Effects of rice straw biochar on CH₄ and N₂O emissions in alternating wetting and drying rice fields

Biochar is widely used as one type of soil amendment to enhance soil properties and carbon sequestration in croplands. But it is still unclear on the influence of rice straw biochar application on CH₄ and N₂O emissions from paddy fields, especially in Northeast China. The typical rice monoculture region is often used under the alternate regime of wetting and drying (I_AWD) with the aging of biochar in soil. This study aims to further investigate the potential of biochar application in I_AWD paddy fields. A split-plot field was designed with two irrigation regimes, two biochar additions, and three replications in 2019-2021. Among them, the irrigation regimes (I) included continuous flooding irrigation (I_CF) and I_AWD. The sub-plots within each main plot were subjected to two biochar (B) additions (biochar-free control and 20 t ha-1 biochar, represented by B₀ and B₂₀, respectively). Prior to the experiment, the surface soil was first sampled in the rice fields and then air-dried to measure the basic physicochemical parameters of the soil. Biochar was applied one day before transplanting in 2019. After that, there was no additional biochar in 2020 and 2021. The opaque static chamber was used to collect the CH₄ and N₂O emissions over 3-4 days after topdressing, and 7-10 days between rice transplanting and harvesting. Gas samples were finally obtained to measure the CH₄ and N₂O concentration using gas chromatograph (Agilent 7890B, Agilent Technologies, Inc., USA). Results showed that the I_AWD significantly reduced the CH₄ emission by 63.03%-78.89%, global warming potential (GWP) by 62.02%-75.21%, greenhouse gas inventory (GHGI) by 62.31%-75.28%, compared with the ICF, although there was an increase in the N₂O emission of 100%-122.67%. The B₂₀ treatment reduced the CH₄ emission by 21.99% in 2020, and 38.21% in 2021, whereas, there was an increase in the soil organic carbon by 24.03%-28.88%, and N₂O emission by 28.26%-33.10% over three years. Moreover, biochar was used to inhibit the negative effect of I_AWD on the increase of N₂O emissions. A correlation analysis showed that there was a significant negative correlation in the cumulative CH₄ emissions with the soil redox potential (Eh) value, and soil organic carbon (SOC), whereas a significant positive correlation was found with the soil temperature. There was a significant negative correlation between the cumulative N₂O emission and soil temperature, whereas, a significant positive correlation was between the soil Eh value. A positive effect of rice production was found in the second and third years, although there was a slight decrease in the first year of biochar application. The main reason was that the early straw biochar was more alkaline with a limited effect. A synergy effect was found in the biochar adsorption emission reduction and slow release with the pH value to the normal, indicating the increasing production. Especially, the B₂₀ treatment significantly reduced GWP by 37.50% and GHGI by 42.86% in 2021, whereas there was an increase of 11.02%, compared with the B₀. The I_AWD B₂₀ treatment reduced the CH₄ cumulative emissions by 83.78%, GWP by 77.98%, and GHGI by 78.95%, compared with the I_CF B₀. It infers that the biochar combined with I_AWD can be used to effectively reduce the GWP and GHGI. Anyway, biochar can be expected to reduce carbon sequestration and CH₄ and N₂O emissions in paddy fields. The finding can also provide a strong theoretical reference to mitigate the CH₄ and N₂O emissions after the biochar application in rice fields.