987). From this perspective, HAs extracted with NaOH were supposed to be relatively easily degradable, and those extracted with Na4P2O7 were supposed to remain in the subsoils because of high stability. The relatively lower HAPP/(HAOH + HAPP) at higher elevation may be result of the suppression of microbial activity (Tan et al. 2010; Wang et al. 2013) and the reduction of the degradation of SOM (Martin et al. 1998), mainly due to the lower temperatures.
3.4. Optical characteristics of HAs
The optical characteristics of HA in each sample in the Cl-OH and the OH-PP method are shown in Table 2. The ∆log K values tended to decline, while the A600/C values tended to increase with soil depth in each profile, as seen from both methods. The lower ∆log K values and the higher A600/C values represent a higher degree of humification of HAs (Ikeya and Watanabe 2003). The degree of humification generally tended to increase with decreasing elevation. This was probably related to temperature (Hargitai 1994). Air temperature and soil temperature both increased with decreasing elevation from 4200 to 3600 m in the same region during summer in 2007: 7.5°C for air temperature and 5.6°C for soil temperature at 4200 m, and 12.0°C for air temperature and 9.9°C for soil temperature at 3600 m (Hirota et al. 2005). Martin et al. (1998) indicated that humic substances extracted from the lower and warmer elevations are more humified than those of higher and cooler elevations. Moreover, A600/CCl increased with decreasing elevation and was the highest at the lowest elevation (4.6–9.3 at 3400 m), which agrees with the distribution of ∆HACl. However, A600/COH showed no remarkable variations along the elevational gradient in comparison with A600/CCl (Table 3). This result suggests that HAs associated with Ca existed preferably and had a higher degree of humification in the deeper soils. Thus, the presence of loess might be contributed to the formation of Ca-humic complexes and the humification of highly humified HAs, due to loess fallout and more deposition at lower elevations.
Table 3. Optical characteristics of humic acid in each sample by the Cl-OH and the OH-PP methods.
The HA was classified into four types, with the degree of humification higher in the order of Type A, Type B, Type Rp, and Type P (Kumada 1987). The humification degree of HACl was higher than that of HAOH and was always classified as Type A from the deeper horizons (Table 3). The lower humification degree of HAs for the surface horizon was probably due to mixing of fresh plant residue from vegetation (Nakamura et al. 2007). The ∆log K-A600/C plots of HAOH and HAPP extracted by the OH-PP method in each horizon are shown in Fig. 2. The HAs extracted with NaOH followed by Na4P2O7 (n = 17) are classified into Type Rp • A (HAOH • HAPP; n = 3), Type Rp • B (n = 1), Type B • A (n = 11), Type B • B (n = 1), and Type A • A (n = 1; Fig. 2). The main HA combination type of this study was Type Rp • A and Type Rp/B • A and often obtained from the Ca-rich soils (Kumada 1987). It is clear that HAPP mostly belonged to Type A. These results are in agreement with those of Kumada (1987), who reported that the combined form of HAs extracted with Na4P2O7 always showed a high degree of humification.
Published online:
22 November 2018Figure 2. Classification diagram of humic acid (HA) extracted by the OH-PP method. Circle and triangle symbols represent HA initially extracted with NaOH (HAOH) and HA subsequently extracted with Na4P2O7 (HAPP), respectively.
![]({"type":"image","src":"/na101/home/literatum/publisher/tandf/journals/content/tssp20/2019/tssp20.v065.i01/00380768.2018.1547098/20190210/images/medium/tssp_a_1547098_f0002_b.gif"})
Figure 2. Classification diagram of humic acid (HA) extracted by the OH-PP method. Circle and triangle symbols represent HA initially extracted with NaOH (HAOH) and HA subsequently extracted with Na4P2O7 (HAPP), respectively.
Type A-HA is commonly observed in the A horizon of Japanese Andosols derived from volcanic ash and charred materials could be an important source (Tate et al. 1990; Shindo et al. 2004; Nishimura et al. 2009). However, in the A horizon of Chernozemic soils, charred organic C was also observed in Europe (Schmidt et al. 1999), which contributes to C storage and the formation of Type A-HA, similar to the humus accumulation in Japanese Andosols (Nishimura et al. 2009). In this study, the soils are neither Andosols nor Chernozems in all the study sites. However, HACl from the subsoils and most of the HAPP belonged to Type A. Tate et al. (1990) also reported similar results in the Foxton soils of New Zealand that contain charcoal particles in the surface soils and fine fragments in the subsoils (Cowie 1968). These findings suggest that fire could also be associated with the formation of type A-HAs obtained from the non-volcanic ash soils. Although the occurrence of fire in the Qinghai–Tibet Plateau is low (Madsen et al. 2006; Rhode et al. 2007), charcoal records reveal the presence of tree or woodland and subsequent conversion to pastureland since at least 4000 cal. BP (Kaiser et al. 2006, 2009). It is possible that environmental changes due to use of fire by humans resulted in an extraordinary increase in CO2 around 8000 cal. BP (Ruddiman 2003). Furthermore, Kaiser et al. (2007) examined paleosols in soil samples from the Qilian Mountains in the Haibei area of Northeast Tibet, Qinghai Province, and their study sites were very close to those of the current study. The soil classification and recent vegetation were similar. They reported that some charcoal particles of Picea were found in three out of five soil profiles, dating to 8267 ± 72 cal. BP (HAB3), 8894 ± 69 cal. BP (HAB4), and 8988 ± 66 cal. BP (HAB5), which reveals that large areas of Qinghai–Tibet Plateau were forest in the past. Therefore, in this study, it is considered that Type A-HA was likely formed after burning of the plants or trees in the past in the surface soils and then buried due to the deposition of loess. Less-oxidized conditions might play a key part in the formation of Type A-HAs (Maie et al. 2002), particularly in the deeper soils of this study. Detailed information on the formation of Type A-HA in this region is scarce, and this subject needs to be revisited in future investigations.
4. Conclusions
Properties of SOM in the alpine grassland in the Qinghai–Tibet plateau were understood based on the humus composition analysis and optical characteristics of HAs as follows:
The percentage of ∆HACl/HACl ranged from 5% to 65%, and the amounts of HAOH were remarkably larger than those of HAPP, indicating that HAs exist mainly in a free form in this region.
The amount of ∆HACl was considerably higher than that of HAPP, indicating that HAs associated with Ca serve as the main combined form of HAs.
The proportion of HAPP/(HAOH + HAPP) increased with soil depth and decreasing elevation, indicating that HAs associated with Al and Fe were distinguished in the subsoils at lower elevations.
HACl from the subsoils and almost all HAPP belonged to Type A.
In this study, SOM quality was evaluated using the humification index. However, it is also important to reveal the structural characteristics of HA considering the SOM properties. Thus, additional studies, such as those with nuclear magnetic resonance spectroscopy, are needed to elucidate the chemical structure of HA.
| Particle size distribution | Exchangeable bases | Citrate | Oxalate | Pyrophosphate | |||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Depth | pH | TC | IC | TN | Sand | Silt | Clay | EC | Ca2+ | Mg2+ | K+ | Na+ | CEC | Base Saturation | Fed | Ald | Fed−Feo | Feo | Alo | Fep | Alp | ||||
| Soil horizon | (cm) | H2O | KCl | (g kg−1 soil) | C/N | (%) | Soil texture | (mS m−1) | (cmolc kg−1) | (%) | (g kg−1) | ||||||||||||||
| 4200 m (N 37º42ʹ42ʹ’, E 101º22ʹ33ʹ’, Regosol) | |||||||||||||||||||||||||
| A1 | 0–10 | 6.6 | 5.8 | 84.9 | N.D. | 6.1 | 13.9 | 25.0 | 47.2 | 27.8 | SiC | 8.7 | N.A. | N.A. | N.A. | N.A. | N.A. | N.A. | N.A. | N.A. | N.A. | N.A. | N.A. | N.A. | N.A. |
| AB | 10–14 | 7.3 | 6.4 | 49.7 | 0.1 | 3.8 | 13.0 | 26.0 | 46.5 | 27.4 | SiC | 6.7 | 53.8 | 3.5 | 0.4 | 0.2 | 36.4 | 159 | 14.3 | 1.9 | 10.7 | 3.6 | 1.6 | 0.8 | 0.8 |
| BC | 14–30 | 7.7 | 6.4 | 21.3 | N.D. | 1.6 | 13.5 | 21.8 | 52.0 | 26.2 | SiC | 3.8 | 39.1 | 3.0 | 0.6 | 0.3 | 37.4 | 115 | 16.9 | 2.1 | 12.2 | 4.8 | 2.1 | 1.0 | 1.0 |
| 4000 m (N 37º42ʹ27ʹ’, E 101º22ʹ19ʹ’, Calcaric-Skeletic Cambisol) | |||||||||||||||||||||||||
| A1 | 0–3 | 6.5 | 5.7 | 55.2 | 1.0 | 4.4 | 12.7 | 14.2 | 53.2 | 32.5 | SiC | 8.8 | 45.3 | 3.7 | 0.9 | 0.3 | 31.4 | 160 | 15.4 | 2.3 | 10.8 | 4.5 | 2.2 | 1.0 | 1.1 |
| AB | 3–10 | 6.7 | 5.5 | 33.7 | N.D. | 2.8 | 12.0 | 15.1 | 52.8 | 32.1 | SiC | 4.6 | 41.1 | 3.0 | 0.5 | 0.2 | 32.4 | 138 | 16.2 | 2.4 | 11.3 | 4.9 | 2.3 | 1.1 | 1.1 |
| Bw1 | 10–20 | 6.8 | 5.6 | 31.1 | N.D. | 2.5 | 12.3 | 15.0 | 53.1 | 31.9 | SiC | 3.9 | 41.2 | 2.8 | 0.5 | 0.2 | 33.4 | 134 | 16.5 | 2.4 | 11.3 | 5.2 | 2.5 | 1.2 | 1.2 |
| Bw2 | 20–30 | 7.3 | 5.7 | 18.3 | N.D. | 1.2 | 14.7 | 19.5 | 54.7 | 25.8 | SiC | 2.4 | 41.3 | 2.8 | 0.5 | 0.2 | 34.4 | 130 | 17.3 | 2.7 | 11.7 | 5.6 | 2.8 | 1.7 | 1.6 |
| Bw3 | 30–38 | 7.4 | 5.8 | 14.1 | N.D. | 0.9 | 15.2 | 21.3 | 54.0 | 24.8 | SiCL | 2.5 | 36.9 | 2.5 | 0.4 | 0.2 | 35.4 | 113 | 16.8 | 2.4 | 11.6 | 5.1 | 2.6 | 1.4 | 1.2 |
| 3800 m (N 37º36ʹ47ʹ’, E 101º18ʹ24ʹ’, Pachic-Calcaric Phaeozem) | |||||||||||||||||||||||||
| A1 | 0–2.5 | 6.0 | 5.4 | 96.4 | 0.6 | 7.8 | 12.4 | 14.7 | 52.5 | 32.8 | SiC | 25.1 | 30.5 | 3.9 | 0.7 | 0.1 | 44.2 | 80 | 13.8 | 2.1 | 8.7 | 5.1 | 2.1 | 1.9 | 1.3 |
| A2 | 2.5–9 | 6.1 | 5.4 | 67.4 | 0.6 | 5.5 | 12.3 | 16.1 | 52.7 | 31.1 | SiC | 13.7 | 26.3 | 3.3 | 0.5 | 0.1 | 39.8 | 76 | 14.4 | 2.2 | 8.7 | 5.7 | 2.3 | 2.1 | 1.5 |
| A3 | 9–20 | 6.5 | 5.6 | 56.5 | N.D. | 4.4 | 12.7 | 18.6 | 50.4 | 31.0 | SiC | 7.7 | 26.6 | 3.1 | 0.4 | 0.1 | 37.3 | 81 | 15.1 | 2.4 | 8.8 | 6.2 | 2.5 | 2.3 | 1.6 |
| A4 | 20–25 | 6.7 | 5.7 | 49.1 | N.D. | 3.7 | 13.2 | 16.3 | 52.7 | 31.0 | SiC | 4.4 | 26.5 | 3.0 | 0.3 | 0.1 | 36.2 | 83 | 15.3 | 2.5 | 8.8 | 6.5 | 2.6 | 2.4 | 1.7 |
| Bw1 | 25–36 | 6.9 | 5.7 | 22.5 | 0.2 | 1.5 | 14.8 | 15.6 | 52.5 | 31.9 | SiC | 4.7 | 18.2 | 2.4 | 0.3 | 0.1 | 30.4 | 69 | 16.4 | 2.6 | 11.4 | 5.0 | 2.8 | 1.7 | 1.6 |
| 3400 m (N 37º39ʹ54ʹ’, E 101º19ʹ51ʹ’, Pachic-Calcaric Phaeozem) | |||||||||||||||||||||||||
| A1 | 0–9 | 6.5 | 5.9 | 116.8 | 0.2 | 8.4 | 13.9 | 17.3 | 58.4 | 24.3 | SiCL | 18.3 | 41.9 | 3.3 | 1.2 | 0.2 | 44.3 | 105 | 13.9 | 1.8 | 11.7 | 2.2 | 1.6 | 0.9 | 0.6 |
| A2 | 9–18 | 6.7 | 5.8 | 58.2 | 0.5 | 4.6 | 12.6 | 17.0 | 58.3 | 24.8 | SiCL | 8.0 | 32.2 | 2.0 | 0.4 | 0.1 | 35.3 | 98 | 16.3 | 2.0 | 13.1 | 3.2 | 2.0 | 1.6 | 1.9 |
| AB | 18–27 | 6.9 | 6.0 | 50.5 | N.D. | 4.2 | 12.1 | 16.6 | 60.8 | 22.6 | SiCL | 7.8 | 31.0 | 1.9 | 0.3 | 0.1 | 30.2 | 110 | 16.6 | 2.0 | 13.6 | 3.0 | 1.8 | 1.5 | 1.6 |
| Bw1 | 27–45 | 6.9 | 6.2 | 35.6 | N.D. | 2.6 | 13.5 | 18.0 | 56.7 | 25.3 | SiC | 4.9 | 24.7 | 3.1 | 0.3 | 0.2 | 28.4 | 100 | 18.6 | 1.9 | 16.1 | 2.5 | 1.9 | 1.3 | 1.7 |
| Bw2 | 45–79 | 7.3 | 6.3 | 27.2 | 0.3 | 1.8 | 14.8 | 23.3 | 52.2 | 24.5 | SiCL | 6.2 | 23.4 | 3.1 | 0.3 | 0.1 | 28.4 | 95 | 18.7 | 2.0 | 15.8 | 2.9 | 1.7 | 1.3 | 0.7 |
| Bw3 | 79–100 | 7.3 | 6.4 | 28.4 | N.D. | 2.0 | 14.3 | 17.3 | 56.5 | 26.2 | SiC | 5.5 | 23.6 | 3.1 | 0.2 | 0.1 | 28.8 | 94 | 17.5 | 2.0 | 14.6 | 3.0 | 1.7 | 1.4 | 0.8 |
N.D.; Not detected, N.A.; Not analyzed, TC; total carbon, IC; Inorganic Carbon.
| Location (Altitude) | Horizon | SOC (TC – IC) | Cl-OH | OH-PP | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Pre-FACl | HACl | FACl | HACl | FACl | HNCl | ∆HACl | ∆HACl/ HACl | HAOH | FAOH | HAPP | FAPP | HAPP/(HAOH+ HAPP) | HAOH-PP | FAOH-PP | HNOH-PP | |||
| (g kg−1) | (C g kg−1) | (%) | (g kg−1) | (%) | (C g kg−1) | (%) | (%) | |||||||||||
| 4200 m | A1 | 84.9 | 2.0 | 21.1 | 15.5 | 25 | 21 | 55 | 3.2 | 15 | 17.9 | 20.4 | 1.4 | 1.9 | 7 | 23 | 26 | 51 |
| AB | 49.6 | 1.2 | 13.2 | 10.4 | 27 | 23 | 50 | 4.1 | 31 | 9.1 | 12.4 | 1.4 | 2.0 | 13 | 21 | 29 | 50 | |
| BC | 21.3 | 0.4 | 5.7 | 4.6 | 27 | 24 | 49 | 1.9 | 33 | 3.8 | 5.0 | 0.7 | 1.3 | 16 | 21 | 30 | 49 | |
| 4000 m | A1 | 54.2 | 1.6 | 12.0 | 13.4 | 22 | 28 | 50 | 2.1 | 18 | 9.9 | 16.5 | 1.7 | 2.0 | 15 | 21 | 34 | 45 |
| AB | 33.7 | 0.7 | 8.6 | 8.5 | 25 | 27 | 47 | 1.7 | 20 | 6.9 | 10.5 | 0.8 | 1.2 | 11 | 23 | 35 | 43 | |
| Bw1 | 31.1 | 0.6 | 8.7 | 8.1 | 28 | 28 | 44 | 0.4 | 5 | 8.2 | 9.3 | 1.0 | 1.3 | 11 | 30 | 34 | 36 | |
| Bw2 | 18.3 | 0.3 | 6.4 | 4.0 | 35 | 23 | 42 | 3.1 | 49 | 3.2 | 4.7 | 0.8 | 0.9 | 20 | 22 | 30 | 47 | |
| Bw3 | 14.1 | 0.2 | 4.3 | 2.9 | 30 | 22 | 48 | 2.1 | 50 | 2.1 | 3.4 | 0.6 | 0.6 | 21 | 19 | 29 | 52 | |
| 3800 m | A1 | 95.8 | 4.1 | 26.7 | 22.7 | 28 | 28 | 44 | 4.1 | 16 | 22.5 | 29.0 | 2.9 | 3.0 | 12 | 27 | 33 | 40 |
| A2 | 66.8 | 2.6 | 21.5 | 16.7 | 32 | 29 | 39 | 4.7 | 22 | 16.9 | 20.0 | 2.0 | 2.2 | 11 | 28 | 33 | 39 | |
| A3 | 56.5 | 1.8 | 18.3 | 12.2 | 32 | 25 | 43 | 3.8 | 21 | 14.5 | 16.8 | 2.2 | 2.5 | 13 | 30 | 34 | 36 | |
| A4 | 49.1 | 1.5 | 17.1 | 11.1 | 35 | 26 | 39 | 5.2 | 30 | 11.9 | 13.4 | 2.4 | 2.5 | 17 | 29 | 32 | 38 | |
| Bw1 | 22.3 | 0.6 | 9.6 | 4.5 | 43 | 23 | 34 | 4.7 | 49 | 4.9 | 5.2 | 2.3 | 1.2 | 32 | 32 | 29 | 39 | |
| 3400 m | A1 | 116.6 | 3.3 | 33.1 | 21.1 | 28 | 21 | 51 | 10.1 | 31 | 23.0 | 24.1 | 2.9 | 2.9 | 11 | 22 | 23 | 55 |
| A2+ AB | 54.0 | 0.8 | 12.4 | 6.2 | 23 | 13 | 64 | 4.7 | 38 | 7.7 | 8.5 | 3.3 | 1.6 | 30 | 20 | 19 | 61 | |
| Bw1 | 35.6 | 0.6 | 15.6 | 5.1 | 44 | 16 | 40 | 8.2 | 52 | 7.4 | 6.2 | 4.9 | 1.6 | 40 | 35 | 22 | 43 | |
| Bw2+ Bw3 | 27.5 | 0.4 | 12.0 | 3.2 | 43 | 13 | 43 | 7.8 | 65 | 4.2 | 3.6 | 6.0 | 1.3 | 59 | 37 | 18 | 45 | |
Pre-FACl: amounts of fulvic acid fractions by HCl pretreatment; HACl and FACl: humic acid fractions and fulvic acid fractions extracted by the Cl-OH method; HAOH and FAOH: amounts of HA and FA extracted with NaOH by the OH-PP method; HAPP and FAPP: amounts of HA and FA extracted with Na4P2O7 by the OH-PP method; ∆HACl: subtracting HAOH from HACl; ∆HACl/HACl: proportion of ∆HACl in HACl by the Cl-OH method; HAPP/(HAOH + HAPP): proportion of HAPP in total of HAOH and HAPP by the OH-PP method; HAOH-PP, FAOH-PP and HNOH-PP were proportion of total HA, FA and HN extracted with NaOH and Na4P2O7 by the OH-PP method in total carbon, respectively.
| Location (Altitude) | Horizon | Cl-OH | OH-PP | |||||||
|---|---|---|---|---|---|---|---|---|---|---|
| ∆log KCl | A600/CCl | TypeCl | ∆log KOH | A600/COH | TypeOH | ∆log KPP | A600/CPP | TypePP | ||
| 4200 m | A1 | 0.751 | 3.0 | B | 0.816 | 2.4 | Rp | 0.591 | 5.4 | A |
| AB | 0.705 | 3.4 | B | 0.800 | 2.6 | B | 0.567 | 5.9 | A | |
| BC | 0.645 | 3.7 | B | 0.794 | 2.3 | Rp | 0.519 | 7.1 | A | |
| 4000 m | A1 | 0.748 | 3.3 | B | 0.825 | 2.4 | Rp | 0.589 | 4.0 | B |
| AB | 0.688 | 3.6 | B | 0.793 | 2.5 | B | 0.558 | 5.0 | A | |
| Bw1 | 0.677 | 3.5 | B | 0.780 | 1.9 | Rp | 0.535 | 5.6 | A | |
| Bw2 | 0.559 | 6.1 | A | 0.672 | 4.0 | B | 0.483 | 10.4 | A | |
| Bw3 | 0.535 | 7.0 | A | 0.659 | 4.1 | B | 0.485 | 10.3 | A | |
| 3800 m | A1 | 0.721 | 3.7 | B | 0.791 | 2.8 | B | 0.569 | 4.8 | B |
| A2 | 0.680 | 4.1 | B | 0.761 | 3.1 | B | 0.536 | 6.1 | A | |
| A3 | 0.643 | 4.4 | B | 0.740 | 3.3 | B | 0.517 | 8.5 | A | |
| A4 | 0.612 | 5.1 | A | 0.723 | 3.4 | B | 0.493 | 10.1 | A | |
| Bw1 | 0.519 | 8.2 | A | 0.623 | 5.5 | A | 0.452 | 13.3 | A | |
| 3400 m | A1 | 0.683 | 4.6 | B | 0.733 | 3.4 | B | 0.542 | 7.2 | A |
| A2+AB | 0.565 | 7.0 | A | 0.670 | 3.9 | B | 0.492 | 9.7 | A | |
| Bw1 | 0.537 | 8.0 | A | 0.631 | 4.9 | B | 0.473 | 11.4 | A | |
| Bw2+Bw3 | 0.522 | 9.3 | A | 0.638 | 4.7 | B | 0.477 | 11.0 | A | |
∆log K: log (A400/A600), where A400 and A600 are absorbances of HA at 400 and 600 nm; A600/C: C is the organic carbon content of HA solution; Subscripts Cl, OH and PP display extraction with NaOH after HCl pretreatment by the Cl-OH method, NaOH and Na4P2O7 by the OH-PP method, respectively.
Acknowledgments
We thank Dr Morimaru Kida and Ms Huiqiao Wu of Graduate school of Agricultural Science, Kobe University for valuable discussion.