Measurements of the amount of organic matter accumulated in the forest floor of plantations of Pinus radiata D. Don in South Australia have shown that there is a considerable variation in stands of the same productivity (Florence & Lamb 1975). The amounts accumulated at various sites ranged from 12 to 28 t ha-' (Table 1). The greatest accumulations were measured at sites with deep sand podzol soils and lesser accumulations were found at sites with humus podzols, or where the common sand podzols were influenced by terra rossa soils. The amount of variation in what otherwise appear to be similar stands is surprising. The material in the forest floor is entirely derived from the needles, twigs, bark, etc. of P. radiata. All the stands were of the same productivity (Site Quality II) and were at an age (30-40 years) when forest floor accumulation is probably approaching a steady state condition (Florence & Lamb 1975). The stands sampled are all within 50 km of each other and, because the topography in the district is generally flat, there are probably only small microclimatic differences between sites. Analysis of the nitrogen and phosphorus contents of the forest floor showed that considerable amounts of these nutrients could be temporarily immobilized in these accumulations (Lamb 1972). The forest floor on the common sand podzol soils, for example, was found to contain c. 680 kg ha-1 of nitrogen, representing c. 55% of the total nitrogen in the aboveground organic matter. The magnitude of these nutrient accumulations is particularly interesting in view of reports of reduced productivity in the second rotation of these forests (Keeves 1966). Replanted stands have been found to be significantly less productive than the first rotation crop. This productivity decline has been observed in sites of high, medium and low fertility. However, most of the plantations are established on deep sand podzols having relatively small nitrogen contents (Stephens et al. 1941) and it is possible that the continued high productivity of these forest ecosystems is closely related to the rate at which the large litter nitrogen accumulations are mineralized and recycled. The present study was carried out to investigate the pattern of nitrogen mineralization in the forest floors from plantations at contrasting sites. Net mineralization was measured in two ways: firstly in a glasshouse pot study by measuring the growth and nitrogen uptake of P. radiata seedlings entirely dependent for nutrition on the nutrients released from forest floor samples, and secondly by standard incubation techniques.