Southern urban forest converted from orchard abandonment: a potentially important N₂O source driven by warm-humid...

In southeastern (SE) China, rapid urban expansion has expanded areas of secondary and plantation forests converted from abandoned orchards, which could be potential hotspots of nitrous oxide (N₂O) emissions due to experiencing historical nitrogen (N) fertilizer applications and high N deposition. However, mechanisms underlying the dynamics of soil N₂O emission from these lands remain poorly investigated, particularly under the significant influences of the SE Asian monsoon climate. In this work, we conducted a year-long observation and a short-term manipulation experiment to explore the factors driving soil N₂O emissions from an urban forest converted from abandoned orchards in a mega city of SE China. Our results highlight its critical role as a significant yet underappreciated source of N₂O, with annual emission of 1.02 kg N ha^-1 yr^-1 which is generally higher than the emission strength typically reported from other subtropical forests. Elevated concentrations of soil nitrate in the studied forest were driven by the combined effects of historical fertilization legacies and continuous N deposition, providing abundant substrates for N₂O emissions. Seasonal rainfall and rewetting events following droughts substantially influenced N dynamics, triggering N₂O flux peaks during periods of rapid soil moisture changes. The short-term drivers, such as rainfall-induced changes in soil moisture, were found to be critical for pulse releases of N₂O, whereas temperature and pH of soil water regulated the seasonal variations of N₂O fluxes. These findings underscore the importance of integrated monitoring to capture both the immediate impacts of extreme climatic events and the long-term trends in emissions. Under sustained N deposition, urban forest soils play a vital role in regional N cycling, emphasizing their significance in global greenhouse gas assessments and strategies for mitigating climate change.