4.1. Conversion to Acacia Plantations Might Boost N2O Flux From Soils
 The conversion to acacia plantations in this region could increase N2O emissions from soils. Annual N2O emissions from acacia plantation soils were 2.56 kg N ha−1, eight times greater than those from secondary forest soils, indicating that conversion of secondary forests into acacia plantations will enhance soil N2O emissions. Because we did not measure the N2O flux in a grassland control, we cannot directly address the effect of grassland conversion on N2O flux. On Sumatra Island, N2O emissions from grassland soils have not been found to exceed those of forest sites [Ishizuka et al., 2002, 2005a; Verchot et al., 2006]. Thus N2O flux from grasslands is likely lower than that from acacia plantations. Further studies are needed to clarify the effect of acacia plantations on soil N2O flux.
 Annual N2O emissions from secondary forest soils (0.33 kg N ha−1) are low relative to other tropical forests; e.g., tropical forest emissions are estimated at 0.01–7.68 kg N ha−1 [Breuer et al., 2000] and Amazon secondary forests at 0.94 kg N ha−1 [Verchot et al., 1999] or 0.80 kg N ha−1 [Palm et al., 2002]. These low secondary forest emissions are comparable to those observed at primary forests in central Sumatra (0.13 kg N ha−1 and 0.39 kg N ha−1 [Ishizuka et al., 2002]), but relatively low compared to emissions from southern Sumatra forests (1.47 and 1.80 kg N ha−1 at sites showing high WFPS (90–100% [Verchot et al., 2006]) and montane forests in central Sulawesi (0.29, 1.01, and 1.11 kg N ha−1 [Purbopuspito et al., 2006]). Ishizuka et al. [2005a] suggested that even within a limited geographical region, N2O flux in soils having a udic moisture regime is higher than that in drier soils. Thus the relatively higher N2O flux observed by Verchot et al.  may be at least partly site specific and reflect exceptionally wet soil moisture conditions. To sum up existing data, N2O flux from Indonesian forest soils ranges from 0.13 to 1.80 kg N ha−1.
 The mechanism boosting N2O emissions from acacia plantations may be an enhanced soil nitrogen cycling rate. The net nitrification rate and nitrate content at acacia plots were greater than those at secondary forest sites (Table 3). These rapid rates of nitrification and high nitrate supply for the denitrification process may promote soil N2O emissions in acacia plantations. The relationship between N2O flux and the ratio of NH4+ to NO3− (a nitrogen limitation index [Verchot et al., 2006]) clearly differed at acacia plantations and secondary forests (Figure 7): acacia plantations were distributed along the Y axis, whereas secondary forests lay along the X axis. Because excess nitrate should be immediately absorbed by plants and microorganisms in the nitrogen limited sites, the ratio of NH4+ to NO3− should be higher. Therefore the low N2O flux in secondary forests might be due to nitrogen limitation, and the high N2O flux in acacia plantations might be partly due to the mitigation of nitrogen limitation. In the drier season, the NO3− content in AM1 and AM2 was high (Table 2) and nitrogen limitation was less severe, possibly due to the lesser demands of plants and microorganisms for nitrate. The net nitrification rate (Table 3) shows that the nitrate supply continued in April at AM2, although soil nitrate content at that time was relatively low (Table 2). Thus the decrease in NO3− content in the wetter season at AM1 and AM2 (Figure 6) may be due to plant uptake, corresponding to high growth in the wetter season and/or the loss by denitrification.
 The significant difference in wetter-season N2O emissions among the acacia plantations (AM3 > AM1 and AM2) indicates that emissions may vary considerably, depending on soil type. Relatively low-clay-content Acrisols have less structure development and high bulk density because soil shrinkage in drier periods is weaker than that of more clayey Acrisols, and particles of sand, silt, and clay are packed closely [Ohta and Effendi, 1992]. Thus in relatively low-clay-content Acrisols, soil porosity declines and drainage is consequently constrained. This may be why bulk density was higher at AM3, regardless of its higher sand content relative to the other acacia soils. Because of this soil physical property, WFPS at AM3 was high and provided ideal conditions for denitrification. This difference in pore space might also affect the proportion of gaseous species; for example, N2O is dominant at relatively high WFPS (70–90% [Davidson et al., 2000]). The low microbial biomass nitrogen and low nitrate content in AM3 soil (Table 2) indicates that nitrogen cycling at AM3 may be lower than that at the other acacia sites, but the proportion of nitrogen coming out as N2O might be larger at AM3 due to the high WFPS.
4.2. How Significant are N2O Emissions From Fast-Growing Leguminous Tree Plantations to Global Warming?
 In terms of global warming potential (GWP), carbon sequestration by acacia trees is far greater than the loss by N2O emission from soils. According to our biomass estimations using an allometric equation formulated by felling trees and excavating their roots and expressed as a function of DBH, the sum of above- and below-ground biomass in 7.7-year-old acacia stands was 136.4 Mg ha−1 for AM1, 154.2 Mg ha−1 for AM2, and 131.9 Mg ha−1 for AM3, respectively, and the mean annual growth in AM1, AM2, and AM3 was 17.7, 20.0, and 17.1 Mg ha−1, respectively (Kaneko et al., personal communication, 2007). The annual carbon uptake at AM1, AM2, and AM3 corresponds to 32.5, 36.7, and 31.4 Mg CO2 ha−1 a−1, respectively. The N2O flux at AM1, AM2, and AM3 (1.75, 2.27, and 3.67 kg N ha−1 a−1, respectively) was equivalent to 0.81, 1.06, and 1.71 Mg CO2 ha−1 a−1, respectively, using 296 as the GWP for N2O in 100 years [Prather et al., 2001]. Thus on a GWP basis, N2O emissions reduced the carbon uptake by plants by 2.5% at AM1, 6.2% at AM2, and 11.7% at AM3. Because the coarser textured soil of AM3 is not common in this plantation area, the N2O emission reduction appears to be less than 10% of the carbon uptake by plants.
 To our knowledge, this study is the first to attempt to clarify the effect of leguminous plantations on atmospheric N2O. In future studies, we plan to consider the following issues. Among the most important is determining how tree stand age affects N2O emissions. This study investigated 7-year-old acacia stands. The growth rate of plantation acacias tends to decrease after 6 years [Yonekawa and Miyawaki, 1988], and nitrogen uptake by trees may decrease at sites older than 7 years. Thus nitrogen limitation is less pronounced in older than in younger acacia stands, indicating that N2O emission rates in younger stands may be lower. Nitrogen input through litterfall also may change with increasing age. We are now monitoring N2O emissions at acacia stands of various ages and rotation stages. We are also assessing the influence of various silvicultural practices, such as weeding, harvesting, and harvest-slash management, on N2O flux fluctuations in acacia plantations.
 Because soil nitrogen content and N2O emissions from soil are affected by previous land use [Erickson et al., 2002], the effect of continuous land use on leguminous tree plantations requires clarification. For example, acacia plantations in our study region are usually rotated at 6- to 8-year intervals, and some plantation areas are now in their second or third rotation. Continuous nitrogen input to the soil from leguminous tree leaf litter maintains high nitrogen cycling, and the nitrogen level in second or third rotations might be greater than that in the first rotation. Thus investigating how N2O emissions and nitrogen cycling from leguminous tree plantations change with each rotation, and during harvests between rotations, is another important issue.
 Other common plantation leguminous trees include Paraserianthes, Leucaena, and Sesbania species, and evaluating the impact of different tree species on N2O emission rates is also important.
 Another critical issue is determining how leguminous plantations stimulate N2O emissions in phosphorus-limited conditions, which are common in the Asian humid tropics. Soil N cycling rates are high in many tropical forest ecosystems, relative to higher latitude forests [Vitousek, 1984], in part due to the presence of leguminous trees and other N-fixing plants, resulting in high nitrogen inputs to forest soils [Vitousek, 1984; Brown and Lugo, 1990]. In the neotropics, where leguminous tree species dominate forests [Primack and Corlett, 2005], the ecosystem is considered to be limited by phosphorus rather than nitrogen [Vitousek, 1984]. Thus inorganic nitrogen in soils dominated by NO3− [Verchot et al., 1999; Reiners et al., 1994] contributes to high N2O emissions from natural forests [Keller and Reiners, 1994; Verchot et al., 1999]. However, in the Asian tropics, natural forests at lower elevations are dominated by non-N-fixing dipterocarp trees [Primack and Corlett, 2005], and secondary forest dominants are not usually legumes either, as in this study. We found NH4+ to be the primary inorganic nitrogen in secondary forest soils, as in other Sumatran forests [Verchot et al., 2006; Ishizuka et al., 2002]. This suggests that mature nonlegume forests in this area may be nitrogen limited, with low N2O flux in these nitrogen-limited systems dependent on the low NO3− production rate. Consequently, an increase in total nitrogen input to such soils, for example by planting leguminous trees, increases nitrogen cycling and results in higher N2O emissions. Because excess nitrogen may easily promote N2O emissions from phosphorus-limited soils [Hall and Matson, 1999], leguminous tree plantations at phosphorus-limited sites may result in higher soil N2O emissions. We have initiated experiments to clarify the impact of leguminous plantations at phosphorus-limited sites.
 Currently, N2O emissions from leguminous plantations do not contribute seriously to global warming. Assuming that acacia plantations were converted from nonleguminous forests or grasslands and that the change in N2O emissions is identical to the difference between acacia plantations and secondary forest in this study, N2O emissions by acacia plantations account for 18.5 Gg N (=2.23 kg ha−1 a−1 × 8,317,000 ha), which corresponds to only 0.6% of N2O emissions from wet tropical forests (3 Tg N [Prather et al., 2001]). However, because of the spread of acacia and other leguminous tree plantations in Asia [FAO, 2001], the importance of N2O emissions from leguminous tree stands will increase in coming decades.