An unprecedented increase in oil palm developments may be underway in Papua New Guinea (PNG) through controversial "special agricultural and business leases" (SABLs) covering over two million hectares. Oil palm development can create societal benefits, but doubt has been raised about whether the SABL developers intend establishing plantations. Here, we examine the development objectives of these proposals through an assessment of their land suitability, developer experience and capacity, and sociolegal constraints. Our review reveals 36 oil palm proposals with plantings planned for 948,000 ha, a sevenfold increase over the existing planted area in PNG. Based on our criteria, however, we estimate that only five plantations covering 181,700 ha might eventuate within the foreseeable future. We conclude that most of the developers are clearing forest with no intention of cultivating oil palm, and that a large-scale land grab is therefore occurring in PNG under the guise of oil palm development.

Nitrogen (N) fertiliser is an important and expensive input to oil palm in Papua New Guinea. Of about 3000 mm/year of rainfall, about 1300 mm is lost as evaporation. This leaves an excess of >1000 mm/year lost as surface runoff and/or deep drainage, and with it the potential for N loss. Approximately 11% of rainfall reached the ground as stem flow. Throughfall was generally lowest near the trunk and highest where canopies overlapped, but random spatial variability was large. The difference between the measured rainfall and stem flow plus throughfall was 6%, indicating relatively little interception. Surface runoff from the volcanic ash soils was 6% of rainfall at one site, but only 1.4% at the other. Less than 2% of the applied N was lost in the surface runoff after an ammonium chloride application. Calculations of N leaching losses made using suction cup data and the water balance indicated that significant losses occur, but the estimates were not reliable due to the huge spatial variability in the suction cup and throughfall data. Therefore, another technique is needed to study N leaching in oil palm plantations.

Knowledge of where roots are active is crucial for efficient management of nutrients in tree crops but measurement of root activity is problematic. Measurement using soil water depletion is an approach that has not been tested in a humid climate. We hypothesised that the three dimensional distribution of root activity of a tree crop in the humid tropics (a) can be determined by measuring soil water depletion during rain-free periods, and (b) is influenced by environment (soil type and climate) and surface management. A field study was carried out in which soil water content was measured and water uptake calculated (by difference between soil water content at beginning and end of rain-free periods) for different surface management zones and depths (0.1 m intervals to 1.6 m depth) under oil palm (Elaeis guineensis Jacq.) at a loam-clay site and a sandy site. Significant differences were measured between sites and between surface management zones at each site. At both sites water uptake was highest under the weeded zone close to the palm stem, slightly lower under the zone where pruned fronds are placed, and lowest under the path used for removing harvested fruit. Vertical distribution of root activity differed between the sites, with higher activity near the surface at the finer textured site. Total water uptake values were lower than estimates of evapotranspiration made using climate data. The difference was probably largely due to water uptake from deeper than 1.6 m. This study showed that the spatial distribution of tree root activity in a humid climate could be quantified using a relatively simple method.

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Oil palm (Elaeis guineensis Jacq.) crops are expanding rapidly in the tropics, with implications for the global carbon cycle. Little is currently known about soil organic carbon (SOC) dynamics following conversion to oil palm and virtually nothing for conversion of grassland. We measured changes in SOC stocks following conversion of tropical grassland to oil palm plantations in Papua New Guinea using a chronosequence of plantations planted over a 25-year period. We further used carbon isotopes to quantify the loss of grassland-derived and gain in oil palm-derived SOC over this period. The grassland and oil palm soils had average SOC stocks of 10.7 and 12.0 kg m À2, respectively, across all the study sites, to a depth of 1.5 m. In the 0-0.05 m depth interval, 0.79 kg m À2 of SOC was gained from oil palm inputs over 25 years and approximately the same amount of the original grass-derived SOC was lost. For the whole soil profile (0-1.5 m), 3.4 kg m À2 of SOC was gained from oil palm inputs with no significant losses of grass-derived SOC. The grass-derived SOC stocks were more resistant to decrease than SOC reported in other studies. Black carbon produced in grassfires could partially but not fully account for the persistence of the original SOC stocks. Oil palm-derived SOC accumulated more slowly where soil nitrogen contents where high. Forest soils in the same region had smaller carbon stocks than the grasslands. In the majority of cases, conversion of grassland to oil palm plantations in this region resulted in net sequestration of soil organic carbon.