Mineral nitrogen cycling through earthworm casts in a grazed pasture under elevated atmospheric CO2.

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. [ABSTRACT FROM AUTHOR]