Alternate wetting and drying irrigation combined with biochar reduces greenhouse gas emissions and improves carbon balance in water‑scarce rice agroecosystems

This 3-year field study investigated the effects of different water management strategies and biochar application on greenhouse gas (GHG) emissions in a Mediterranean rice agroecosystem. The experiment compared a conventional flooding (CF) rice cultivation agroecosystem, CF cultivation amended with holm oak ( Quercus ilex L.) biochar (CF-B), alternate wetting and drying (AWD) managed at –20 kPa with (AWD-B) and without biochar (AWD), and severely stressed AWD managed at –70 kPa with (AWDS-B) and without (AWDS) biochar. Biochar was applied at a rate of 35 t ha −1 during the first year of the study. Methane (CH 4 ), carbon dioxide (CO 2 ) and nitrous oxide (N 2 O) emissions were monitored throughout the experimental period, and global warming potential (GWP), yield-scaled GWP (GWP-y), and net ecosystem carbon balance (NECB) were calculated for each treatment. CF rice cultivation exhibited the highest CH 4 emissions, which were markedly reduced under AWD, regardless of the severity threshold. Furthermore, under traditional flooding regimes, the biochar incorporation markedly reduced CH 4 emissions. In addition, the transition from continuous flooding to intermittent irrigation did not result in substantial changes in N 2 O emissions, with values < 3.62 N 2 O-N kg ha −1 . Biochar application effectively mitigated these N 2 O emissions, particularly in CF-B and AWD-B treatments. Over the final 2 years of the study, AWD-B exhibited the lowest GWP and GWP-y values. After 3 years of study, regardless of the irrigation regime, biochar ageing decreased carbon outputs and enhanced NECB values (33% compared with the initial treatments). These results suggest that integrating agroecosystem management practices, such as intermittent irrigation and holm oak biochar application, is an effective approach for mitigating GHG emissions and improving the carbon balance of Mediterranean rice systems while enhancing adaptation to the availability and efficient use of local natural water resources.