Nitric oxide (NO) and nitrous oxide (N2O) are important air pollutants and precursors of acid rain. Subtropical forest soils in southern China were hotspots of nitrogen oxides release.

Soil moisture is a major driver of NO and N2O release. However, little is known about the response of the NO and N2O emissions to soil drying in subtropical forest soils in South China.

Dr. Kang Ronghua, research scientist of the Institute of Applied Ecology of the Chinese Academy of Sciences (CAS), together with Thomas Behrendt of the Max Planck Institute for Biogeochemistry and Peter Dorsch of the Norwegian University of Life Sciences, collected well-drained hill slope soils (HS soils) and wet soils of the groundwater discharge zone (GDZ soils) from a subtropical forest in southwest China, and evaluated the rates of NO and N2O release following soil drying.

The researchers observed that, for soils from both hydrological environments, the maximum NO release peaked at water filled pore space (WFPS, the ratio of volumetric soil water content to total soil porosity) level of approximately 20%, i.e., within the dry to intermediate soil moisture range, which suggested that nitrification was a dominant process for NO formation in the subtropical forest soils. For GDZ soils, NO production and NO consumption peaked at different WFPS levels, and they concluded that denitrification exerted additional control on NO flux in the groundwater influenced surface soils. In addition, the researchers found that net N2O release peaked at WFPS level of 100% and the value decreased during soil drying.

This study suggests that changes in soil moisture caused by global warming and forest management may affect the magnitude of regional nitrogen oxide emissions.

This study has been published in Forests, entitled “Soil Moisture Control of NO Turnover and N2O Release in Nitrogen-Saturated Subtropical Forest Soils”, and it was funded by CAS "Hundred Talent Program" and the National Natural Science Foundation of Norway.