Replacement of high-input N fertilized pastures with low-input grass-legume pastures may provide a mitigation option to reduce agricultural N(2)O emissions. This study examined the relationship between N-cycling rates and N(2)O production and evolution from the root zone of grass-clover pastures of various ages (production year 1, 2 and 8). The experimental approach included cross-labelling pasture monoliths with (15)N-enriched substrates to identify sources of N(2)O, in combination with assessment of gross N mineralization and nitrification. Nitrous oxide emissions were generally low, fluctuating between 82 and 136 mu g N(2)O-N m(-2) d(-1), independent of pasture age. The (15)N labelling indicated that at least 50% of the N(2)O was derived from the soil NH(4)(+) pool, approaching 100% in June. In the two year old pasture the NH(4)(+) pool contributions to N(2)O emissions varied significantly with sampling time. Emission rates of N(2)O correlated positively with soil NH(4)(+) concentrations and the NH(4)(+) supply as expressed by gross mineralization. The N(2)O emissions showed a significant inverse relationship with soil NO(3)(-), but was not correlated with the supply of NO(3)(-) as expressed by gross nitrification. The ratio N(2)O vs. nitrification averaged 0.05% (range 0.004 to 0.29%) and varied with sampling time showing the lowest value in wet soil conditions.