Effects of biochar addition and deficit irrigation with brackish water on yield-scaled N2O emissions under drip irrigation with mulching

Biochar has been proven to have great potential for mitigating greenhouse gas emissions and improving crop growth. However, the effects of biochar addition on yield-scaled N2O emissions in a mulched and drip-irrigated maize field using different irrigation water qualities and quantities remain unclear. A two-year field experiment was conducted to examine the effects of biochar addition, irrigation level and water salinity on soil N2O emissions, maize yield and grain N uptake in Northwest China. Eight treatments in total included the combination of two biochar addition rates of 0 t/ha (B0) and 60 t/ha (B1), two irrigation levels of full (W1) and deficit irrigation (W2, W2 = 1/2 W1) and two water salinity levels of freshwater (S0, 0.71 g/L) and brackish water (S1, 4 g/L). The soil properties, N2O emission fluxes, maize yield and grain N uptake, along with the abundances of N-cycle functional marker genes were measured during the two growing seasons. Soil water-filled pore space (WFPS), soil NH4+-N and NO3--N contents accounted for the majority of the variation in the dynamic changes of N2O fluxes. Deficit irrigation had lower N2O emissions compared with full irrigation, which was more pronounced in the second year. Relative to freshwater irrigation, brackish water irrigation increased N2O emissions under W1, but freshwater and brackish water irrigation had similar N2O emissions under W2 in both years. Biochar addition effectively mitigated N2O emissions by 29.4–31.0% and 17.9–29.2% under different irrigation treatments in 2020 and 2021, respectively. The continuous drought induced by deficit irrigation weakened the effectiveness of biochar in reducing N2O emissions. The differences in N-cycle functional marker genes could not reflect the variation in cumulative N2O emissions among treatments. Under all irrigation treatments excluding W2S1, biochar addition improved the total maize yield and grain N uptake over the two-year period by 4.06–10.74% and 5.57–8.63%, respectively. Brackish water and deficit irrigation increased grain and grain N yield-scaled N2O emissions, whereas biochar addition reduced both of them. Overall, these findings suggest that biochar addition can effectively reduce N2O emissions and increase yield and grain N uptake under brackish water and deficit drip irrigation with mulching in a maize field system, and thus adding biochar can help with achieving the sustainability of agricultural production in areas with limited freshwater resources.