Sub-surface drip irrigation reduced N2O emissions via inhibiting denitrification pathways in northern China

Water-saving irrigation has become an important method for alleviating water scarcity in semi-arid regions. However, the lack of knowledge about the effects of different water-saving irrigation techniques on soil nitrogen conversion limits the understanding of the underlying mechanisms involved in soil N2O emissions. To determine the effects of different irrigation methods on N2O emissions, we conducted a two-season experiment under four different irrigation methods (DI, surface drip irrigation; SDI, sub-surface drip irrigation; PRI, partial rootzone irrigation; FP, flood irrigation). Also, we studied the dynamics of nitrifier and denitrifier community structures and compositions and their associations and effects on N2O emissions. The results showed that seasonal N2O emissions were affected by irrigation practices and significantly correlated with the amount of irrigation and soil moisture. Drip irrigation significantly reduced N2O emissions, with 38-45 % (DI), 51-59 % (SDI), and 64-68 % (PRI) decreases compared to FP treatment. Under SDI, the copy numbers of ammonia-oxidizing archaea amoA significantly decreased and nirK displayed an increase compared to FP. The abundance of unclassified_Proteo-bacteria dominant among nirK-type denitrifiers under FP was observed to be strongly and positively correlated with N2O emissions, and the dominant bacteria shifted to Bosea under drip irrigation. Irrigation practices had a strong effect on the Shannon and ACE indexes of five key genes. PRI showed a similar effect under SDI treatment on nitrifier and denitrifier community structures. Furthermore, PRI, with only a 38 % reduction of irrigation water consumption, produced a 4.4-11.3 % lower yield than SDI. Therefore, SDI treatment can inhibit the denitrification process led by nirS-type denitrifiers and reduce the abundance of the dominant genus of nirK and promote nosZ abundance, thus decreasing N2O emissions. Our findings indicate that (i) drip irrigation increased ammonia-oxidizing bacteria amoA abundance and diversity but decreased nirS abundance, which led to N2O mitigation and (ii) SDI might be the best method for maintaining high crop yield with lower N2O emissions in intensively farmed fields.