Rapid soil rewetting promotes limited N₂O emissions and suppresses NH₃ volatilization under urea addition

The alternation of dry and wet is an important environmental factor affecting the emission of nitrous oxide from soil. However, the consistent or opposite effects on NH₃ and N₂O emissions caused by adding exogenous urea in this process have not been fully considered. Here, we controlled the initial (slow drying) and final (adding water) water-filled pore space (WFPS) at 70%, 60%, or 50% through microculture experiment to simulate a process of slow drying-fertilization and rapid wetting of the soil from rice harvest to dryland crop fertilization. Through measuring soil chemical properties and the abundance and composition of related microbial communities during drying process, we studied the pathways of influence of drying and rewetting on the emission of N₂O and NH₃ after urea application. During the progressive drying process (WFPS decreasing from 70% to 60% and 50%), soil N₂O and NH₃ emissions decreased by 49.77%-72.13% and 17.89%-42.19%, respectively. After rapid rewetting (WFPS increasing from 60% to 70%, 50%-60% and 70%), N₂O emissions showed a slight increase, while NH₃ volatilization continued to decrease. Soil NH₄⁺-N and DOC contents both decreased during progressive drying, while the soil NO₃^--N content was enhanced. The drying process changed the community structure of ureC and amoA-b and reduced their abundance but had no effect on amoA-a, nirK or nirS. Correlation analysis indicated that the reductions in NH₄⁺-N content and the abundances of ureC and amoA-b were the main factors suppressing N₂O and NH₃ emissions. We believe that drying process limits the related microbial activity and substrate supply during ammonia oxidation process in terms of N₂O emissions, while in terms of NH₃ volatilization, it reduces the related microbial activity of urea hydrolysis process and increases the ammonium adsorption to the soil.