Greenhouse Gas Emissions from Deferred Ipyporã Grass Pastures Fertilized with N-Enriched Organomineral Biochar

Sustainable pasture intensification requires efficient nitrogen (N) use while limiting greenhouse gas (GHG) emissions. In this study, the effects of biochar-based nitrogenous organomineral fertilizers (OMF-N) on GHG emissions, nitrogen use efficiency (NUE), and forage accumulation of Ipyporã grass ( Urochloa ruziziensis × Urochloa brizantha ) under deferred grazing management were evaluated. The study was conducted in 2022 and 2023 using a randomized complete block design with four replications. Seven treatments were evaluated: an unfertilized control, urea at 40 and 80 kg N ha⁻ 1 , OMF-N 1:2 at 40 and 80 kg N ha⁻ 1 , and OMF-N 1:4 at 40 and 80 kg N ha⁻ 1 . Nitrous oxide (N₂O) and methane (CH₄) fluxes were monitored using static chambers for 30 days following fertilization. CH₄ fluxes remained low and were frequently negative, indicating net soil CH₄ uptake on several sampling dates. N₂O emissions increase after N fertilization, with greater cumulative emissions in 2023 than in 2022. Compared with urea, OMF-N did not reduce cumulative N₂O emissions at equivalent N rates. Emission factors (EFs) averaged 0.31% in 2022 and 0.78% in 2023, both below the 1% IPCC default value. Forage accumulation (FA) during the gas monitoring period was greater with OMF-N ratios of 1:2 and 1:4 at 80 kg N ha⁻ 1 than in the unfertilized control. NUE was unaffected by the fertilization strategy but was higher in 2022 than in 2023. Overall, OMF-N increased short-term forage accumulation at 80 kg N ha⁻ 1 but provided no clear advantage over urea for N₂O mitigation or NUE under the tropical conditions evaluated in the Amazon biome.