The Effect of Ammonical N Fertilizer Concentration Soil O2 Consumption and N2O Production Pathways.

When aiming to develop mitigation strategies for nitrous oxide (N₂O) emissions from soils, an accurate understanding of N₂O production pathways and the management and edaphic conditions that evokes them is needed.  Ammonia oxidation (nitrifier nitrification, nitrifier denitrification and nitrification coupled denitrification) and heterotrophic denitrification are the main pathways of N₂O production, depending on soil conditions such as soil pH, oxygen (O₂) content and substrate. Many researchers have reported that N2O production increased as substrate concentration increased resulting in an exponential increase in N₂O emissions to fertilizer N additions. However, there is only limited understanding of how N fertilizer concentration affects the mechanisms of N₂O production. To investigate the effect of N fertilizer concentration on N₂O production, we incubated a soil (clay loam) under various O₂ concentrations after adding ammonium sulfate ((NH₄)₂SO₄) or urea at different concentrations. The results showed that the application of these fertilizers significantly promoted N₂O production under 21% O₂ concentration, but no N₂O from these substrates was produced at 0% O₂. Under all aerobic conditions (0.5 – 21% O₂), N₂O production increased as the N concentration increased from 0 - 50 µg N g^-1. However, no significant difference was found among the treatments with 50, 100 and 200 µg N g^-1 (NH₄)₂SO₄ or urea added while N₂O production in the 500 and 1000 µg N g^-1 (NH₄)₂SO₄ treatments was significantly higher. In contrast, less N₂O was produced in the 500 and 1000 µg N g^-1 urea than in the other urea treatments. To further elucidate the relationship between O₂ consumption and N fertilizer concentration, two agricultural soils (clay loam and clay) were incubated with different concentrations of (NH₄)₂SO₄ under 50 % of water holding capacity. In both clay loam and clay soils, the total amount of O₂ consumption and N₂O production were increased as soil ammonical N concentration increased. These results indicate that a non-linear response of soil N₂O emissions to fertilizer N can be due to the soil O₂ limitation caused by ammonia oxidation.