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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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