Reactive gaseous N losses from an alternate wetting and drying paddy field as affected by biochar application

Alternate wetting and drying irrigation (I_AWD) is one types of water-saving technologies to repeatedly dry and re-flood the fields. However, the frequent alternate aerobic and anaerobic environment under I_AWD has increased the reactive nitrogen gases emission in rice fields. Fortunately, biochar has been widely used to improve the land productivity (including the crop production, carbon sequestration, mitigation of GHG, and remediation of heavy metal pollution) in the agricultural soil amendment. However, little information is available on the effects of biochar on the NH₃ volatilization, N₂O emissions, and reactive nitrogen gas emissions in paddy rice systems, especially under I_AWD. This study aims to explore the response mechanism of reactive nitrogen gas emissions to the biochar application under I_AWD conditions. A two-year field split-plot experiment was conducted with the biochar application rates (0 and 20 t/hm²) under two irrigation regimes (continuously flooded irrigation, I_CF and I_AWD). Two irrigation regimes were used as the main plots, whereas, two biochar additions were the subplots. Each plot was in the size of 3 m (width)×6 m (length) that separated by the PVC barrier at a depth of 30 cm. in order to avoid the lateral flow of nutrients and water. The results showed that the I_AWD increased yields by 1.7% to 5.1% (P>0.05), NH₃ volatilization 8.9%（only in 2020）, and N₂O emissions by 105.0% to 115.0%, compared with the I_CF (P<0.05). Biochar addition at the rate of 20 t/hm² was significantly reduced the NH₃ volatilization by 8.7% to 20.5% and N₂O emissions by 21.6% to 24.2% (P<0.05). There was an increase in the yield by 0.2% to 12.5% and a reduction in the reactive nitrogen gas emissions by 6.1% to 11.7% in biochar combined with the I_AWD, compared with the control (I_CFB₀). Furthermore, the inorganic nitrogen existed mainly in the form of NH₄⁺-N in the soil environment of rice field, accounting for 88.6% to 94.7%. The I_AWD had no significant some impact on the soil NH₄⁺-N, compared with the I_CF, whereas, the significant increase was found in the content of surface soil NO₃⁻-N by 50.3% to 74.0% and the oxidation-reduction potential by 47.6% to 54.1% (P<0.05). Moreover, the biochar addition increased the soil NH₄⁺-N by 25.9% to 52.41% (P<0.05), and pH by 0.6% to 1.0%, compared with the biochar free treatment. Principal component analysis showed a strong negative correlation between NH₃ volatilization and soil NH₄⁺-N content, yield, and aboveground nitrogen accumulation. It indicated that the biochar addition was significantly improved the soil adsorption capacity for the inorganic nitrogen, thereby reducing the NH₃ volatilization losses and increasing aboveground nitrogen accumulation, ultimately leading to increase the yield. N₂O emissions were strongly positively correlated with the oxidation-reduction potential and soil NO₃⁻-N. Therefore, the frequent alternate aerobic and anaerobic environment under I_AWD increased the concentration of soil NO₃⁻-N and the oxidation-reduction potential, thereby increasing denitrification substrates and N₂O emissions. Anyway, the biochar addition coupled with alternate wetting and drying can be expected to promote the rice production with water saving, carbon sequestration and emission reduction, while reduce the environmental costs associated with the reactive nitrogen emissions in the paddy field.