Nitrous oxide emissions from a legume pasture and the influences of liming and urine addition

Nitrous oxide (N2O) is a potent greenhouse gas contributing to global radiative forcing. Grazed legume pasture is one of the major agricultural land uses in southern Australia and no N2O emission studies have been undertaken previously on this system. A study over 1 year has been undertaken measuring N2O emissions from legume pasture at Wagga Wagga, Australia. The study was embedded within a larger on-farm study of the management of acid soils. Sampling involved simultaneous N2O emission measurements on two paddocks each of: acidic soil with pH 4.0; limed soil with soil pH 5.5, and limed soil with addition of 20 g N m−2 of animal urine. The average emission of N2O from the acidic legume pasture was 0.35 kg N ha−1 y−1 with a standard error of 0.03 kg N ha−1 y−1. Isotope measurements indicated that in aerobic conditions the ratio of N2O emissions to gross mineralisation was 0.01–0.05%. Liming did not make a significant change to the average N2O emissions. The average of daily N2O emissions are: for the acidic plots 0.96 ± 0.07 mg N m−2 d−1 and for the limed plots 0.88 ± 0.04 mg N m−2 d−1. We conclude that under the conditions experienced at Wagga Wagga, which are typical of many Australian pastures, liming does not change the N2O emissions. Measurements from the urine-treated soil showed that between 0.1 and 0.2% of the nitrogen in the urine was released as N2O. These and a compilation of other emission factors for N2O emissions from urine addition to pasture give a median emission factor of 0.7%, significantly smaller than the two default values of 2% and 1% recommended by the Revised 1996 and the 2006 IPCC Guidelines for National Greenhouse Gas Inventories.