Researchers from the Institute of Applied Ecology of the Chinese Academy of Sciences and the University of California, Riverside, have reported pronounced spatial heterogeneity in forest soil nitrogen emissions across hillslopes, revealing that topography-mediated soil moisture gradients play a key role in regulating gaseous nitrogen losses from forest soils.

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

Soil emissions of nitric oxide (NO) and nitrous oxide (N₂O) are highly sensitive to soil moisture conditions and represent important pathways of forest nitrogen (N) loss. However, the interaction between topography and seasonal variation through hillslope moisture gradients that shapes these emissions is poorly understood.

To address this knowledge gap, the researchers conducted a two-year hillslope monitoring experiment at the Qingyuan Forest Ecosystem National Observation and Research Station in northeast China. This study is the first to continuously and frequently investigate soil NO and N₂O emissions across forest hillslope positions, including upper slope, backslope, footslope and toeslope.

The researchers observed pronounced spatial heterogeneity in gaseous N emissions. Lower slope positions exhibited substantially higher NO and N₂O emissions than upper slope positions, with NO emissions increasing by approximately 1.5 to 2 times and N₂O emissions increasing by 1.3 to 7.2 times. 

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 revealed that topography regulates soil moisture gradients and N substrate availability, thereby shaping the 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. These findings highlight 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 the spatial patterns of NO and N₂O emissions.

The findings fill an important knowledge gap in high-frequency observations of forest hillslope N emissions and provide new insights into the spatial heterogeneity of forest N cycling under changing environmental conditions.