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Soils of arid and semi-arid Australia are typically low in both N and P relative to arid zones elsewhere. We examined relative limitations of N and P on productivity of two native perennial grasslands of sub-tropical, semi-arid, north-west Australia. Effects of single and combined additions of N (50 kg ha(-1)) and P (25 kg ha(-1)) on growth and plant nutrients were documented over 2 years. Addition of N alone significantly increased total cover, above-ground biomass, N concentrations and N content, in the first year following above-average summer rains. These effects persisted into the second growing season but were of reduced magnitude. Addition of P alone increased P concentrations and aboveground P content but had negligible effects on biomass, indicating luxury consumption. Combined addition of N and P had similar effects to addition of N alone, strengthening the argument that N and not P was limiting to growth. Nitrogen limitation was clearly indicated by low N to P ratios (< 7) in aboveground plant components. (C) 2001 Academic Press.