Global warming potential of a Mediterranean irrigated forage system: Implications for designing the fertilization strategy

Under Mediterranean conditions, the impacts of both organic and mineral N fertilization on soil Greenhouse Gases (GHG) emission can be controversial. The aim of this study was to assess the soil GHG emissions and the net Global Warming Potential (GWP) in a Mediterranean irrigated forage system under different fertilization treatments. Three N fertilization options were compared for two years in a double-crop rotation of silage maize and Italian ryegrass for hay: cattle slurry (SL), solid fraction of slurry (SO) and mineral fertilizer with a nitrification inhibitor (MI). The soil CO2, N2O and CH4 fluxes were highly influenced by the interaction between treatment and date. The maximum values of GHG emissions were observed after fertilizations, to a different extent depending on the fertilizer. In the net GWP reference year, soil respiration (SR) was higher in SO (46.26 ± 3.26 Mg ha−1 yr−1 of CO2) than SL (30.03 ± 0.40 Mg ha−1 yr−1) and MI (23.71 ± 0.57 Mg ha−1 yr−1). However, the C sequestration was higher in SO than in the other treatments. The N2O fluxes were higher in SL (11.5 ± 5.2 kg ha−1 yr−1 of N2O) than in SO (3.4 ± 1.8 kg ha−1 yr−1), while the MI had intermediate values (6.5 ± 1.4 kg ha−1 yr−1). No differences were observed in cumulative CH4 emissions. The SO resulted as a net GWP sink (-9.86 ± 3.05 Mg yr−1 of CO2eq based on SR), while the SL and MI (9.79 ± 1.41 and 1.34 ± 1.87 Mg yr−1, respectively, based on SR) resulted as a source. The SO seemed to have a higher potential in terms of reducing GHG emissions by maintaining adequate levels of agronomic efficiency. This study put in evidence how different organic fertilizers can have contrasting impacts on GHG emissions providing some insights on their different potential mitigation roles under Mediterranean conditions.