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

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 h-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 ha-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 CO2into 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.