Background <p>Soil organic phosphorus (P) and its chemical nature change markedly during long-term pedogenesis, but how variation in ecosystem water balance and associated differences in vegetation impact such transformations remain unclear.</p> Methods <p>We used solution ³¹P-nuclear magnetic resonance (³¹P-NMR) spectroscopy to assess the chemical nature of soil organic P along two &gt; 2-million-year coastal sand dune chronosequences in south-western Australia characterised by contrasting ecosystem water balance. We sampled soils from the progressive and retrogressive stages of the ecosystem along the wetter Warren and drier Jurien Bay chronosequences.</p> Results <p>Organic P was a much greater proportion of the total soil P in the wetter Warren than the drier Jurien Bay chronosequence. However, the composition of soil organic and inorganic P detected by ³¹P-NMR spectroscopy was similar in the two chronosequences. Orthophosphate and simple phosphomonoesters were the dominant P species, and their proportional importance increased as soils aged, constituting &gt; 80% of soil total P in the late stages of pedogenesis. However, no higher-order inositol phosphates were detected along either chronosequence, presumably due to the sandy texture and limited sorption capacity of the soils.</p> Conclusion <p>Our results provide evidence that ecosystem water balance has little impact on the long-term soil organic P transformations during pedogenesis in south-western Australian dune sequences.</p>