Greenhouse gas fluxes from a grazed grassland soil after slurry injections and mineral fertilizer applications under the Atlantic climatic conditions of NW Spain

The number of studies that investigate how agricultural practices on dairy farms in the North West (NW) of Spain affect greenhouse gas (GHG) fluxes from soils is limited. Thus, the objective of this study was to quantify the effects of the application of mineral fertilizers and cattle slurry injections on GHG fluxes from a grassland soil with grazing dairy cattle, in Galicia (NW Spain). We also aimed to identify the type of fertilizer associated with high grass production and low GHG fluxes. To achieve this, fluxes of nitrous oxide (N₂O), methane (CH₄) and carbon dioxide (CO₂)_, grass yields and soil mineral nitrogen (N) contents were monitored after three applications (in spring, summer and autumn) of surface broadcasted mineral fertilizer (MN) and injected cattle slurry (CS) and compared with no fertilization (zero N). Dry soil conditions (<60% water-filled pore space (WFPS)) were observed during the spring and summer, contrasting with the higher soil WFPS (>60%) in autumn due to the more frequent rainfall. Overall, total cumulative N₂O fluxes from CS were similar than from MN (P>0.05), indicating that denitrification in this C-rich soil was not stimulated by slurry-carbon applications. Large losses of CH₄ and CO₂ were related to CS, but overall only total cumulative CH₄ fluxes were larger with respect to MN (P<0.05). Dry weather conditions would have stimulated organic matter mineralization in this soil, which resulted in the low efficiency of both fertilizers to increase yields. As we obtained similar total CO₂ equivalents to produce same yields with both types of fertilization (P>0.05), this study did not show a clear type of fertilization related to low GHG fluxes and high yields. We believe that longer-term studies are required to provide more robust estimations and conclusions about the effect of fertilizer applications on GHG fluxes from grassland soils in NW Spain.