Relationship Between Gross Nitrogen Cycling and Nitrous Oxide Emissionin Grass-clover Pasture

Abstract

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 ¹⁵N-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 136μg N₂O–N m⁻² d⁻¹, independent of pasture age. The ¹⁵N labelling indicated that at least 50% of the N₂O was derived from the soil NH ⁺₄ pool, approaching 100% in June. In the two year old pasture the NH ⁺₄ pool contributions to N₂O emissions varied significantly with sampling time. Emission rates of N₂O correlated positively with soil NH ⁺₄ concentrations and the NH ⁺₄ supply as expressed by gross mineralization. The N₂O 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 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.