The amount of tropical plantations is increasing due to the demand for woody biomass. Acacia is one of the major plantation genera which is planted as frequently as Eucalyptus. Acacia mangium has been planted in more than 2 million ha around the world, which is nearly a quarter of the total planted area of all Acacias [1]. A. mangium is one of the major fast-growing species which yields 31–128 kg ha⁻¹ year⁻¹ of nitrogen (N) [2,3], and which grows well in the wet tropics even under acidic and poor nutrient conditions [4]. However, such fast-growing plantations have been criticized because of their resource consumption, such as of water and nutrients, their lower biodiversity and vulnerability to pests and disease [5]. In addition to these problems, A. mangium is also criticized for extracting large nitrous oxide (N₂O) from soil [6,7].

Symbiotic nitrogen fixation is one of the major pathways of N input to forest ecosystems enriching N availability, particularly in lowland tropics. Tropical forests generally are large N₂O and NO sources compared to other forests in upper latitudes, because of higher N availability [8]. As compared to carbon dioxide (CO₂), N₂O is a 300-fold stronger green house gas, and is considered the most dominant ozone depleting substance of the 21st century [9]. Arai et al. [6] reported that A. mangium plantations produced 8-fold larger N₂O emissions compared to adjacent secondary forests during wet seasons and these results were supported by Konda et al. [7]. They attributed the reasons for the larger emissions to dinitrification [6] and N mineralization [7], due to higher N availability under A. mangium soil, in addition to sufficient soil water conditions. Previous studies on N input through litterfall and N mineralization in A. mangium plantations have reported a large N availability [10-13]. In addition to standing plantations, logging of tropical forests can increase N₂O emissions by promoting soil N mineralization [14]. Therefore such large areas of short-rotation A. mangium plantations can be a crucial source of N₂O, which may reduce CO₂ mitigation effects by accumulating C in their biomass.

In contrast to the large N cycling in A. mangium plantations, phosphorus (P) cycling is unclear. Some tropical N₂-fixing trees exhibit specific P acquisition abilities by producing larger extracellular phosphatase [15]. This phosphatase production was larger in soils under A. mangium than those under secondary forests [16]. Houlton et al. [17] suggested that N₂-fixing plants could thrive under P-deficit conditions, such as those encountered in the wet tropics, in part by enhanced phosphatase production using the abundant N resources. A. mangium growing in solution cultures and pots were found to use P efficiently under P-deficit conditions [18]. Hence, the phosphorus cycling in A. mangium plantations is expected to be different from that of others.

In this study, we reviewed the difference in N and P retranslocation and in N and P demand in the case of the A. mangium plantation compared with two other plantations of Swetenia macrophylla and Araucaria cunninghamii; as well as litterfall data from adjacent primary forests [19]. We also discussed how A. mangium plantations can influence the regional and global N and P cycling and nutrient conditions for other plants.

This study was conducted at the Gum Gum Forest Reserve in Sandakan, Sabah, Malaysia (5°52′N, 117°54′E). The average annual precipitation was 2,572 mm, and the average annual temperature was 27.9 °C during the measurement period. This area has a wet tropical climate, with no month receiving a long-term average of less than 100 mm of rain. Fluctuations in the mean monthly temperature are less than ±1.5 °C, and no obvious long droughts have occurred. The research sites were on gently sloping alluvial plains at an altitude of less than 40 m above sea level. Litterfall, fresh leaves and fine-root ingrowth by nutrient applications were examined at three plantations: (i) 20-year-old Acacia mangium Willd., (ii) 34-year-old S. macrophylla King, and (iii) 27-year-old Araucaria cunninghamii Sweet., as of 2002, located within 1 km of each other. A. mangium is a N₂-fixing evergreen tree from northern Australia and Papua New Guinea, S. macrophylla is a non-N₂-fixing evergreen tree from Central and South America, and Ar. cunninghamii is a conifer from Australia and Papua New Guinea, respectively. The forest conditions of each plot are described by Inagaki et al. [20]. The soil is Haplic Alisol, and the chemical conditions in the surface soil (0–5 cm, 5–15 cm, and 15–30 cm) were virtually the same among the sites, particularly in the sub-surface soil [21].

All litterfall data was obtained from Inagaki et al. [13]. Litterfall samples were collected from March 2002 to February 2005 (3 years), twice per month on the 1st and 15th days, using 1 m × 1 m square litter traps. The litter traps were randomly placed at each site, corresponding to the density of one trap per 0.01 ha. Litter samples were dried and separated into five fractions: species leaves, other leaves, fine wood (<20 mm in diameter), reproductive parts, and miscellaneous. Large wood samples (≥20 mm) were omitted from the analysis because of large fluctuations in values. Fresh leaf data was obtained from Inagaki et al. [22]. Fresh leaf samples were collected in July 2008 using slingshots from as high as possible on five different trees of each species. All samples were dried at 70 °C to a constant weight, and were ground before chemical analyses. Total C and N concentrations were measured using the dry combustion method (NC-22F, Sumitomo chemical, Tokyo, Japan). For P analyses, samples were tested on an ICP-AES (Optima 4300 DV, PerkinElmer, Waltham, MA, USA) using the wet combustion method.

The root-ingrowth core method was used for evaluating the N and P limitations at the plantations [20]. Expanded vermiculite was immersed in a 0.1 M NH₄NO₃ for N application and in a 0.1 M KH₂PO₄ solution for P application for 24 h. For combined treatments, a 0.1 M mixed solution of both NH₄NO₃ and KH₂PO₄ was used. As the control, vermiculite was immersed in water. We filled 1-mm-mesh polyethylene bags with approximately 100 g of moist vermiculite, that is called as root-ingrowth core [23,24]. Six replicates were prepared for each treatment, and 24 cores were placed at each site. We installed the cores on October 2002. Soil monolyths of 5 cm in diameter and 10 cm in depth (ca. 200 mL) were excavated vertically from the soil surface using augers, and the cores were positioned in the soil surface void. The depth was almost equivalent to the Ah horizon, where most fine roots were found to be distributed. The in-growth cores were collected after 5 months. Penetrating roots were carefully cut away from the cores on site using scissors. At the laboratory, vermiculite and roots were separated using tweezers. The root samples were dried at 70 °C for 3 days and then measured for dry weight.