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[1] We compared nitrous oxide (N 2 O) emissions over 1 year from soils of plantations growing acacia, which is a leguminous plant capable of symbiotic nitrogen fixation in root nodules, and secondary forests in Sumatra, Indonesia. N 2 O emissions from acacia plantation soils fluctuated seasonally, from high in the wetter season to low in the drier season, whereas N 2 O emissions from secondary forest soils were low throughout the year. Water-filled-pore-space data showed that denitrification contributed substantially to N 2 O emissions from soils at acacia sites. The average annual N 2 O flux in acacia plantations was 2.56 kg N ha À1 a À1 , which was eight times higher than that from secondary forest soils (0.33 kg N ha À1 a À1 ). In secondary forests, NH 4 + was the dominant form of inorganic nitrogen. However, in acacia plantations, the NH 4 + : NO 3 À ratio was relatively lower than that in secondary forests. These results suggest that secondary forests were nitrogen limited, but acacia plantations were less nitrogen limited. Leguminous tree plantations may increase nitrogen cycling, resulting in greater N 2 O emissions from the soil. However, on a global warming potential basis, N 2 O emissions from acacia plantation soils accounted for less than 10% of the carbon uptake by plants. Nevertheless, because of the spread of leguminous tree plantations in Asia, the importance of N 2 O emissions from leguminous tree stands will increase in the coming decades.

Globally, tropical deforestation is often followed by the establishment of fire-prone grassland.

To clarify the reason for the higher CH 4 uptake rate in Japanese forest soils, twenty-seven sites were established for CH 4 flux measurement. The first order rate constant for CH 4 uptake was also determined using soil core incubation at 14 sites. The CH 4 uptake rate had a seasonal fluctuation, high in summer and low in winter, and the rate correlated with soil temperature at 17 sites. The annual CH 4 uptake rates ranged from 2.7 to 24.8 kg CH 4 ha -1 y -1 (the average of these rates was 9.7 or 10.9 kg CH 4 ha -1 y -1 , depending on method of calculation), which is somewhat higher than the uptake rates reported in previous literature. The averaged CH 4 uptake rate correlated closely with the CH 4 oxidation rate of the topsoil (0-5 cm) in the study sites. The CH 4 oxidation constant of the topsoil was explained by a multiple regression model using total pore volume of the soil, nitrate content, and C/N ratio (p \ 0.05, R 2 = 0.684). This result and comparison with literature data suggest that the high CH 4 uptake rate in Japanese forest soils depends on the high porosity probably due to volcanic ash parent materials. According to our review of the literature, the CH 4 uptake rate in temperate forests in Europe is significantly different from that in Asia and North America. A new global CH 4 uptake rate in temperate forests was estimated to be 5.4 Tg y -1 (1 SE is 1.1 Tg y -1 ) on a continental basis.

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Root biomass and root distribution were studied in Entisols derived from the thick deposition of volcanic pumice on Hokkaido Island, Japan, to examine the effect of soil conditions on tree root development. The soil had a thin (<10 cm) A horizon and thick coarse pumiceous gravel layers with low levels of available nutrients and water. Two stands were studied: a Picea glehnii-Abies sachalinensis stand (PA stand) and a Larix kaempferi-Betula platyphylla var. japonica stand (LB stand). The allometric relationships between diameter at breast height (DBH) and aboveground and belowground biomass of these species were obtained to estimate stand biomass. The belowground biomass was small: 30.6 Mg ha −1 for the PA stand and 24.3 Mg ha −1 for the LB stand. The trunk/root ratios of study stands were 4.8 for the PA stand and 4.3 for the LB stand, which were higher than those from previous studies in boreal and temperate forests. All species developed shallow root systems, and fi ne roots were spread densely in the shallow A horizon, suggesting that physical obstruction by the pumiceous layers and their low levels of available water and nutrients restricted downward root elongation. The high trunk/root ratios of the trees may also have resulted from the limited available rooting space in the study sites.

The general chemical properties of Brown Forest Soils (BFS) differ according to the nature of their parent materials. The influence of volcanic ash as a soil parent material must be regarded as a factor producing differences in the submontane zone of the Kanto and Chubu districts in Japan. To determine the influence of volcanic ash, the free oxides and short-range ordered mineral compositions of these soils were examined. Brown Forest Soils, excluding the soils derived from volcanic ash in these regions, had crystalline iron oxide as the main fraction of the free oxides. Iron/aluminum-humus complexes were formed in these surface horizons. In BFS derived from volcanic ash, parameters such as the indexes of allophane and imogolite increased with depth, and became dominant at deeper horizons. Black Soils derived from volcanic ash had chiefly non-crystalline iron oxide and non-crystalline Al-hydrous oxide. Even most BFS derived from nonvolcanic ash in the Kanto and Chubu districts had a little volcanic ash influence. Therefore, the possibility for these soils to be classified as Andisols was suggested, regardless of the surface geological features in the Kanto and Chubu districts.

It is essential to analyze chemical properties including the amount of various materials and the soil colloid characteristics in forest soils to forecast wood production and the distribution of and variations in the environmental functions of forest soils, such as conservation of stream water and carbon sequestration. Approximately 70% of the forest soils in Japan consists of Brown Forest Soils (BFS), which are considered to be typical zonal soils under the humid‐temperate and warm‐temperate regime of Japan. BFS were subclassifled into several groups according to the soil moisture environment along the slope and morphological properties. However, even the same type of soil may display different properties depending on the climatic conditions, parent materials and vegetation types. In the present study, the variations in the carbon content, nitrogen content, cation exchange capacity and some properties depending on the parent materials, were clarified by using 34 sola of BFS, and 3 sola of black soils (BLS) for comparison, which were distributed in the submontane zone of the Kanto and Chubu districts in central Japan under the same climatic conditions. We observed differences in the pH, cation exchange capacity, base saturation and clay content among BFS samples derived from various parent materials. The BFS derived from volcanic ash contained obviously larger amounts of carbon and nitrogen than the BFS derived from other parent materials. However, the BFS derived from volcanic ash differed from the BLS derived from volcanic ash in the vertical distribution pattern of carbon and nitrogen. Thus, even in the submontane zone of the Kanto and Chubu districts, the chemical properties of BFS varied considerably with the parent materials. It was concluded that the classification of BFS by the parent materials was useful for evaluating the ability of the BFS, that cover 70% of the forests in Japan, to store various materials.