Irrigation of grazed pasture significantly increases plant and animal production, which may in turn increase soil organic carbon (SOC), depending on the balance between primary production and below-ground allocation of C on the one hand, and the decomposition and export of C from the soil on the other. To evaluate the effect of irrigation on SOC we sampled a grazed pasture field experiment maintained under different irrigation treatments for 62years. The dry-land treatment in this experiment only received rainfall at an average of 740mmyear-1. The 10 and 20% irrigation treatments involved application of 100mm of irrigation when the soil reached 10 and 20% gravimetric moisture content, respectively. The 10 and 20% irrigation treatments received average total annual irrigation inputs of 260 and 770mmyear-1, respectively. The 10 and 20% irrigation treatments increased pasture production by 44 and 74%, respectively, compared with that from the dry-land. Analysis of soils taken to 1-m depth revealed that amounts of SOC were not significantly different between the dry-land (125.5Mgha-1) and 10% irrigation (117.8Mgha-1) treatments, but these were significantly greater than the 20% irrigation treatment (93.0Mgha-1). At 50-100cm, SOC was also less (34%) for the 20% irrigation treatment than for the 10% irrigation treatment. The relative quantities of carbon (C) and nitrogen (N) in the light fraction (LF) at all soil depths decreased successively from dry-land to the 20% irrigation treatment, suggesting that wetter soil conditions accelerated decomposition of the LF fraction, a comparatively labile SOC fraction. The C-to-N ratio of the bulk soil was also less for the 20% irrigation treatment, indicating more decomposed SOM in the irrigated than in the dry-land treatment. There were no significant differences in the microbial biomass between the three different irrigation treatments, but the respiration rate (CO2 production) of soil organisms in the 20% irrigation treatment was consistently greater than in the other two treatments. It was concluded that large increases in plant productivity as a result of irrigation had either no effect or significantly reduced SOC stocks under grazed pasture. The reduced SOC content observed in the 20% irrigation treatment was attributed to a combination of increased C losses in animal products and drainage associated with greater stocking, together with accelerated decomposition of organic C resulting from elevated soil moisture maintained throughout the growing season. © 2014 British Society of Soil Science.