Models are promising tools for evaluating the importance of drying-rewetting and freeze-thaw events (Groffman et al., 2009). Simple regression and empirical models have been developed based on the relationships between environmental factors including soil moisture and/or soil temperature and biogenic soil gas fluxes (Roelandt et al., 2005; Flechard et al., 2007). Some rely on empirical observations but fail under rewetting or thawing conditions (Borken et al., 2003; Lawrence et al., 2009). Further work in this area will increasingly have to incorporate the actual substrate and microbial dynamics occurring (Davidson and Janssens, 2006; Vargas et al., 2011).
Process-based models have been developed with the objective of simulating terrestrial ecosystem C and N biogeochemistry including GHGs (e.g. DAYCENT, Parton et al., 2001; DNDC, Li et al., 1992; ecosys, Grant and Pattey, 2003). Most existing process-based models require additional work to improve simulating rewetting and thawing effect on biogenic gas fluxes (Jarecki et al., 2009; Norman et al., 2008). Groffman et al. (2009) suggested that modelling peak fluxes associated with drying and rewetting events requires: 1) accurate simulation of moisture changes in different soil layers and complex shifts in utilisation of fast- and slow-cycling soil organic matter pools by microbes that take place during these events (Miller et al., 2005) and 2) daily or sub-daily simulations of both physical and biological processes (Kiese et al., 2005). They also suggested that modeling of freeze-thaw induced N2O fluxes requires consideration of the increase in easily degradable substrates following freezing, tight coupling of nitrification and denitrification in the water saturated topsoil, and the breakdown of N2O reductase activity at low temperature (Holtan-Hartwig et al., 2002). Regardless of the specific processed under consideration, it is critical to enhance the communication between field scientists and the modelling community, as models can be use to generate hypotheses (de Bruijn et al., 2009) to be tested in the field and lab.
Wolf B, Kiese R, Chen W, Grote R, Zheng X, Butterbach-Bahl K (2012) Modeling N2O emissions from steppe in Inner Mongolia, China, with consideration of spring thaw and grazing intensity. Plant Soil 350 (1):297-310. doi:10.1007/s11104-011-0908-6
ReplyDeleteAims
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.