Modeling N2O emissions from steppe in Inner Mongolia, China, with consideration of spring thaw and grazing intensity

AIMS: Temperate grassland is one of the major global biome types and is widely used as rangeland. Typically, cold winters are followed by a transition period with soil thawing that may last from days to weeks. Pulse N2O emissions during freeze-thaw events have been observed in a range of temperate ecosystem types and may contribute significantly to annual N2O emissions. It was shown recently that spring thaw pulse N2O emissions dominated annual N2O emissions in a steppe region of Inner Mongolia. Even though biogeochemical models are increasingly used for up scaling of N2O emissions from terrestrial ecosystems, they still need to be further developed to be capable of both simulating pulse N2O emission during spring thaw and accounting for the impact of grazing on soil N2O emissions in general. METHODS: In this study, we modified an existing biogeochemical model, Mobile-DNDC, to allow an improved simulation of plant production, snow height, and soil moisture for steppe in Inner Mongolia exposed to different grazing intensities. The newly introduced routines relate maximum snow height to end-of-season biomass (ESSB), to account for decreased plant productivity due to grazing and consider the increase of resistance (impedance) of soil ice on the soil hydraulic conductivity. RESULTS: The implementation of the impedance concept, which means the consideration of decreased hydraulic conductivity in frozen soil, resulted in an improved simulation of soil water content and decreased simulated oxygen content in the top soil during freeze-thaw periods. Increased soil moisture and associated oxygen limitation stimulated N2O emission by enhanced denitrification. Based on observations in the field, maximum snow height was limited by ESSB, protecting snow against erosion by wind. Since grazing reduced biomass and thereby snow cover, water availability during spring thaw was smaller at grazed sites as compared to ungrazed sites. In agreement with field observations, lower water content and anaerobiosis resulted in decreased N2O emissions during spring thaw. CONCLUSIONS: The introduction of the impedance concept into Mobile-DNDC is a major step forward in simulating pulse N2O emissions from soils during spring-thaw.