Carbon stock in litter, deadwood and soil in Japan’s forest sector and its comparison with carbon stock in agricultural soils
M. TAKAHASHI, Forestry and Forest Products Research Institute, 1 Matsunosato, Tsukuba, Ibaraki 305-8687, Japan. Email: firstname.lastname@example.org
Estimation of carbon sequestration in the forest sector should take into consideration changes in carbon stock in all carbon pools, including above-ground and below-ground biomass, litter, deadwood and soil. In this review, we discuss current knowledge of carbon stocks in litter, deadwood and soil in Japan’s forest sector. According to data from published reports and nationwide surveys, the carbon stock in forest litter is less than that indicated in the Intergovernmental Panel on Climate Change (IPCC) guidelines for temperate and cool temperate forests; for example, coniferous species showed 4.4 Mg C ha−1 for Cryptomeria japonica and 3.1 Mg C ha−1 for Chamaecyparis obtusa, and broad-leaved species ranged from 3.5 Mg C ha−1 for Castanopsis spp. to 7.3 Mg C ha−1 for Fagus spp. For deadwood carbon stock, coniferous plantations with a record of non-commercial thinning showed 17.1 Mg C ha−1 and semi-natural broad-leaved forests showed 5.3 Mg C ha−1 on average, although only limited data were available. The black soil group (comparable to Andosols and Andisols) showed large carbon stocks in soil layers 0–30 cm deep (130 Mg C ha−1). The brown forest soil group (Cambisols and Inceptisols), occupying the most dominant area, showed a carbon stock of 87.0 Mg C ha−1 on average, which was similar to the data shown in the IPCC guidelines. In a comparison of land use between the forest sector and the agricultural sector for the same soil group, the carbon stock in the agricultural soil was 21% lower and in the grassland soil it was 18% higher than the stock in the forest soil. In this review, we also discuss issues for improving the estimation method and inventory of carbon stock in litter, deadwood and soil in Japan.
Global warming is a major concern in both the Japanese domestic and international arena. According to the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (IPCC 2007), continuous greenhouse gas emissions at or above the current rates will cause further warming and induce serious changes in the global climate system in the 21st century. Although improved sequestration of carbon dioxide is expected through innovative engineering technology (e.g. Matysek et al. 2005), forests play an important role as a cost-effective carbon sink that absorbs carbon dioxide (Lal 2005; Stern 2007).
Soil is the largest carbon pool in the terrestrial ecosystem. Global carbon storage in soil is estimated to be 1500–2000 Gt C, which is three–fourfold greater than that in vegetation (Batjes 1996; Watson et al. 2000). In forest ecosystems, dead organic matter, such as litter and deadwood, forms specific carbon pools. Although there is no doubt that growing trees function as an active carbon sink, large emissions from dead organic matter and soil would count as a reduction in the amount of sequestrated carbon in the forest (Randerson et al. 2002). Thus, the National Inventory Report and the Kyoto Protocol report under the United Nations Framework Convention on Climate Change (UNFCCC) require that parties estimate the carbon stock not only in above-ground and below-ground biomass, but also in deadwood, litter and soil, separately.
Dead organic matter and soil carbon stock are influenced by vegetation, site conditions and forest management practices. For example, leaf litter from coniferous species usually decomposes more slowly to accumulate thicker organic layers than broad-leaved species towing to the higher lignin content (Berg and McClaugherty 2003; IPCC 2006). Mesic and productive sites have thin organic layers, referred to as mull-type humus, in temperate and cool temperate forests (Green et al. 1993; Uchida 1959). Deadwood stock is also influenced by forest type; old-growth forests usually show large carbon stock in decaying boles (Harmon and Hua 1991; Takahashi et al. 2000). Natural and anthropogenic disturbances, such as typhoons and non-commercial thinning operations, result in an immediate increase in deadwood stock (Harmon et al. 1986). Regarding soil carbon, soil type and soil texture are the decisive factors for carbon stock level (IPCC 2003; Parton et al. 1994). Andosols, particularly those with thick, fine-textured A horizons, sequestrate large amounts of carbon in the soil (Morisada et al. 2004; Shoji et al. 1993). Wide Andosol distribution is a unique characteristic of the soil cover in Japan.
Related to the effects of human activity in terrestrial ecosystems, land-use category is a key factor for determining the equilibrium level of carbon stock in the soil (Paul et al. 2002; Post and Kwon 2000). In general, forests and grasslands show high soil carbon stock as a result of the high input of dead organic matter from the vegetation. The conversion of forest land to agricultural land usually reduces the soil carbon stock and this has been a major source of carbon dioxide (CO2) emission throughout human history (Houghton 2003), although the application of organic manure and no-till farming maintain or enhance the soil carbon level (Lal 2004). Thus, we need to understand the steady-state carbon level in the land-use categories for each soil type.
Since 2007, the Japanese government has submitted an annual National Inventory Report (NIR) and Kyoto Protocol (KP) report to the UNFCCC secretariat (Ministry of the Environment, Japan 2008). The accounting methods for estimating the emission and removal (absorption or uptake) of greenhouse gases (GHG) in Japan’s forest sector can be referenced in previous reports (Fang et al. 2005; Matsumoto et al. 2007). In the present paper, we review the characteristics of litter, deadwood and soil carbon stock in the forest sector using data from the NIR and KP reports (Ministry of the Environment, Japan 2008) and data from other sources (e.g. Morisada et al. 2004; Takahashi 1995). Among the GHG, we focus on CO2 emission and removal in the present review; non-CO2 gases, such as methane and nitrous oxides, have been reported elsewhere (Ishizuka et al. 2009; Morishita et al. 2007). To understand the effect of changing land use on the carbon balance in ecosystems, a comparison of soil carbon stock is made between agricultural land and forest land. We also discuss methods for improving the estimation of carbon stock in dead organic matter and soil in the forest sector.
Intergovernmental Panel on Climate Change guidelines and reporting format
Annex I Parties of the UNFCCC are required to submit information on their national inventory of carbon emission and removal every year using the common reporting format provided by the UNFCCC office. To assist this process, the IPCC has prepared several guidelines: the Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories for Land Use, Land-Use Change and Forestry (IPCC 1996), the Good Practice Guidance for Land Use, Land-Use Change and Forestry (IPCC 2003) and the 2006 IPCC Guidelines for National Greenhouse Gas Inventories for Agriculture, Forestry and Other Land Use (IPCC 2006). Using these guidelines, most parties can calculate CO2 emission and removal in agriculture, forestry and other land-use sectors if they at least have data on the areas of the land-use categories (see below). In this review, the CO2 accounting methods are not explained, but the referenced IPCC guidelines can be downloaded from the UNFCCC website (http://unfccc.int/; June 2009).
Land-use categories and changes
All land in Japan is classified into six land-use categories (Forest Land, Cropland, Grassland, Wetland, Settlement and Other Land; Table 1) (IPCC 2006), and the emission and uptake of CO2 are calculated for each land-use category. These categories are further subdivided into land remaining in the same category and land converted from one category to another (Table 2). As the conversion of land-use often leads to high emissions of CO2, particularly, for example, in forest clearing for agricultural use (Guo and Gifford 2002; Houghton 2003; Murty et al. 2002), estimating the area of land-use change is the first step in the accounting procedure for CO2 emission and uptake (IPCC 2006).
Table 1. Land-use categories, their definitions and a comparison of areas and area percentages between 1990 and 2005 in Japan
|Forest land||Forests under Forest Law Articles 5 and 7.2||24,950 (66.1)||24,992 (66.1)|
|Cropland||Rice fields, crop fields and orchards||4,596 (12.2)||4,061 (10.7)|
|Grassland||Pasture land and grazed meadow land||660 (1.7)||638 (1.7)|
|Wetland||Bodies of water (such as dams), rivers and waterways||1,320 (3.5)||1,340 (3.5)|
|Settlement||Urban areas that do not constitute forest land, cropland, grassland or wetlands Urban green areas that are all wooded and planted areas that do not constitute forest land||2,750 (7.3)||3,170 (8.4)|
|Other land||Any land that does not belong to the above land-use categories||3,493 (9.2)||3,588 (9.5)|
|Total|| ||37,769 (100)||37,789 (100)|
Table 2. Matrix of land-use change areas between 1990 and 1991
| Forest land||23,632||167||51||57||375||69|
| Other land||1,188||251||258||0||IE||2,981|
As a definition of land-use categories is not provided by the IPCC guidelines, parties need to establish their own definition (IPCC 2006). This may cause some discrepancy in reported estimates among parties, but it is a practical solution for implementing the UNFCCC process. In Japan, forest land is defined by the following criteria: minimum area 0.3 ha, minimum tree crown cover 30%, minimum tree height 5 m and minimum width of forest land 20 m (Ministry of the Environment, Japan 2008). The area fulfilling these criteria covers 24,992 kha, which is 66.1% of Japan’s total land area (Table 1). Following forest land is cropland, covering 10.7% in 2005. The large forest cover over Japanese land means that the forests are expected to function as a national carbon sink. As conversion of land use has not been active in modern Japan, the proportion of land-use change was very small between 1990 and 2005, although land-use changes occurred between most land-use categories (Table 2). In fact, it has been reported that significant land-use change has not occurred in more than 100 years (Himiyama 1995). These data suggest that the emission and uptake of CO2 under afforestation, reforestation and deforestation (ARD) activities set out in Article 3.3 of the KP as significant causes of change in soil carbon stock (Watson et al. 2000), do not become weighty values in Japan compared with newly industrializing and developing countries.
Dead organic matter and soil carbon stock
Definition of litter, deadwood and soil
Dead organic matter is composed of litter and deadwood. Although the IPCC guidelines imply that litter is organic layers on the mineral soil surface, the term “litter” is equivocal in soil science because it is restricted to freshly fallen dead leaves (Wild 1971). Soil science conventionally describes decomposing dead leaves as humus (organic) layers, for example, an A0 layer consisting of L, F and H layers (Forest Soil Division 1976) or an O layer consisting of Oi, Oe and Oa layers (Soil Survey Staff 2006). There is no analytical definition for these layers and the boundary between “litter” and mineral soil can sometimes be ambiguous in the field, particularly when the site has an H (Oa) layer, even though the carbon content usually exceeds 20% of the weight (IUSS Working Group WRB 2006; Takahashi 1998). In Japan’s forest sector, “litter” includes the L, F and H layers on the mineral soil.
Deadwood, often referred to as coarse woody debris (CWD) (Harmon et al. 1986), is defined as non-living woody biomass and includes dead boles, stumps and snags. In Japan, the minimum size of deadwood is defined as 5 cm in diameter, which is smaller than the criteria (10 cm) indicated in the IPCC guidelines (IPCC 1996, 2003, 2006), because non-commercial thinning fall stunted trees which are usually smaller than 10 cm in diameter in Japan’s forest plantation (Sakai et al. 2008).
In Japan, carbon stock in the soil is defined as the organic carbon in the mineral soil from a depth of 0 to 30 cm. The stipulated thickness of the soil is rather shallow if we consider that a large carbon stock exists in deep soil layers (Batjes 1996); however, this value is in accordance with the IPCC guidelines. In addition, a uniform thickness among all land-use categories is indispensable for calculating soil carbon in the case of land-use change. In Japan’s forest sector, organic soil, such as peat, is not an important pool for carbon storage because peat soil exists in only 0.3% of the forest sector (Morisada et al. 2004).
Carbon stock in litter
The dry weight of litter in Japan’s forests has been compiled from various published and unpublished reports (Ono et al. 2001; Takahashi 1995). In cases where data on the carbon content of the litter were not available, a conversion equation was applied to obtain the carbon content from the dry weight (Takahashi 2005). Statistics on the major tree species are shown in Table 3. According to the IPCC guidelines (IPCC 2003, 2006), coniferous species have a carbon stock of 22 Mg C ha−1 and broad-leaved species have a carbon stock of 13 Mg C ha−1 on average in warm temperate moist climates. However, most Japanese tree species, including conifers, have litter pools with a low carbon stock of less than 10 Mg C ha−1. The widely planted coniferous species Cryptomeria japonica (Japanese cedar, Sugi) and Chamaecyparis obtusa (Japanese cypress, Hinoki) show quite low litter stocks of 4.35 and 3.11 Mg C ha−1, respectively. As C. japonica plantations are selected from among sites with moist and fertile soils (Hirai et al. 2006; Mashimo 1960), litter decomposition would be quick. For C. obtusa, fragments of decomposing litter are easily eroded on slopes (Miura 2000) or incorporated into mineral soil (Sakai et al. 1987; Tsukamoto 1991). Moreover, C. obtusa and C. japonica are planted in warm temperate areas and not in cool temperate zones, such as Hokkaido Island and high subalpine mountains. These environmental conditions and species characteristics of the litter appear to result in the accumulation of a small amount of litter in Japanese coniferous plantations. In cool temperate zones, however, coniferous species, including both natural and planted stands, show a larger carbon stock, 9.49 Mg C ha−1 for Abies and 11.53 Mg C ha−1 for Picea spp.
Table 3. Statistics on carbon stock in litter of major tree species (Mg C ha−1)
Carbon stock variations in broad-leaved species are also influenced by climatic conditions. Evergreen broad-leaved species such as Castanopsis spp., distributed in warmer climate areas, have lower carbon stock (5.11 Mg C ha−1), whereas deciduous species show relatively larger stock, particularly for Fagus spp. (10.0 Mg C ha−1) and Betula spp. (9.0 Mg C ha−1), in cool climates. The low carbon stock in the litter appears to be reflected in the quick litter decomposition rate, probably because of the warm and humid climatic conditions in Japan (Takeda et al. 1987).
Carbon stock in deadwood
Deadwood dynamics are closely related to forest management, such as thinning and harvesting operations. After pre-commercial and non-commercial thinning, living trees immediately change to deadwood and litter. Similarly, tree harvesting produces deadwood as stumps and slashes. According to the root/shoot ratio (R), which is also called the below-ground/above-ground biomass ratio (IPCC 2006), 1/4 to 1/3 of the total living biomass turns to deadwood after harvesting. This is the largest event creating deadwood carbon in plantation forestry.
The Forestry Agency of Japan conducted a survey on the amount of deadwood carbon stock in plantations with a record of non-commercial thinning (Sakai et al. 2008; Takahashi and Sakai 2006). Plantations that had been thinned over the previous 20 years had a carbon stock ranging from 6.7 Mg C ha−1 for Larix kaempferi (Japanese larch) to 22.3 Mg C ha−1 for Cryptomeria japonica on average (Takahashi and Sakai 2006)(Table 4). Owing to large variation in the size of the stands and the intensity of thinning, no clear relationship was found between deadwood carbon stock and years after thinning. It can be concluded that plantations sometimes show large carbon stock in the deadwood carbon pool, particularly after non-commercial thinning.
Table 4. Carbon stock in deadwood in Japanese forests (Mg C ha−1)
|Coniferous plantations with a record of non-commercial thinning†|
| Picea glehnii ||8||1.4–20.7||8.9||6.7||8.0||1–7|
| Abies sachalinensis||8||2.5–41.8||12.2||13.1||8.5||1–11|
| Larix kaempferi||16||0.6–29.4||6.7||7.1||3.5||1–14|
| Cryptomeria japonica||34||0.4–71.5||22.3||16.8||19.6||1–20|
| Chamaecyparis obtusa||36||1.5–68.7||19.6||15.4||15.6||1–20|
| Total||102||0.36–71.5||17.1||15.3||12.4|| |
|Natural and semi-natural forests|
| Broad-leaved spp.|
| Fagus crenata‡||6||2.9–5.4||4.2||0.9||4.3|| |
| Evergreen broad-leaved forests§||4||3.8–18.5||9.2||6.9||n.d.|| |
| Deciduous broad-leaved forests¶||3||1.1–9.3||5.0||4.2||n.d.|| |
| Total||13||1.1–18.5||5.3||4.8||3.8|| |
| Coniferous spp.|| || || || || || |
| C. japonica††||1||19.4|| || || || |
| Abies-Picea forests‡‡||3||18.3–36.8||24.7||10.5||n.d.|| |
Several reports have examined deadwood accumulation in natural and semi-natural forests of Japan (Matsuura et al. 2001; Jia et al. 2002; Jomura et al. 2007; Kawaguchi and Yoda 1986; Yoneda 1982) (Table 4), although the measurement methods used differed among the studies; for example, the smallest size of deadwood measured ranged from >1 cm to >10 cm and standing snags and stamps were sometimes ignored. Coniferous old-growth forests tend to show large carbon stock, such as Hokkaido Island’s Picea and Abies forests (24.7 Mg C ha−1) (Takahashi 1995) and a C. japonica forest in southern Yakushima Island (19.4 Mg C ha−1) (Yoneda 1982). Broad-leaved species usually show carbon stock lower than 10 Mg C ha−1. These field data indicate that the carbon stock in deadwood is generally smaller than that indicated in the IPCC Good Practice Guidance (IPCC 2003). Sakai et al. (2008) suggested that the warm and humid climate, which induces quick decomposition of deadwood, and small deadwood size may result in low accumulation of deadwood carbon in Japan’s forests.
It should also be noted that windblown disturbances caused by the many typhoons that occur around Japan and monsoon Asia can have a drastic effect on dead organic matter dynamics (Harmon and Hua 1991; Sato 2004; Takahashi et al. 2000) and can result in the unpredictable accumulation of deadwood in the stands. Dead and broken boles are usually withdrawn from plantation stands after several years (Yamaguchi et al. 1963), but in a natural forest on Hokkaido Island deadwood carbon still remained at 24.7 Mg C ha−1 42 years after a typhoon event (Takahashi 1995).
Carbon stock in the soil
All soil in Japan’s forest sector has been classified by the Japanese Forest Soil Classification System (Forest Soil Division 1976). Morisada et al. (2004) compiled data on the soil survey reports used to calculate soil carbon stock in Japan’s forests. There is wide variation among the soil groups, ranging from 39 (immature soil group) to 172 Mg C ha−1 (peat soil group) in soil layers 0–30 cm deep on average. The predominant soil group is brown forest soil, which covers 70% of the forest sector and has a carbon stock of 87 Mg C ha−1 (Table 5). Japanese brown forest soils are often strongly influenced by volcanic ash (Imaya et al. 2007) and are classified as Andisols in some cases (Soil Survey Staff 2006), but the mean value of all soil groups (90 Mg C ha−1) is not as high as we expected and is similar to the value (88 Mg C ha−1) indicated in the IPCC guidelines (IPCC 2006). Morisada et al. (2004) also reported that variation exists in the carbon stock in the brown forest soil group among soil types: drier soil moisture types, for example, dry brown forest soils, have a lower soil carbon stock compared with soil in moist sites, for example, slightly wet brown forest soils.
Table 5. Soil carbon stock and distribution area of forest soil groups in Japan calculated from data in Morisada et al. (2004)
|Brown forest soils|
(Inceptisols, Andisols/ Cambisols, Andosols)
(Ultisols, Inceptisol/Acrisols, Cambisol)
Black soil, referred to mainly as Andisols in Soil Taxonomy (Soil Survey Staff 2006) or Andosols in the World Reference Base (WRB) (IUSS Working Group WRB 2006), covers the second largest area in the forest sector. This is because Japanese soil is distinctively influenced by volcanic activity (Shoji et al. 1993; Takahashi et al. 2001; Wada 1986). The black soil group, derived from fine-textured volcanic ash, shows a large amount of soil carbon (130 Mg C ha−1), and this is far larger than the value of 80 Mg C ha−1 indicated by the IPCC guidelines for volcanic soil. This might be the result of humus characteristics, for example, characteristics of humus considered to be derived from charred plant materials (Shindo et al. 2004) and the low decomposability of the humus through the formation of alumino-humus bindings in weathering volcanic ash (Shirato et al. 2004).
With regard to volcanic soil, part of the immature soil group also derived from volcanic materials, such as slightly weathered pumice and scoria, is distributed around the mouths of active volcanoes. These soils contain only a small amount of carbon in the surface soil, for instance 31.9 Mg C ha−1 on Hokkaido Island (Sanada et al. 1995), although they often have a buried A horizon under the deposits of volcanic materials. This means that the influence of volcanic activity varies widely in the soil carbon stock, ranging from a minimum to a maximum influence, according to the deposition thickness and particle size of the volcanic products.
Another major soil type in the immature soil groups is eroded soil (weathered granite soil), which is distributed in western Japan where forest resources have been intensively used for several centuries (Totman 1999; Tsukimori et al. 1992). The other soil groups (podzol, red yellow soil, gley soil and peat soil) have minor distributions in Japan.
Distributions of soil carbon stock were depicted using 1 km × 1 km resolution maps in Hokkaido (Hakamata et al. 2000; Takahashi 2000) and in the forest sector of Japan (Morisada 2004). According to these maps, we conclude that wide variations in carbon stock and in the heterogeneity of the soil types in the mountainous landscape have created a diverse soil carbon distribution in Japan’s forest sector.
Forest and agricultural sectors in the same soil group
For soil comparisons between the forest and agricultural sectors, data on agricultural soil were obtained from the National Greenhouse Gas Inventory Report (Ministry of the Environment, Japan 2008); data were calculated from countrywide monitoring of soil characteristics in arable land conducted by the Ministry of Agriculture, Forestry and Fisheries of Japan and cooperating prefectural governments since 1979 (Nakai and Obara 2003). Monitoring sites were selected according to the area proportion of the soil groups and the number of sites totaled approximately 20,000 and covered the entire agricultural sector in Japan. Therefore, the average features of the agricultural soils could be represented using the monitoring dataset. For the Kyoto Protocol, the soil carbon stock in each land-use category of the agricultural sector was calculated using data surveyed around 1990, which is the base year of the Kyoto Protocol.
The IPCC Guidelines (e.g. IPCC 2006) recommend that where land-use change has occurred, carbon stock change is to be calculated in accordance with the linear change in carbon stock in a land-use category compared with that in the other land-use categories over a transition period of 20 years. However, a direct comparison of carbon stock between pre-land-use and post-land-use categories is not appropriate because the dominant soil type in each land-use category is biased. As shown in Table 6, the dominant soil types differ between the forest sector and the agricultural sector (Ministry of the Environment, Japan 2008). Brown forest soil is the main soil type in the forest sector, whereas lowland soil and gley soil dominant paddy fields, and Andosols dominant in cropland and grassland. In this mountainous country, agricultural lands have long been concentrated on flat land and on gentle slopes (Himiyama 1995). Grassland in Japan is mostly managed as grazing land and is very small in area; 1.7% of the total land area in 2005 (Table 1).
Table 6. Soil group composition (%) and land area in the land-use categories
|Black soils and Kuroboku soils|
|Brown forest soils|
(Inceptisols, Andisols/Cambisols, Andosols)
|Upland soils (Inceptisols, Ultisols, Entisols/Anthrosols, Gleysols, Planosols)||0.0||4.1||4.1||1.6||9.6|
|Red-yellow soils and dark red soils|
(Ultisols, Alfisols, Inceptisols/Acrisols, Cambisols, Luvisols)
|Lowland soils (Inceptisols, Entisols/Anthrosols, Fluvisols)||0.0||41.5||16.7||11.2||12.3|
|Gley soils (Inceptisols, Entisols/Anthrosols, Fluvisols)||1.7||30.8||0.7||0.5||0.0|
Carbon stock data were prepared for each soil group present in every land-use category, that is, Andosols, brown forest soils, red-yellow soils, dark red soils, gley soils and peat soils (Table 7). These soil groups cover 77.5–87.7% of each land-use category, except for rice fields, suggesting that the results of the comparison can be applied to most cases of land-use conversion from forest to agriculture in Japan. In rice fields, the soil groups for comparison cover 51.8% of the total area because the most dominant group, lowland soil, is not included.
Table 7. Average carbon stock (0–30 cm) in the soil groups, the percentage area of soil type in each land-use type and the emission factor
|Soil group (area of soil type in each land-use category (%))|
| Brown forest soils||70.9||0.2||15.7||36.9||17.5|
| Red-yellow soils||1.6||5.0||7.2||23.7||3.4|
| Dark red soils||0.2||0.1||1.6||1.5||3.0|
| Gley soils||1.4||30.8||0.7||0.0||0.0|
| Peat soils||0.3||3.8||1.8||0.0||2.8|
| Other soil groups||12.3||48.2||22.5||16.0||22.5|
|Mean carbon stock (Mg C ha−1)†|
| Brown forest soils||87||59.5||65.2||68.4||101.3|
| Red-yellow soils||69||63.2§||46.2¶||64.3¶||74.4§|
| Dark red soils||89||56.3||45.2||54.6||54.6|
| Gley soils||92||64.8||65.9|| || |
| Peat soils||172||115.0||184.9|| ||325.2|
| Brown forest soils||1.00||0.68||0.75||0.79||1.16|
| Red-yellow soils||1.00||0.92||0.67||0.93||1.08|
| Dark red soils||1.00||0.63||0.51||0.61||0.61|
| Gley soils||1.00||0.71||0.72|| || |
| Peat soils||1.00||0.67||1.08|| ||1.89|
|Weighted average emission factor||1.00||0.76||0.82||0.86||1.18|
| || || ||0.79‡‡|| || |
Emission from land-use change
The emission factor is the emission or removal of CO2 after conversion of land-use by human activities. To understand the effects of land-use change from forest to agriculture on soil carbon stock, the emission factor was determined by calculating the relative amounts of soil carbon stock in the agricultural sector to that in the forest sector. Table 7 shows that rice fields, croplands and orchards have a lower carbon stock than forest land in most soil groups and that the emission factors are similar irrespective of the soil group. This suggests that forest land in the soil groups compared contains similar proportions of labile and active soil carbon pools (Leifeld and Kögel-Knabner 2005; Parton et al. 1994), which could be considered easily decomposing organic carbon by disturbances for agricultural land use. The emission factor for rice fields was the lowest, 0.76, followed by crop fields and orchards. Converting from forest to agriculture land use, the area-weighted average of the emission factor was 0.79, meaning that 21% of the carbon stock in the soil would be released after the conversion. In contrast, the conversion to grassland accumulated carbon in the soil and its increment rate relative to forest soil was 1.18. Although dark red soil and peat soil showed a somewhat different trend, which may have resulted from statistics based on a small number (n = 9 for dark red soil and n = 3 for peat soil), the narrow distribution of the dark red soil and peat soil groups did not have a significant effect on the weighted average of the emission factor.
Land-use change from forest to agriculture usually results in large carbon emissions from dead organic matter and soil (Guo and Gifford 2002). Murty et al. (2002) reported that the change in soil carbon after land-use conversion from forest to agriculture was 22%. This rate is comparable to the reduction rate (21%) in our analyses. In rice fields, organic matter under anaerobic conditions is often considered to be recalcitrant (Kilham and Alexander 1984; Shirato 2006). Indeed, soil monitoring of Japan’s arable land has shown that the soil carbon concentration in the rice fields has been stable for 25 years in most soil groups, whereas that in cropland decreased with time (Nakai 2008). However, the emission factor for conversion from forest to paddy field is similar to that for crop fields in our analyses. This may be the result of the long-term use of land for rice or the artificial effects of soil dressing by land improvement activities in rice fields. Nakai (2008) suggested that some monitoring sites were influenced by soil dressing over the plowed layer (Ap horizon).
In grassland, there are large variations in carbon stock among soil groups, but appropriate management of grassland appears to result in the accumulation of soil carbon. Increased rates for soil carbon in grasslands are similar to the rates observed in forests (Post and Kwon 2000). Moreover, the maximum potential carbon stock in grassland and pasture soil is often larger than that in forest soil (Cerri et al. 2003; Halliday et al. 2003). In New Zealand, conversion from pasture to pine plantation could result in a 15% reduction of mineral soil carbon, except in high clay activity soils (Scott et al. 1999). As in New Zealand, Japan’s grassland management system appears to have the potential to increase the soil carbon stock.
The difference in carbon stock between the forest and agricultural sectors in this comparison would mostly be the maximum potential carbon emission or removal after the land-use change because soil carbon in each land-use category is considered to have almost reached equilibrium under continuous land use for several decades or centuries under the usual management system. However, soil carbon stock in agricultural soil varies with changes in management. Soil monitoring in the agricultural sector has revealed that some properties in the top layer have changed over the past 25 years (Nakai and Obara 2003; Obara 2000; Obara and Nakai 2003, 2004). A large input of organic manure in tea soil and greenhouse soil tended to accumulate carbon in the soil (Nakai 2008). It has also been suggested that the soil carbon concentration in orchard soil tends to increase, probably as a result of no-tillage management. Thus, agricultural soils have the potential to accumulate soil carbon through an improved soil management system and the emission factors in Table 7 may need to be revised in future.
Improvement in the estimating methods and inventory
The forest data analyzed were collected from published and unpublished reports on soil surveys conducted mainly in the late 20th century (Morisada et al. 2004). For the following reasons, caution should be used when evaluating these data. First, the data that we collected from representative soil profiles in the soil map reports might have been overestimated or underestimated because the representative soil pit was often selected at a point that had the most typically developed soil profile around the area. The soil profile of the representative pit was often emphasized by developing soil layers and characteristics compared with the average features of the soil around the area. Second, the soil carbon balance might have changed from the noontide period of the soil survey (1950s–1970s) because broad-leaved natural forests have been converted to coniferous plantation forests in a large area of Japan’s forest sector over the past 40 years (Handa 1988). The equilibrium of carbon stock under plantations could still be shifting even now. Third, the evaluation of gravel and stone content was not quantitative in the soil mapping survey, and this appears to influence accurate calculation of soil carbon stock.
Similar issues, such as biased site selection, can be pointed out when the carbon stock in litter is evaluated. For deadwood, nationwide data must be collected from a wide range of forest vegetation and management practices. As shown in Table 4, deadwood stock, which would be large carbon stock in managed forests under Article 3.4 in the Kyoto Protocol, is sometimes large and varies with the management methods of the forest. These issues would be solved by systematic and uniform sampling protocols. In 2006, a soil carbon inventory under a strategic soil survey project was launched to evaluate soil carbon stock in Japan’s forest sector (Takahashi and Moridasa 2008). We expect to be able to update the values on carbon stock in soil and dead organic matter in the future. In addition, repeated sampling should be organized using the same sampling protocols to monitor the effect of global warming on soil carbon stock in Japan.
To evaluate land-use change, we should establish long-term monitoring sites in each land-use category under different climate conditions. Soil carbon stocks in settlement and other land are still rather limited in Japan. Comparisons will need to be drawn between land influenced and not influenced by volcanic soil. Plantation forests should be included to detect changes in carbon balance in accordance with the management system. The modeling approach would also be highly variable (Liski et al. 2005; Parton et al. 1994). To understand the full flux of CO2 and carbon stock in all pools in the forest ecosystems, continuous efforts must be made in the future.
We reviewed the data on carbon stock in deadwood, litter and soil in Japan’s forest sector. The carbon stock in dominant coniferous plantation species litter, such as Cryptomeria japonica and Chamaecyparis obtusa, is small compared with that indicated in the IPCC guidelines (IPCC 2003, 2006). Deadwood accumulation is low in semi-natural forests, generally <10 MgC ha−1. Deadwood carbon stock in natural forests is also small, although plantations sometimes have large carbon stock after pre-commercial thinning. The black soil group has high carbon stock, 130 MgC ha−1 in layers 0–30 cm deep, although the brown forest soil group, which is the predominant soil type in the forest sector, has a carbon stock (87 MgC ha−1) comparable to the values indicated in the IPCC guidelines. Soil carbon stock in the agricultural sector is 21% lower than that in the forest sector except for grassland, which is 18% higher in soil carbon compared with forests. These values should be revised by systematic survey and monitoring in the future.
This study was funded by the Forestry Agency of Japan, the Ministry of the Environment Japan (Global Environment Research Fund B082 and Global Environment Research Account for National Institute) and the Ministry of Agriculture, Forestry and Fisheries of Japan (the Evaluation, Adaptation and Mitigation of Global Warming 11070). We thank Dr Masahiro Amano, Dr Mitsuo Matsumoto, Dr Yoshiyuki Kiyono and Dr Tamotsu Sato for their invaluable support and suggestions. We also thank Dr Hiroshi Obara of the National Institute for Agro-Environmental Sciences for his valuable guidance on arable soils in Japan.