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

Abstract

Nitrous oxide (N 2O) 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 N 2O emission studies have been undertaken previously on this system. A study over 1 year has been undertaken measuring N 2O 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 N 2O 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 N 2O 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 N 2O emissions to gross mineralisation was 0.01–0.05%. Liming did not make a significant change to the average N 2O emissions. The average of daily N 2O 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 N 2O emissions. Measurements from the urine-treated soil showed that between 0.1 and 0.2% of the nitrogen in the urine was released as N 2O. These and a compilation of other emission factors for N 2O 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.