Abstract
This study aimed to clarify pedogenetic processes and classification of yellowish Brown Forest Soils according to the Classification of Forest Soils in Japan and the Yellow Brown Forest soils according to the Unified Soil Classification System of Japan in the warm and cool temperate forest of Kyushu district, Japan. In addition, the study aimed to clarify a problem with the Unified Soil Classification System of Japan. Thirty-six soil profiles of Brown Forest Soils, including 13 yellowish Brown Forest Soils and 15 Yellow Brown Forest soils, were compared with regard to their chemical properties and the relationship with climatic conditions was assessed. The yellowish Brown Forest Soils had thin A horizons, low pH and low levels of free oxides in the B horizons, and a low amount of silica and a high aluminum and iron to silica ratio. These features were related to the paleo reddish weathering. The immaturely developed A horizon of the yellowish Brown Forest Soils was caused by these weathered, low-activity substances. The Yellow Brown Forest soils had low levels of active iron oxides and a low activity ratio of free iron oxides compared with the Haplic Brown Forest soils in the same thermal climatic conditions. The activity ratio of free iron oxides was correlated to mean annual air temperature with the carbon stocks and with many other chemical properties. Accordingly, classification of Brown Forest Soils was clearer according to thermal climatic conditions. The activity ratio of free iron oxides can become an effective index that distinguishes Yellow Brown Forest soils under warm temperate lucidophyllous forest and Haplic Brown Forest soils under cool temperate broad-leaved deciduous forest with considerable vertical soil zonality.
INTRODUCTION
Yellow Brown Forest soils (CitationMatsui 1964) or warm-temperate forest soils (CitationEndo 1966) are assumed to be distributed in warm temperate lucidophyllous forest climate conditions, and are different from the Brown Forest Soils in the cool temperate broad-leaved deciduous forest zone in Japan. These soils show features such as a thin organic layer and a slightly reddish or yellowish color in the B horizons compared with Brown Forest Soils, although these features of Yellow Brown Forest soils and warm-temperate forest soils are not enough to be Red Soils (CitationEndo 1966). According to the Unified Soil Classification System of Japan – 2nd Approximation (2002) – (USCSJ 2nd) (CitationThe Fourth Committee for Soil Classification and Nomenclature, The Japanese Society of Pedology 2003), these soils are classified as Yellow Brown Forest (YB) soils within the Brown Forest soils great group, and correspond to the yellowish Brown Forest Soils in the Classification of Forest Soils in Japan – 1975 – (CFSJ) (CitationForest Soil Division 1976). The yellowish Brown Forest Soil (yBFS) subgroup in the CFSJ has thin and lighter colored A horizons, stronger yellowish colored B and C horizons, stronger acidity and a lower ratio of cation exchange capacity to clay content compared with the typical Brown Forest Soil (BFS) subgroup (CitationForest Soil Division 1976). The yBFSs have been assumed to be the intermediate soils between the paleo reddish or yellowish weathering materials and the Brown Forest Soils under present climate conditions (CitationOhmasa 1977). The yBFSs account for approximately 10% of the Brown Forest Soils group and are widely distributed in the western part of Japan (CitationEditorial Committee of the Forest Soils of Japan 1983). In contrast, YB soils have been assumed to be zonal soils present in the warm temperature soil forming conditions in southwestern Japan, between the BFSs in the cool-temperate zone and the Red-Yellow soils in the subtropical zone (CitationMatsui 1964). The question remains as to which soils evolve to become Brown Forest Soils or YB soils in the warm-temperate zone in southwestern Japan. However, in the CFSJ, there was no separation of the soils in warm-temperate lucidophyllous forests and cool-temperate broad-leaved deciduous forests. Furthermore, each Brown Forest Soil and related soil corresponded to Cambisol (CitationThe Fourth Committee for Soil Classification and Nomenclature, The Japanese Society of Pedology 2003).
The difference between the YB soils in a warm-temperate zone and the Brown Forest Soils under a cool-temperate zone is shown by the activity ratios of free iron oxides (CitationNagatsuka 1975). The activity ratio of free iron oxides gives a comparative maturity and crystallinity of free iron oxides (CitationSchwertmann 1964). The Brown Forest Soils in the cool-temperate zone had high activity ratios of free iron oxides because the crystallization of free iron oxides is inhibited with mostly non-crystalline iron oxides binding with low polycondensation humic substances. In contrast, the YB soils in the warm-temperate zone had low activity ratios of free iron oxides because the crystallization of free iron oxides progresses without controlling the non-crystalline iron oxides by binding with the small amount of humic substance that is present (CitationNagatsuka 1975). The activity ratio of free iron oxides was adopted as a diagnostic criteria for classification to the YB group from the Brown Forest soils great group according to the Unified Soil Classification System of Japan (1st Approximation) (CitationCommittee for Soil Classification and Nomenclature 1986). However, this classification could not be used for boundary soils as it could for forest soils (CitationYambe and Yagi 1983) and cultivated soils (CitationMitsuchi 1985). Therefore, carbon content and soil color were used as other criteria to distinguish between YB and Haplic Brown Forest (HB) soils in the USCSJ 2nd (CitationThe Fourth Committee for Soil Classification and Nomenclature, The Japanese Society of Pedology 2003). In the reports by CitationYambe and Yagi (1983) and CitationYambe and Kurotori (1970), the activity ratios of free iron oxides in the forest soils were found to be Red Soils < reddish or yellowish Brown Forest Soils < BFS regardless of present bio-climatic conditions. In contrast, the activity ratios of free iron oxides in the BFS in the warm-temperate zone in the southwestern part of the Shikoku district had various ratios for each profile (CitationYambe and Kurotori 1972). Moreover, the yBFS in the intermediate zone between the warm-temperate zone and cool-temperate zone in Kitaibaraki had high activity ratios of free iron oxides (CitationYambe and Yagi 1988). There is the possibility that the soils with high activity ratios of free iron oxides in these reports were influenced by mixed volcanic ash. Studies have shown that mixed volcanic ash influenced the properties of the free oxides of BFSs in the Kanto district (CitationImaya et al. 2007). Therefore, it is possible that the activity ratios of free iron oxides have been changed by the volcanic ash influence. Thus, the classification of Brown Forest Soils and related soils was complicated by the volcanic ash influence and by present and past bio-climatic conditions.
In the present study, the Brown Forest Soils and related soils in submontane regions as a warm-temperate zone and in the mountains as a cool-temperate zone in the Kyushu district were investigated. These soils were classified according to the CFSJ and the USCSJ 2nd, and the chemical properties of each soil group were estimated. The influences of present thermal climatic conditions on the soils were considered according to the relationship between the soil chemical properties and climatic conditions. The present study aimed to clarify the pedogenetic processes and classification of yBFSs in a forest in the Kyushu district, as well as clarify a problem with the USCSJ 2nd.
MATERIALS AND METHODS
Study sites and soil sampling
shows the geological and vegetation characteristics of the soil sampling sites used in the present study. The study areas were located in southwestern Japan, Kyushu district, between 32″17′Ν and 33″51′Ν latitude, 130″05′Ε and 131″29′Ε longitude, and between 61 and 1,270 m a.s.l. The mean annual air temperature and precipitation were estimated from 1-km2 mesh data supplied by the Japan Meteorological Agency (CitationJapan Meteorological Agency 1996). The mean annual air temperature (MAT) ranged from 9.2 to 16.2°C. OSZ-1, OSZ-2, OSZ-3 and TMC-1, TMC-2 and TMC-3 belonged to the cool-temperate broad-leaved deciduous forest zone because the warmth indices were less than 85 (CitationKira 1945). Annual precipitation ranged from 1,661 to 1,856 mm year−1 at GKI, INO, KSI and YKY in Fukuoka Prefecture, from 1,820 to 2,560 mm year−1 at TMC, AMK, TMI, TUR and YTS in Kumamoto Prefecture, from 2,220 to 2,562 mm year−1 at KNZ and SFR in Saga Prefecture, and from 3,037 to 3,070 mm year−1 at OSZ in Miyazaki Prefecture. Most sites consisted of plantation forests of Japanese cypress (Chamaecyparis obtuse) or Japanese cedar (Cryptomeria japonica), and the remaining sites were secondary forests with evergreen or deciduous broad-leaved trees.
Table 1 Site descriptions
The soil profiles were examined according to the Guidelines for Soil Description, 3rd edn (CitationFood and Agriculture Organization 1990), and the soil types were classified based on the morphological features of the profile in accordance with the CFSJ, and based on the chemical features in accordance with the USCSJ 2nd.
Thirty-six soil profiles of Brown Forest Soils were studied. The soil samples were classified into 13 yBFSs and 23 BFSs according to the CFSJ, and into nine HB soils, 15 YB soils, five Red-Yellow soils, six Kuroboku soils and one Regosol according to the USCSJ 2nd. OSZ-1, OSZ-2, OSZ-3 and TMC-1, TMC-2 and TMC-3 in the cool-temperate zone were classified as BFSs, except that TMC-3 was yBFS according to the CFSJ, and Kuroboku soils other than TMC-1 were HB according to the USCSJ 2nd. Meanwhile, 30 soil profiles in the warm-temperate zone were classified into 12 yBFSs according to the CFSJ and 15 YB soils according to the USCSJ 2nd. Of these soils, three profiles were classified as both yBFS and YB soil. Another nine profiles of yBFSs in the warm-temperate zone were classified as five Red-Yellow soils, three HB soils and one Regosol. The profiles of Red-Yellow soils had warmth indices of 115 or more and were derived from sandstone. The 18 soil profiles of the BFSs in the warm-temperate zone were classified into five HB soils, 12 YB soils and one Kuroboku soil.
The soil pits were dug to a depth of 2 m or to the base rock when the base rock was shallower than 2 m. Soil samples for chemical analysis and cylinder samples for measuring the dry bulk density were collected from each soil horizon. Bulk samples of the mineral soil were air-dried and passed through a 2-mm sieve. Visible root fragments were removed manually.
Analytical methods
Selective dissolutions of aluminum (Al), iron (Fe) and silica (Si) compounds were carried out using three chemical reactants to obtain the non-crystalline components. “Active” or short range order Al, Fe and Si compounds (Alo, Feo, Sio) were extracted after shaking the soil with an ammonium oxalate solution buffered at pH 3. To extract Fe and Al complexed with organic matter compounds (Fep, Alp), the soil was shaken with a sodium pyrophosphate solution (CitationBlakemore et al. 1987). To extract “free” Fe and Al compounds (Fed, Ald), a citrate dithionite solution was used (CitationMehra and Jackson 1960). Concentrations of Fe, Al and Si in the supernatants were determined using inductively coupled plasma–atomic emission spectrometry (Optima-4300DV; Perkin Elmer, Waltham, MA, USA). Soil pH was measured both in water and in 1 mol L−1 KCl suspensions using a 1:2.5 w/v soil : solution ratio. The pH (NaF) was measured in a suspension of 1 g soil mixed with a 50 mL 1 mol L−1 NaF solution after 2 min of stirring (CitationFieldes and Perrot 1966). Total soil organic carbon and nitrogen were analyzed using the dry combustion method (MT-600 CN Corder Yanaco, Kyoto, Japan). The carbon stocks in the A horizons and above a depth of 30 cm were calculated using the total soil organic carbon content, dry bulk density and the abundance of stones in the field descriptions.
RESULTS
Comparison of the chemical properties for soil groups of each soil classification system
shows a comparison of the chemical properties of the BFS and the yBFS in the CFSJ, and the HB soils, YB soils, Red-Yellow soils and Kuroboku soils in the USCSJ 2nd.
In the CFSJ, there was no difference in climatic conditions such as MAT, warmth index and annual precipitation and altitude between the BFS and the yBFS. The values for thickness, pH (KCl), carbon stock and Sio in the A horizons, and thickness, pH (H2O), pH (KCl), Ald, Fed, Feo and Sio in the B horizons of the BFS were significantly higher than the values recorded for the yBFS (Steel–Dwass multiple comparison, P < 0.05).
In contrast, in the USCSJ 2nd, there was no difference in MAT, warmth index, annual precipitation and altitude between the HB, YB and Red-Yellow soils. The ranges of MAT, warmth index and altitude of the Kuroboku soils were comparable to those of the HB soils. The range of annual precipitation for the Kuroboku soils was significantly higher than that for the other soil groups (Steel–Dwass multiple comparison, P < 0.05). The Kuroboku soils had significantly higher pH (NaF), total carbon, total nitrogen, carbon stock, Ald, Alo, Alp, Fed, Feo, Fep, Sio and Alo + 1/2Feo in the A horizons, and pH (H2O), pH (KCl), pH (NaF), Alo, Sio, Alo + 1/2Feo and Alo/Ald in the B horizons than the HB soils (Steel–Dwass multiple comparison, P < 0.05). The HB soils had significantly higher Feo, Fep, Alo + 1/2Feo and Feo/Fed in the A horizons, and total carbon, total nitrogen, Feo, Fep, Alo + 1/2Feo and Feo/Fed in the B horizons than the YB soils. There was no difference in the chemical properties between the YB and the Red-Yellow soils. The Red-Yellow soils had lower mean values for thickness, pH (H2O), pH (KCl), total carbon and Fed in the A and B horizons than the YB soils, although the difference was not statistically significant.
Relationship between the chemical properties and mean annual air temperature
shows the relationship between carbon stock above a depth of 30 cm and MAT. Carbon stocks were higher in the Kuroboku soils with lower MAT and lower in the Red-Yellow soils with higher MAT. The distributional areas were comparable between the YB and the HB soils. Moreover, the distributional areas were comparable between the yBFSs and the BFSs. There was a significant negative correlation between carbon stocks and MAT as a whole profile (r = –0.69, P < 0.001). shows the relationship between the activity ratio of free iron oxides (Feo/Fed) and MAT at each of the B1 horizons. The distributional areas overlap, but group each soil type. There was a significant negative correlation between the activity ratio of free iron oxides and MAT as a whole profile (r = –0.76, P < 0.001). shows the correlation coefficient between other chemical properties and MAT for the A1 and B1 horizons as whole profiles. The pH (H2O), pH (KCl), total carbon, total nitrogen, Ald, Alo, Alp, Feo, Fep, Alo + 1/2Feo and Feo/Fed of the A1 horizons, and the pH (NaF), total carbon, total nitrogen, Ald, Alo, Alp, Feo, Fep, Sio and Alo + 1/2Feo of the B1 horizons were significantly correlated to MAT (P < 0.05).
Table 2 Comparison of the chemical properties of the soil groups in each soil classification system
Figure 1 Relationship between mean annual air temperature and carbon stocks above a depth of 30 cm. Typical Brown Forest Soils are classified as (▀) Haplic Brown Forest soils, (•) Yellow Brown Forest soils and (▴) Kuroboku soils. The yellowish Brown Forest Soils are classified as (□) Haplic Brown Forest soils, (○) Yellow Brown Forest soils, (▿) Red-Yellow soils and (▵) Kuroboku soils.
Figure 2 Relationship between mean annual air temperature and the activity ratio of free iron oxides in the B1 horizons. Typical Brown Forest Soils are classified as (▀) Haplic Brown Forest soils, (•) Yellow Brown Forest soils and (▴) Kuroboku soils. The yellowish Brown Forest Soils are classified as (□) Haplic Brown Forest soils, (○) Yellow Brown Forest soils, (▿) Red-Yellow soils and (▵) Kuroboku soils.
Table 3 Correlation coefficients of mean annual air temperature and some chemical properties of the samples
DISCUSSION
Characteristics and pedogenetic process of yellowish Brown Forest Soils
The yBFSs were characterized as having thin and light-colored A horizons and stronger acidity than the BFS according to the CFSJ. In the present study, the yBFSs had lower amounts of carbon stocks in the thin A horizons than the BFSs, although there was no difference in the total carbon content (). Meanwhile, the B horizons of the yBFSs had low pH, and low levels of free aluminum and iron. Moreover, the yBFSs had a low amount of silica and a high ratio of aluminum and iron to silica. These phenomena are similar to the genesis of red weathering crust in a warm and humid paleoclimate or from continuous weathering (CitationMatsui 1989). Moreover, CitationNishida (1977) reported that the yBFSs, reddish Brown Forest Soils and Red Soils were clearly recognized eluviations of SiO2, and enrichment of sesquioxides. Thus, it is considered that yBFSs are immaturely developed compared with BFSs under the same climatic conditions, as soil development is delayed by these weathering and low-activity substances.
The soil colors for the diagnostic criteria had different ranges for the yBFSs in the CFSJ and the YB soils in the USCSJ 2nd. Therefore, some yBFSs with a strong yellowish color were classified as Red-Yellow soils (CitationImaya 2003). It is assumed that Red-Yellow soils are strongly weathered (CitationThe Fourth Committee for Soil Classification and Nomenclature, The Japanese Society of Pedology 2003). The ratio of Alo/Ald was helpful in separating Red-Yellow soils and YB soils (CitationHirai et al. 1991). In the present study, ratios of Alo/Ald were no different between the YB soils and the Red-Yellow soils. Although it might be that the cation exchange capacity of the clay, clay mineral composition and humic substances were different in both soils. These strong yellowish-colored yBFSs that were classified as Red-Yellow soils were distributed on the Mesozoic sandstone at a low altitude around the Yatsushiro Sea of Kumamoto Prefecture. It is considered that the parent material influenced the soil colors.
Characteristics and pedogenetic process of Yellow Brown Forest soils
In general, Brown Forest soils in the cool-temperate zone have higher amounts of organic carbon in the deeper horizons than the YB soils in the warm-temperate zone (CitationThe Fourth Committee for Soil Classification and Nomenclature, The Japanese Society of Pedology 2003). The free iron oxides were associated with carbon storage (CitationEusterhues et al. 2005). The Brown Forest soils in the cool-temperate zone had high activity ratios of free iron oxides because the crystallization of free iron oxides is inhibited as mostly non-crystalline iron oxides bind with low polycondensation humic substances. In contrast, the Yellow Brown Forest soils in the warm-temperate zone had low activity ratios of free iron oxides because the crystallization of free iron oxides progresses without controlling the non-crystalline iron oxides by binding with the little humic substance that is present (CitationNagatsuka 1975). In the present study, there were higher amounts of total carbon in the B horizons for the HB soils than for the YB soils, although the thickness and amount of total carbon were comparable in the A horizons. Thus, there was deeper percolation of organic matter in the HB soils than in the YB soils. The activity ratios of free iron oxides were higher in the HB than the YB soils, although there was no difference in the amounts of Fed in the HB and the YB soils. Furthermore, HB soils had higher amounts of Fep as organically bound Fe than did the YB soils. In the HB soils, the ratios of Fep to Feo were high at approximately 70%. However, there was no difference in the MAT, warmth index, annual precipitation and the actual vegetation types for both areas. For this reason, the tentative theory that both soil groups divided under the present bio-climatic conditions would be inappropriate. However, in the USCSJ 2nd, it was noted that the soil groups were separated only by the contents of organic carbon and by soil color (CitationImaya 2003). The soil colors reflect the parent materials (CitationImaya et al. 2005). Furthermore, it was considered that a manganese coating inhibited the bright reddish or yellowish brown coloration of crystalline free iron oxides in the BFSs producing a mismatch with the activity ratio of free iron oxides (CitationYambe and Yagi 1983). Therefore, some HB soil fulfilled the diagnostic properties of the YB soil, except for the criteria of soil color. As a result, the thermal climatic conditions for HB soil were expanded. In addition, CitationEndo (1984) chose the warm-temperate forest soil classification and not the YB soil classification because the soil colors were controlled by the color tone of the parent materials. The characteristics of the YB soil in the present study are still in discrepancy with the warm-temperate forest soils of CitationEndo (1984), which have a thick horizon and a high pH value.
Effectiveness of the activity ratio of free iron oxides to distinguish between YB and HB soils
CitationNagatsuka (1975) proposed that YB soils in a warm-temperate zone have an activity ratio of free iron oxides of less than 0.4 in the subsurface horizons, and are different from soils under a cool-temperate zone. Zonal soils in a warm-temperate deciduous broad-leaved forest climate and in a warm-temperate lucidophyllous forest climate were classified as YB soils according to Nagatsuka's scheme (CitationKurihara et al. 2002). If the soil color in the diagnostic criteria of the Yellow Brown property in the USCSJ 2nd is replaced by the activity ratio of free iron oxides of less than 0.4, the KNZ-1, KNZ-4, KNZ-6, OSZ-4 and GKI-1 of the YB soils are reclassified as HB soils, and TUR-1 of the HB soils is reclassified as a YB soil. Although, in this case, a division of both soil groups was also a mismatch with regard to the thermal climate according to the warmth index. It was considered that the carbon contents in the upper part of the cambic horizons, not necessarily in the B horizons, were comparable between the YB and HB soils. In contrast, soils at 700 m or more altitude with a warmth index of 90 or less had one or more subsurface horizons with an activity ratio of free iron oxides of 0.4 or more (,). Thus, the activity ratio of free iron oxides is beneficial for classification according to thermal climatic conditions in cool-temperate forest and warm-temperate forest soils with a few exceptions. Moreover, the activity ratio of free iron oxides, total carbon, carbon stock and any other chemical properties had a significant correlation to the MAT as a whole profile. Therefore, these soils were under the influence of present thermal climatic conditions. These results suggest that thermal climatic conditions are important in separating Brown Forest Soils and related soils.
Table 4 Number of B horizons in the Brown Forest Soils that fit reference values of the activity ratio of free iron oxides (Feo/Fed)
Effect of volcanic ash on the activity ratio of free iron oxides
In a previous study, the properties of free oxides of the BFSs differed according to the influence of volcanic ash in the Kanto district (CitationImaya et al. 2007). Accordingly, there was a concern that the activity ratio of free iron oxides was different from the value predicted by thermal climatic conditions to the value predicted by volcanic ash influences. Volcanic ash influences were estimated from values of pH (NaF) and Alo + 1/2Feo as the Kuroboku properties in the USCSJ 2nd. The HB soils had higher activity ratios of free iron oxides and higher values of Alo + 1/2Feo than the YB soils. However, values of pH (NaF) and Alo were comparable between the HB and YB soils. Therefore, it is difficult to say whether both soils had different influences of volcanic ash. It was concluded that the activity ratio of free iron oxides was not under the influence of volcanic ash.
Classification of yBFSs under warm-temperate forest in the Kyushu District
In the submontane and mountain zones of the Kyushu district, with regard to vertical soil distributions, YB soils under the warm-temperate lucidophyllous forest were divided into HB soils under the cool-temperate broad-leaved deciduous forest using the activity ratio of free iron oxides. In contrast, separation of yBFSs and BFSs is attributed to a difference in the degree of weathering of the parent materials and is designated by the thickness and amount of carbon stocks in the A horizons and the pH value and amounts of free oxides in the B horizons. Thus, it was possible to estimate the genetic process of the Brown Forest Soils under the warm-temperate zone in the Kyushu district according to varied indices of each soil classification system. Furthermore, soils under the warm-temperate forest soils in the Kyushu district had been subject to each genetic process that has been considered for the yBFSs and the YB soils. The meanings of the classification of the Brown Forest Soils under the warm-temperate zone in the Kyushu district are different according to which process is assumed to be important. CitationMatsui (1964) regarded the Red Soils in southwestern Japan as special soil genera in the YB soils derived from paleo red weathering crust. Laterization, such as reddish and yellowish weathering, has been received though the yBFSs and is considerably weak compared with the Red Soils and the reddish Brown Forest Soils (CitationNishida 1977). In contrast, the Kuroboku soils as interzonal soils are separate from the YB soils according to their unique properties under the same bioclimatic conditions. According to this ideology, yBFSs under the warm-temperate zone in the Kyushu district will be regarded as one soil genus of the YB soils derived from weathering materials under the paleoclimate because these soils do not have characteristics markedly different from those of the BFSs. Thus, the yBFSs in the warm-temperate zone are immature soils derived from weathering materials under the paleoclimate, and are currently in the YB soil forming process.
Conclusions
The chemical properties of Brown Forest Soils and related soils in the submontane and mountain zones in the Kyushu District were different according to the thermal climatic conditions at these altitudes. When vertical soil zonality was considered, the activity ratio of free iron oxides was shown to be an effective index to distinguish between YB soils under warm-temperate forests and HB soils under cool-temperate forests.
The yBFSs in the warm-temperate zone were immature soils derived from weathering materials affected under the paleoclimate, and currently in the YB soil forming process.
It is possible to subdivide Brown Forest Soils and related soils according to a combination of soil groups from the different soil classification systems, even though these soils have been classified by each classification system. Further study is necessary to evaluate differences in productivity and tree growth among the soil groups.
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