VARIABLES CONTROLLING DENITRIFICATION FROM EARTHWORM CASTS AND SOIL IN PERMANENT PASTURES

Denitrification (using the acetylene block method) was determined in earthworn casts and soils from permanent, drained or undrained pasture plots fertilized with 0 or 200 kg N ha-1 year-1 as ammonium nitrate. Rates of N2O production from soil cores were about three times higher from the fertilized than from the unfertilized plots while drainage had a relatively small effect. Denitrification rates from casts were 3-5 times higher than those from soil irrespective of the drainage treatment. Casts generally had higher NO3-, NH4+, and moisture contents, and higher microbial respiration rates than soil. Rates of N2O production were determined primarily by NO3- supply, secondarily by moisture; available C did not appear to limit denitrification in these pastures. Estimates of the potential contribution of casts to denitrification ranges from 10.1% of 29.3 kg ha-1 year-1 from the unfertilized, drained plot to 22% of 82.5 kg ha-1 year-1 from the fertilized undrained plot.