Relationship between gross nitrogen cycling and nitrous oxide emissionin grass-clover pasture

Replacement of high-input N fertilized pastures with low-input grass-legume pastures may provide a mitigation option to reduce agricultural N2O emissions. This study examined the relationship between N-cycling rates and N2O 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 N-15-enriched substrates to identify sources of N2O, in combination with assessment of gross N mineralization and nitrification. Nitrous oxide emissions were generally low, fluctuating between 82 and 136 mu g N2O-N m(-2) d(-1), independent of pasture age. The N-15 labelling indicated that at least 50% of the N2O was derived from the soil NH4+ pool, approaching 100% in June. In the two year old pasture the NH4+ pool contributions to N2O emissions varied significantly with sampling time. Emission rates of N2O correlated positively with soil NH4+ concentrations and the NH4+ supply as expressed by gross mineralization. The N2O emissions showed a significant inverse relationship with soil NO3-, but was not correlated with the supply of NO3- as expressed by gross nitrification. The ratio N2O vs. nitrification averaged 0.05% (range 0.004 to 0.29%) and varied with sampling time showing the lowest value in wet soil conditions.