Rapid soil rewetting promotes limited NO emissions and suppresses NH 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 NH and NO 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 NO and NH after urea application. During the progressive drying process (WFPS decreasing from 70% to 60% and 50%), soil NO and NH 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%), NO emissions showed a slight increase, while NH volatilization continued to decrease. Soil NH-N and DOC contents both decreased during progressive drying, while the soil NO-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 NH-N content and the abundances of ureC and amoA-b were the main factors suppressing NO and NH emissions. We believe that drying process limits the related microbial activity and substrate supply during ammonia oxidation process in terms of NO emissions, while in terms of NH volatilization, it reduces the related microbial activity of urea hydrolysis process and increases the ammonium adsorption to the soil.