Nitrogen oxide emissions from an irrigated maize crop amended with treated pig slurries and composts in a Mediterranean climate

Organic fertilizers may differ greatly in composition and as a result there may also be differences in nitrogen oxides emissions following their application to soils. The aim of this study was to evaluate the influence of mineral and organic N fertilizers on the nitrification and denitrification processes, and consequently on N2O and NO emissions. Therefore, a field experiment was carried out on an irrigated sandy loam soil under Mediterranean conditions during the maize (Zea mays L.) growing season. Untreated pig slurry (UPS) both with and without the nitrification inhibitor dicyandiamide (UPS + DCD), digested thin pig slurry fraction (DTP), composted solid fraction of slurry mixed with urea (CPS + U) and composted municipal solid waste mixed with urea (MSW + U) were applied at a rate of 175 kg available N ha−1. Their emissions were compared with those from urea (U) and a control treatment to which no nitrogen fertilization was administered (Control). Accumulated nitrous oxide losses during the crop season ranged from 6.0 to 9.3 kg N2O-N ha−1 for the Control and CPS + U, respectively, whereas nitric oxide losses ranged from 0.01 to 0.23 kg NO-N ha−1, for the Control and U, respectively. The use of digested slurries mitigated N2O emission by 25% in relation to untreated pig slurry, but NO emissions were similar for both treatments. Dicyandiamide reduced N2O and NO emissions by 64 and 78% with respect to slurry without the inhibitor. An indirect effect of DCD on denitrification was also observed, with a reduction of 32% in denitrification with respect to the slurry without the inhibitor. In this case, the greatest reduction in denitrification losses occurred during the irrigation period. Composts mixed with urea reduced NO emissions by 56% (CPS + U) and 85% (MSW + U) in relation to the urea treatment, but its effect on N2O depended on the type of compost involved: CPS + U increased N2O emission by 27%, whereas MSW + U reduced it by 55% in relation to urea. Denitrification was the most important process responsible for N2O emissions when organic fertilizers were applied to the soil, while nitrification was the most important for the inorganic fertilizer. The C:N ratio of fertilizers was a good predictor of their NO emissions, denitrification losses and N2O/N2 ratio. On the other hand, added soluble N was a good predictor for cumulative N2O emissions during the period before irrigation. This work shows that an appropriate selection of organic fertilizers based on their composition could be used to mitigate emissions of the atmospheric pollutants NO and N2O in comparison with urea.