Abstract
We used the New Zealand grazed pasture free air CO 2 enrichment experiment to determine the effects of elevated CO 2 on earthworm ( Aporrectodea caliginosa and Lumbricus rubellus ) cast production and mineral nitrogen (N) concentration over a 5‐week period in the spring. Elevated CO 2 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 CO 2 . An analysis of the earthworms' potential diet showed that a reduction in the N content of sheep dung at elevated CO 2 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 CO 2 which is consistent with the hypothesised process of progressive N limitation.