Residual effects of slow-release nitrogen fertilizer application during the wheat growing season on N₂O emission in soybean season under co-elevated atmospheric CO₂ concentration and temperature

Farmland N₂O emission is a popular topic in climate change research. Climate change (elevated CO₂ concentrations and increased temperatures) and farmland management measures are key factors affecting N₂O emissions in soil. Slow-release fertilizers are considered to have a positive effect on reducing N₂O emissions. It is unclear how the interaction between climate change and slow-release fertilizers affects N₂O emissions. Thus, we combined the local wheat-soybean cultivation model to investigate the potential impact of applying slow-release fertilizers on soil N₂O emissions during the growth season of subsequent crop under elevated CO₂ concentrations and increased temperatures. In this study, two air temperatures (ambient temperature and ambient temperature + 2 ℃) and two CO₂ concentrations (400 and 600 μmol∙mol⁻¹) were set in the controlled chambers. Two fertilizers (conventional urea and slow-release urea) were applied during the wheat season under various environmental treatments in a wheat-soybean rotation system. Still, no fertilizer was used during the soybean growing season. CK referred to the ambient CO₂ concentration and temperature simulated in the controlled chamber (the ambient environmental CO₂ concentration in this experiment was 400 μmol∙mol⁻¹, and the environmental temperature was controlled by the intelligent control system to be consistent with the external environment). N₂O emissions during the entire growth period of soybean, soil ammonium nitrogen and nitrate nitrogen contents, and activities of soil enzymes involved in nitrification and denitrification during the seed filling and mature stages of soybean were measured. The results indicated that N₂O emissions from soybean-grown soil were mainly concentrated during the later growth period of soybean. Compared to the application of conventional urea, the application of slow-release fertilizer in wheat-grown soil reduced total N₂O emissions by 38.46% during the soybean growth period under increased temperature treratment (ET), whereas the application of slow-release fertilizer increased total N₂O emissions in soybean-grown soil by 80.76%, 82.35%, and 11.96% under the CK, elevated CO₂ concentration (EC), and co-elevation of atmospheric CO₂ concentration and temperature (ECT) treatments, respectively. Compared to CK treatment, ET treatment accelerated the conversion of ammonium nitrogen to nitrate nitrogen in soybean-grown soil at the seed filling stage, enhancing nitrate reductase activity and increasing N₂O emissions. In addition, EC treatment had no significant effect on N₂O emissions from soybean-grown soil compared to the CK treatment. Compared with CK, the nitrate nitrogen content in soybean-grown soil under ECT treatment increased slightly at the seed filling stage and mature stage, and the ammonium nitrogen content increased significantly at the mature stage. In the seed filling stage, the increase of nitrate reductase activity promoted N₂O emissions, and in the mature stage, the increase of nitrate nitrogen and ammonium nitrogen promoted N₂O emissions. However, with the application of conventional urea, there was no significant difference in N₂O emissions in soybean-grown soil between ECT and ET treatments. In conclusion, N₂O emissions did not increase with elevated CO₂ concentration alone. N₂O emissions were expected to increase under elevated temperatures in the future, and the rational application of slow-release fertilizers in wheat-grown soil can reduce N₂O emissions in soybean-grown soil. Our study highlights that slow-release fertilizers have greater potential to reduce N₂O emissions under climate change scenarios.