Combined application of urea and cow manure results in similar cumulative N2O emissions relative to conventional fertilization, in two types of soil.

Agriculture is the second-largest contributor of greenhouse gasses (GHGs) globally, after fossil fuels combustion. The excessive application of mineral fertilizers and the inadequate disposal of large amounts of livestock waste in agricultural soils result in elevated N₂O and CO₂ emissions, which surpass 17% of the global GHG emissions.

Approximately 1.4 billion tons of cow manure (CM) are produced every year in the EU and current EU policies promote CM incorporation into the soil, as a cost-efficient and sustainable agronomic practice. The European Green Deal urges a 20% reduction in chemical fertilization by 2030 and reuse of organic fertilizers, i.e. cow manure The beneficial use of CM is linked to enhanced soil fertility, soil organic matter content and carbon sequestration. However, soil organic amendments may fuel soil nutrient transformations and potentially increase nutrient losses i.e. GHG emissions.

To test the short-term effects of combined organic and inorganic fertilization on GHG emissions, we conducted a mesocosm experiment using two soil types (Sandy-loam (SL) & Loamy (L)) and including five treatments: Control (C: No fertilization), Urea as Chemical Fertilization in two rates (100U:200 kg/ha & 80U:160 kg/ha), Cow Manure (CM:50 Mg/ha) and the combination of 80U and CM (80U-CM:160 kg/ha Urea & 50 Mg/ha Cow Manure). During a 90-days incubation period, CO₂ and N₂O flux rates and soil NO^-₃, NO^-₂ and NH⁺₄ were measured regularly. 

Soil type was the only significant factor (p≤0.05) driving cum. CO₂ emissions. A 20% increase of cum. CO₂ was found for L soil treatments than SL. The combined treatment 80U-CM had similar emissions to conventional fertilization (100U) that were on average 762 mg/kg C-CO₂, approx. 28.5% greater than C (591 mg/kg). CM incorporation led to 19% increase in cum. CO₂ emissions than C.

Contrary to CO₂, soil (p<0.001), fertilization (p<0.001) and their interaction (p=0.002) were significant factors explaining cum. N₂O emissions. The SL soil had 60% higher cum. N₂O emissions compared to L. The use of CM in L soil decreased (39%), while in SL soil increased (5%) cum N₂O emissions, relative to C. A 20% reduction in urea application resulted in 90% and 19% reduction for SL and L soil, respectively when compared to 100U. The combined application 80U-CM increased cum. N2O emissions than CM and 80U and had lower cum. N₂O emissions than 100U, for both soils. Soil, fertilization, and their interaction were accounted for statistically significant (p≤0.05) differences in soil NO₃^- , NO₂^- and NH₄⁺ availability (AUC) .

According to our study, the combined application of 80U-CM cannot be an effective alternative to conventional fertilization (100U), as it generates similar levels of GHG emissions and has lower nutrient (N) supply potential. Furthermore, our preliminary research indicates the need to further quantify the effects of different organic amendments on GHG soil emissions and soil microbial communities.

Funding: The BSc and MSc research work by George Kourtidis and Elpida Pasvadoglou, respectively, was supported in part by the Hellenic Foundation for Research and Innovation (HFRI) Post-Doctoral Grant #1053 awarded to Principal Investigator Dr. Georgios Giannopoulos.