Nitrous oxide (N2O) emissions to the atmosphere from grazed pasture can be high, especially from urine-affected areas. When pastoral soils are damaged by animal treading, N2O emissions may increase. In New Zealand, autumn-sown winter forage crops are often grown as a break-crop prior to re-sowing pasture. When these crops are grazed in situ over winter (as is common in New Zealand) there is high risk of soil damage from animal treading as soil moisture contents are often high at this time of year. Moreover, the risk of soil damage during grazing increases when intensive tillage practices are used to establish these forage crops. Consequently, winter grazed forage crops may be an important source of N2O emissions from intensive pastoral farming systems, and these emissions may be affected by the type of tillage used to establish them. We conducted a replicated field experiment to measure the effects of simulated cattle grazing (mowing followed by simulated treading and the application of synthetic urine) at three soil moisture contents ( field capacity) on measured N2O emissions from soil under an autumn (March) sown winter forage crop (triticale) established with three levels of tillage intensity: (a) intensive, IT, (b) minimum, MT, or (c) no tillage, NT. In all treatments, bulk density in the top 7.5 cm of the soil was unaffected by treading when simulated grazing occurred at field capacity, and by 10% in the MT plots trodden at > field capacity. Treading did not significantly increase the bulk density in the NT plots. Emissions of N2O from the tillage treatments decreased in the order IT > MT > NT. N2O emissions were greatest from plots that were trodden at > field capacity and least from plots trodden at field capacity. The N2O emission from urine-amended NT plots that were trodden at < field capacity was 2.0 kg ha(-1) over 90 days (0.25% of the total urine N applied). Decreasing the intensity of tillage used to establish crops and restricting grazing when soils are wet are two of the most effective ways to minimise the risk of high N2O emissions from grazed winter forage crops.
