Soil NO emission in Cinnamomum camphora plantations along an urbanization gradient altered by changes in litter input and microbial community composition.

Urbanization alters land use, increasing the rate of greenhouse gas (GHG) emissions and hence atmospheric compositions. Nitrous oxide (NO) is a major GHG that contributes substantially to global warming. NO emissions are sensitive to changes in substrate availabilities, such as litter and N input, as well as micro-environmental factors caused by land-use change upon urbanization. However, the potential impacts of changing litter and N on soil NO emissions along urban-rural gradients is not well understood. Here, we conducted an in situ study over 19 months in Cinnamomum camphora plantations along an urban-rural gradient, to examine the effects of the urban-rural gradient, N and litter input on NO emissions from C. camphora plantation soils and the underlying mechanisms via N, litter and microbial communities. The results showed that urban soil NO emissions were 105% and 196% higher than those from suburban and rural soil, respectively, and co-occurred with a higher abundance of AOA, nirS and nirK genes. Litter removal increased cumulative NO emissions by 59.7%, 50.9% and 43.3% from urban, suburban and rural soils, respectively. Compared with litter kept treatment, increases in AOA and nirK abundance were observed in urban soil, and higher rural nirS abundance occurred following litter removal. Additionally, the relatively higher soil temperature and available N content in the urban soil increased NO emissions compared with the suburban and rural soil. Therefore, in addition to changes in microbial communities and abiotic environmental factors, litter kept in C. camphora plantations along an urban-rural gradient is also important in mitigating NO emissions, providing a potential strategy for the mitigation of NO emissions.