Biological amendments reduce soil N2O and CH4 emissions from slurry application under field conditions.

Managed grasslands influence global warming by the exchange of the greenhouse gases (GHG) like carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄). Application of animal waste, such as slurry, rich in inorganic nitrogen (N), may escalate soil processes and thus soil GHG emissions, particularly in organic systems that rely on input of animal manures without chemical inputs.

The objective of this study was to evaluate GHG mitigation potential of biological amendments that might be relevant to organic systems and their effects on soil N and N leaching over a 2-month period. To achieve this a plot-scale field experiment on a grassland site in Rosemount (Dublin, Ireland) was conducted over a period from May to July 2023. Closed static chamber technique was used to measure soil emissions of N₂O, CH₄ and CO₂ with an increased sampling frequency after the slurry application. The dynamics of soil ammonium, nitrate and dissolved organic N were evaluated weekly in soil surface samples from 0-15 cm and in a 10-day interval in the leachate collected at a 50 cm depth. The grass yield was assessed twice during the course of the experiment. The plots were equally irrigated to stimulate soil processes during the dry periods. The treatments assigned to the plots in a randomised complete block design with 5 replicates included control (CON), cattle slurry (SLU) and slurry mixed with biochar (BIO; added at 2 kg/m²), neem oil high with slurry (NEEM H; added at 100% of N applied) and neem oil low with slurry (NEEM L; added at 20% of N applied). The slurry was applied at 50 kg N ha^-1 to all plots apart from CON.

The application of neem oil at both levels of input consistently reduced soil N₂O and CH₄ daily emissions (p<0.001), while NEEM H at the same time increased soil CO₂ daily emissions (p<0.001), compared to the SLU treatment. Biochar reduced soil CH₄ daily emissions (p<0.001), but did not influence soil N₂O and CO₂ daily emissions relative to the SLU treatment.  

These results might be highly relevant for the climate-change policies relevant to organic farming systems and achievements of the national and international climate goals.