The effects and mechanisms of deep straw incorporation and denitrifying bacterial agents on mitigating nitrate leaching and N2O emissions in four soil types in the North China Plain

The agricultural practice of straw incorporation is often employed to enhance soil fertility, improve soil properties and reduce nitrate leaching via deep burying. Nevertheless, the tradeoff between nitrate leaching and N2O emissions caused by deep straw incorporation in various soil types remains uncertain. To address this knowledge gap, a mesocosm column experiment was established to evaluate the impact of straw amendment (NS) and of straw plus application of denitrifying bacterial agents (NSS) at column depths of 20-60 cm on NO3--N leaching and N2O emission across four soil types (brown, cinnamon, fluvo-aquic and mortar black soil) in the North China plain. The results showed that the NS and NSS treatments significantly reduced NO3--N leaching by 47%-73% and 37%-81%, respectively, across four soil types, in comparison to the control without straw and bacterial agents (N), with the greatest reduction recorded in the mortar black soil (by 73% for NS and 81% for NSS). Meanwhile, the NS and NSS treatments resulted in a significant increase in cumulative N2O emissions (by 289% for NS and 598% for NSS) in the acidic brown soil with a soil pH ranging between 4.8 and 6.5, but showed no significant effect in the remaining three soil types in which soil pH is above 7.5. When compared to the NS treatment, NSS treatment reduced the cumulative N2O emissions in the fluvo-aquic soil by 42%. We further showed that soil type is the primary driver of bacterial community composition (R-2 = 62.3%; P < 0.001), followed by soil depth (R-2 = 5.2%; P < 0.001). The incorporation of straw and denitrifying bacterial agents induced significant change in the bacterial community in the middle depth layers, and increases in nosZ gene abundance and the relative abundance of inoculant-similar OTUs significantly by 0.5-37 folds and 2.8-6900 folds, respectively, in all soil types, suggesting the colonization of denitrifying bacterial agents. Our results further suggested that the Streptomycetales were the most important predictors for N2O emissions, and the relative abundance of Streptomycetales was highly positively correlated with N2O emissions across the four soil types. Overall, the study suggests that deep straw incorporation as soil amendment is effective in mitigating nitrate leaching in all four soil types, especially in the fluvo-aquic soil, due to its dual effects on controlling nitrate leaching and N2O emissions. However, it is not recommended for the acidic brown soils as it may cause increased N2O emissions.