Modeling N2O emissions from steppe in Inner Mongolia, China, with consideration of spring thaw and grazing intensity
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.
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.
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.
The introduction of the impedance concept into Mobile-DNDC is a major step forward in simulating pulse N2O emissions from soils during spring-thaw.
KeywordsFreeze-thaw Impedance concept N2O Steppe Biogeochemical modelling Grazing intensity
- Allard V, Soussana JF, Falcimagne R, Berbigier P, Bonnefond JM, Ceschia E, D’hour P, Henault C, Laville P, Martin C, Pinares-Patino C (2007) The role of grazing management for the net biome productivity and greenhouse gas budget (CO2, N2O and CH4) of semi-natural grassland. Agric Ecosyst Environ 121:47–58CrossRefGoogle Scholar
- Butterbach-Bahl K, Stange F, Papen H (2001) Regional inventory of nitric oxide and nitrous oxide emissions for forest soils of southeast Germany using the biogeochemical model PnET-N-DNDC. J. Geophys. Res. 106: 34,155–34, 166Google Scholar
- Frolking SE, Mosier AR, Ojima DS, Li C, Parton WJ, Potter CS, Priesack E, Stenger R, Haberbosch C, Dorsch P, Flessa H, Smith KA (1998) Comparison of N2O emissions from soils at three temperate agricultural sites: simulations of year-round measurements by four models. Nutr Cycl Agroecosyst 52:77–105CrossRefGoogle Scholar
- Hansson K, Simunek J, Mizoguchi M, Lundin LC, van Genuchten MT (2004) Water flow and heat transport in frozen soil: numerical solution and freeze-thaw applications. Vad Zone J 3:693–704Google Scholar
- Li CS, Frolking S, Frolking TA (1992a) A model of nitrous-oxide evolution from soil driven by rainfall events. 1. Model structure and sensitivity. J Geophy Res Atmos 97:9759–9776Google Scholar
- Li CS, Frolking S, Frolking TA (1992b) A model of nitrous-oxide evolution from soil driven by rainfall events. 2. Model applications. J Geophy Res Atmos 97:9777–9783Google Scholar
- Lu CQ, Tian HQ (2007) Spatial and temporal patterns of nitrogen deposition in China: synthesis of observational data. J Geophy Res Atmos 112:DD22S05Google Scholar
- Mummey DL, Smith JL and Bluhm G (2000) Estimation of nitrous oxide emissions from US grasslands. Environmental Management 25:169–175 Google Scholar
- Olson DM, Dinerstein E, Wikramanayake ED, Burgess AD, Powell GVN, Underwood EC, Itoua I, Strand HE, Morrison JC, Loucks JL, Allnutt TF, Ricketts TH, Kura Y, Lamoreux JF, Wettengel WW, Hedao P, Kassem KR (2001) Terrestrial ecoregions of the world: a new map of live on earth. Bioscience 51:933–938CrossRefGoogle Scholar
- Saggar S, Hedley CB, Giltrap DL and Lambie SM (2007) Measured and modelled estimates of nitrous oxide emission and methane consumption from a sheep-grazed pasture. Agr. Ecosyst. Environ. 122:357–365Google Scholar
- Zhang Y, Li CS, Trettin CC, Li H, Sun G (2002) An integrated model of soil, hydrology, and vegetation for carbon dynamics in wetland ecosystems. Global Biogeochem Cy 16Google Scholar