Decreased soil N₂O and N₂ emissions during the succession of subtropical forests

Natural forest succession may modify soil nitrogen (N) cycling and N gas emissions. However, little is known about how this ecological succession modulates soil N₂O and N₂ emissions. We focused on three typical succession chronsequences of subtropical forests: the early stage of an Alnus nepalensis forest (~ 60 years), the intermediate stage of a Populus bonatii forest (~ 100 years), and the late stage of an evergreen broad-leaved forest (> 300 years).

The acetylene inhibition technique and molecular method were used to investigate the changing patterns of soil N₂O and N₂ emissions, as well as the key abiotic and biotic factors that regulate gas emissions.

The highest rates of soil N₂O and N₂ emissions were observed in the early-successional stage, which were 10–21 times and 6–12 times higher than those of the intermediate and late stages, respectively. This stimulation in the early stage was mainly related to the pure stands of N-fixing trees, thus amplifying soil inorganic N pools and providing additional substrates for nitrification- and denitrification- driven N₂O. Although N₂O emissions under denitrifying conditions were 2–131 times higher than those under nitrifying conditions, N₂ was the dominant N gas loss in subtropical forests. Changes in nirK-denitrifier abundance with forest succession were closely related to N₂O emissions.

Our findings suggest that variations in soil active nitrogen pools and nirK abundance associated with subtropical forest succession could reduce N₂O and N₂ emissions, thus resulting in positive feedbacks for climate change mitigation.