We used the New Zealand grazed pasture free air CO2 enrichment experiment to determine the effects of elevated CO2 on earthworm (Aporrectodea caliginosa and Lumbricus rubellus) cast production and mineral nitrogen (N) concentration over a 5-week period in the spring. Elevated CO2 did not affect earthworm biomass or the amount of cast material produced. However, cast mineral N concentrations were 18% lower, resulting in 27% less mineral N being deposited on the soil surface under elevated CO2. An analysis of the earthworms' potential diet showed that a reduction in the N content of sheep dung at elevated CO2 was the most likely cause of the lower cast N concentrations. Earthworm casts made only a small contribution to mineral N cycling in our system; however, their quality may act as a sensitive indicator of reduced N availability under elevated CO2 which is consistent with the hypothesised process of progressive N limitation.