N₂O emissions from spring barley production as influenced by fertilizer nitrogen rate

Usage of mineral nitrogen (N) fertilizers for agricultural crop production systems is a major contributor to anthropogenic nitrous oxide (N 2 O) emissions. As part of a national study to quantify N 2 O emissions under different cropping systems and in different eco-regions, this study quantified the effect of fertilizer N rate on spring barley (Hordeum vulgare L.) on N 2 O emissions in 3 yr in a cool maritime climate with humid soil moisture regimes. Treatments were 0, 75 and 150 kg N ha -1 as ammonium nitrate applied as a pre-plant broadcast. N 2 O emissions were increased by fertilizer N application in each year. In 2003 and 2005, elevated N 2 O emissions occurred in the 6-wk period following fertilizer application when soil NO 3 -N concentrations were high. However, in 2004 and 2005, peak N 2 O emissions occurred near crop harvest. Elevated N 2 O emissions at this time were attributed to increased carbon availability due to re-wetting of dry soil. Therefore, the effect of fertilizer N management on N 2 O emissions may not necessarily occur immediately after treatment application. This emphasizes the importance of measuring N 2 O emissions outside of the crop growth period. Fertilizer-induced cumulative N 2 O emissions averaged 0.011 and 0.021 kg N kg -1 N when fertilizer N rate was increased from 0 to 75 kg N ha -1 and from 75 to 150 kg N ha -1 , respectively, indicating increased N 2 O emissions when fertilizer is applied at above optimal rates. N 2 O emissions increased linearly with nitrate intensity, the summation of daily NO 3 -N concentrations for 0- to 15-cm depth. This suggests that the non-linearity in the relationship between fertilizer N rate and N 2 O emissions can be explained by the decreasing efficiency in crop NO 3 -N uptake at high fertilizer N rates.