Impact of maize-cover crop sequence on N₂O mitigation in response to nitrogen in a long-term no-tillage system

Atmospheric concentrations of nitrous oxide (N₂O) have increased in recent decades, contributing significantly to climate change. In Brazil, the agricultural sector is the main source of N₂O emissions, and sustainable practices such as no-tillage and cover crops (CCs) have been adopted to reduce national emissions of N₂O. Although the inclusion of CCs in no-tillage systems has been suggested as a strategy to mitigate N₂O emissions from tropical soils, the magnitude and impacts remain uncertain and highly dependent on CCs species, residue quality, and soil-climate interactions. This study compared soil N₂O emissions from a maize-cover crop sequence in response to nitrogen in a long-term no-tillage system in the Cerrado. The study was conducted at Embrapa Cerrados, in the Federal District, Brazil, on plots where maize with and without N side-dressing, was followed by different CCs (Cajanus cajan, Crotalaria juncea, Raphanus sativus and Mucuna aterrima). The experiment was arranged in a randomized complete block design with split plots and three replications. Fluxes of N₂O were measured using the static chamber method and the N₂O concentration was analyzed by gas chromatography. To indicate the action of nitrifying and denitrifying microorganisms, mineral-N (NH₄⁺ and NO₃^-) analyses were performed. Daily N₂O fluxes increased by 38% and 23% following N side-dressing application to maize in the first and second years, respectively. Cumulative N₂O emissions were ≤2.98 kg ha^-1 and emission factor ranged from 0.13% in Cajanus cajan to 0.49% in Mucuna aterrima. N₂O emissions in treatments with N topdressing enhanced nitrifying activity, mediated by nitrifiers microorganisms. The succession of maize with Mucuna aterrima in the first year and Crotalaria juncea in the second year increased N₂O emissions. In contrast, during the first year, the maize-Crotalaria juncea sequence exhibited higher nitrogen fertilizer use efficiency (NFUE) than the maize-Mucuna aterrima and maize-Raphanus sativus systems. Notably, the maize-Cajanus cajan sequence showed the lowest cumulative N₂O emissions, indicating its superior effectiveness in mitigating N₂O emissions.