Long-term effects of contrasting tillage on soil organic carbon, nitrous oxide and ammonia emissions in a Mediterranean Vertisol under different crop sequences

Highlights: • Tillage effects on soil organic C and N gas emissions were assessed in two crop sequences. • N₂O emissions, but not total NH₃ emissions, were higher in NT than CT. • Continuous application of NT for 23 years increased bulk density, WFPS, TOC and EOC in the topsoil compared to CT. • Denitrifying enzyme activity and nosZ gene were enhanced by long term NT. This 2-year study aimed to verify whether the continuous application of no tillage (NT) for over 20 years, in comparison with conventional tillage (CT), affects nitrous oxide (N₂O) and ammonia (NH₃) emissions from a Vertisol and, if so, whether such an effect varies with crop sequence (continuous wheat, WW and wheat after faba bean, FW). To shed light on the mechanisms involved in determining N-gas emissions, soil bulk density, water filled pore space (WFPS), some carbon (C) and nitrogen (N) pools, denitrifying enzyme activity (DEA), and nitrous oxide reductase gene abundance (nosZ gene) were also assessed at 0–15 and 15–30 cm soil depth. Tillage system had no significant effect on total NH₃ emissions. On average, total N₂O emissions were higher under NT (2.45 kg N₂O-N ha^{− 1}) than CT (1.72 kg N₂O-N ha^{− 1}), being the differences between the two tillage systems greater in FW than WW. The higher N₂O emissions in NT treatments were ascribed to the increased bulk density, WFPS, and extractable organic C under NT compared to CT, all factors that generally promote the production of N₂O. Moreover, compared to CT, NT enhanced the potential DEA (114 vs 16 μg N kg^{− 1} h^{− 1}) and nosZ gene abundance (116 vs 69 copy number mg^{− 1} dry soil) in the topsoil. Finally, NT compared to CT led to an average annual increase in C stock of 0.70 Mg C ha^{− 1} year^{− 1}. Though NT can increase the amount os soil organic matter so storing CO₂ into soil, some criticisms related to the increase of N₂O emission arise, thereby suggesting the need for defining management strategies to mitigate such a negative effect.