Researchers from the Institute of Applied Ecology of the Chinese Academy of Sciences and the University of California, Riverside, report pronounced spatial heterogeneity in forest soil nitrogen emissions (NO and N₂O) across hillslopes, identifying topography-mediated soil moisture gradients as a key control of gaseous N losses.

This study was published in Global Change Biology on April 17.

Soil NO and N₂O emissions are highly sensitive to soil moisture conditions and represent important pathways of forest N loss. However, how topography and seasonal variation interact through hillslope moisture gradients to shape these emissions remains poorly understood.

To address this knowledge gap, researchers conducted a two-year hillslope monitoring experiment at the Qingyuan Forest Ecosystem National Observation and Research Station in Northeast China. The study represents the first continuous high-frequency investigation of soil NO and N₂O emissions across forest hillslope positions, including upper slope, backslope, footslope and toeslope.

Their results showed pronounced spatial heterogeneity in gaseous N emissions. Lower slope positions exhibited substantially higher NO and N₂O emissions than upper slope positions, with emissions increasing by approximately 1.5–2 times for NO and 1.3–7.2 times for N₂O. Across the entire hillslope, annual soil NO and N₂O emissions reached 0.2 kg N ha⁻¹ and 1.0 kg N ha⁻¹, respectively.

Further analyses demonstrated that topography regulates soil moisture gradients and N substrate availability, thereby shaping nitrification and denitrification processes that govern gaseous N emissions. Downslope positions favored denitrification and higher N₂O emissions, whereas upper slopes limited both NO and N₂O emissions under drier conditions.

These contrasting patterns indicate that NO emissions are primarily driven by nitrification-related substrate availability, while N₂O emissions are more strongly linked to denitrification-related microbial functional genes, highlighting the combined influence of environmental gradients and microbial functional potential.

Overall, the study provides a process-based explanation for hillslope-scale gas N emissions, linking topography-driven hydrological gradients with microbial processes and spatial patterns of NO and N₂O emissions. The study fills an important knowledge gap in high-frequency observations of forest hillslope N emissions and provides new insights into the spatial heterogeneity of forest N cycling under changing environmental conditions.