Researchers from the Institute of Applied Ecology of the Chinese Academy of Sciences and the University of California, Riverside, have investigated how the loss of forest soil gaseous nitrogen (NO, N₂O, and N₂) is affected by climate warming, highlighting the critical role of these gases in regulating forest nutrient cycling and ecosystem functioning.

The study was published in PNAS on November 24.

For decades, climate models have predicted that warming would accelerate nitrogen (N) cycling and increase gaseous nitrogen losses. However, most previous warming studies relied on manual or low-frequency sampling and often focused only on N₂O, overlooking NO and N₂. Consequently, the complete picture of gaseous nitrogen loss under climate warming remained unclear.

To address this gap, the researchers conducted research at the Qingyuan Forest Warming Experiment in Northeast China, the world's largest infrared forest-warming platform. This site offers an unparalleled opportunity to examine soil gaseous nitrogen emissions under controlled, long-term warming conditions.

Their study represents the first in situ quantification of warming effects on both NO and N₂O emissions in a forest ecosystem. Remarkably, the researchers collected over 200,000 automated, real-time gas measurements over six years, far exceeding the temporal resolution of any previous study.

Contrary to longstanding expectations based on laboratory-derived temperature sensitivities (Q₁₀), experimental warming of 2 °C reduced, rather than increased, soil gaseous nitrogen emissions. Annual NO emissions declined by 19%, while N₂O emissions decreased by 16%.

This reduction in nitrogen emissions could not be explained by enhanced microbial reduction to N₂, greater plant nitrogen uptake or leaching, or changes in nitrogen cycling gene abundance. Instead, the researchers found that warming-induced soil drying, though modest, was sufficient to suppress the microbial nitrogen transformations that generate gaseous losses.

Overall, the study challenges key assumptions in many ecosystem and climate models that often rely on laboratory parameters which do not fully capture field conditions. These results highlight the importance of long-term, high-resolution field observations for understanding how ecosystems respond to a warming world.

These results address a critical knowledge gap with broad implications for climate science and ecosystem management. They will help promote a new wave of ecosystem modeling that accounts for these unexpected changes in nitrogen cycling.