Soil organic carbon (SOC) is both a source and sink of atmospheric carbon dioxide (C[O.sub.2]), with important implications for global climate change. Irrigation of grazed pastures has reportedly increased, reduced or made no difference to SOC stocks relative to dryland management. This study examined, over an annual plant growth cycle, the persistence of photosynthate-derived carbon (C) previously allocated to the plant-soil system during summer, under irrigated or dryland conditions. A continuous [sup.13]C[O.sub.2] pulse labelling method was used to label ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) mesocosms under simulated dryland or irrigated conditions. Plant and soil [sup.13]C was traced over 349 days using destructive sampling on days 1, 12, 125, 237 and 349 (T1-T5 respectively). After the cessation of labelling and summer irrigation, the mesocosms were maintained under the same seasonal soil moisture conditions. The persistence of [sup.13]C in the aboveground plant C pools was lower than in the root C pool. Approximately 50% of the initial mass of [sup.13]C recovered in roots at T1 remained by T5. There was no difference between the summer irrigated and dryland treatments in terms of the [sup.13]C recovered from the soil over the subsequent annual growing season. There was also no significant change in the [sup.13]C recovered in the soil between T1 and T5. However, summer irrigation did affect the spatial and temporal distribution of the photosynthate-derived C within the soil size fractions relative to summer dryland conditions. Summer irrigation promoted the transfer and storage of [sup.13]C in the fine particulate organic matter and clay size fractions. Keywords: irrigation, pasture, photosynthate C, soil particle size fractionation.