Guano from breeding seabirds provides a large external source of nutrients to the soils of breeding colonies. However, little is known of guano P retention relative to N or the relative importance of guano and soil parent material as P sources. Soil profile N and P inventories (0?0.60m, n = 4; 0?0.36m, n = 1) and guano N and P concentrations were measured at a Westland petrel colony, and the parent material contributions of P were calculated using Ca, Al, Fe, Ti, and Zr as reference elements. Median inventories (0?0.60m) were 1.49kgN/m2 and 332 10?3kgP/m2, the N result being similar to that from a seabird colony on peat soil where N retention was very low. Calculated parent material contributions were smallest (32?66% of soil P) when based on Ca and largest (47?102% of soil P) when based on Zr. Contributions were similar for Al, Fe, and Ti; Al (41?87% soil P) was selected for subsequent calculations. Regardless of the reference element, parent material therefore contributed a large part of soil P. Phosphorus in excess of parent material supply (Pexcess) was significantly correlated with soil C, implying that guano P is held primarily in organic form. The median soil N : P molar ratios were 9.6 : 1 based on total P and 11.2 : 1 based on Pexcess, compared with ratios for Westland petrel guano of 4.1 : 1 (when birds were consuming fisheries waste) and 16.4 : 1 (when fisheries waste was replaced by fish). The similarity between soil and guano N : P ratios implies that both N and P are lost from soil at similar rates, although volatilisation of N would enrich soil drainage water in P. Calculations using guano deposition rates from the literature yielded P residence times of 4?15 years (Pexcess) and 11?41 years (total P), consistent with a highly dynamic soil system.