N_xO emissions in response to the irrigation lower limits under different irrigation modes in a lettuce field

Irrigation has a significant impact on N_xO (N₂O and NO) emissions from cultivated land, yet the N₂O or NO emission among the irrigation lower limits under different irrigation modes has not been well compared. In an irrigated lettuce field, three DR (drip irrigation) lower limits were designed, including 75% (DR1), 65% (DR2) and 55% (DR3) field capacity, and one FI (furrow irrigation) lower limit (65% field capacity). The N₂O and NO emission fluxes and soil nitrogen (N) forms were determined, and the linear correlation between these indicators was analyzed. Results showed that under the same irrigation regime, the N₂O and NO emissions from furrow irrigation treatment increased by 36.8% and 45.2% respectively compared to that from drip irrigation treatment. The cumulative N₂O and NO emissions under DR3 were 30.2% and 28.6% higher than under DR1, respectively. Moreover, DR1 was also the lowest among the four treatments in soil \rm NO_3^- -N concentration. The N₂O and NO emission fluxes were more correlated to soil \rm NH_4^+ -N (r=0.88 and 0.76) or \rm NO_2^- -N (r=0.90 and 0.80) concentration than soil \rm NO_3^- -N and soluble organic N, indicating that N₂O and NO were mainly produced by the soil nitrification process. When the irrigation regime was the same, N₂O and NO emissions were lower with drip irrigation than with furrow irrigation. Besides, drip irrigation with small quota but high frequency reduced N₂O and NO emission compared to that with large quota but low frequency.