Warming reduces nitrogen emissions in some forests ^{CEFR B2}

Researchers report in the Proceedings of the National Academy of Sciences that modest warming can reduce nitrogen gas emissions from some forest soils, overturning a common assumption. The study grew from a University of California, Riverside collaboration with graduate students and postdoctoral researchers based in Shenyang City, China. Over six years at a forest site in Qingyuan County the team collected more than 200,000 soil gas measurements.

To mimic mid-century warming, infrared heaters raised temperatures above forest plots by about 2°C (3.6°F). Six plots of 108 square metres each were equipped with automated chambers that opened and sealed to capture gas fluxes. The detailed dataset showed that warming reduced emissions where soils became drier: nitric oxide emissions fell 19% and nitrous oxide fell 16%. The researchers identified a moisture threshold around 1,000 millimetres of annual rainfall; below that, drying tended to cut emissions, while wetter forests showed the opposite pattern, consistent with laboratory studies.

Pete Homyak, an associate professor at UCR, said the results "flip our assumptions," and lead author Kai Huang emphasised that moisture, not just heat, is key. Although nitrogen appeared to remain in drier soils, tree growth did not increase; unpublished measurements suggest trees in warmed plots may grow more slowly from drought stress. The team is continuing to track microbial responses, soil chemistry and forest health in this and other experimental plots worldwide to improve models of forest responses to warming.