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In this study, a process-based denitrification-decomposition (DNDC) model was parameterized and validated with CO2 fluxes data measured using eddy covariance (EC) technique in the meadow and desert steppes of Inner Mongolia grasslands, China. The validated model was applied to predict the influence of moderate climate change on carbon (C) budget of the two steppes over the next 30 years.

A series of parameters associated with plant growth including maximum biomass production, C/N ratio of plant tissue, and accumulating thermal degree days based on local observations were adjusted to meet the requirements of adapting DNDC for the two targeted steppes. Daily weather data for the studied year together with soil properties for the study sites were employed as inputs to simulate the grass growth and soil C dynamics. A baseline and five alternative climate scenarios were utilized to estimate the effect of climate change on the ecosystem C dynamics for the next 30 years. The coefficient of determination (R (2)), root mean square error (RMSE), relative mean deviation (RMD), and modeling efficiency (EF) were used to assess the DNDC model based on the EC measurements.

The DNDC model successfully captured the maximum and minimum soil surface temperature and the effects of rainfall pulse on soil water content (SWC). Validation of ecosystem daily CO2 fluxes against the EC observed values resulted in satisfactory results in both of the meadow and the desert steppe. Additionally, the comparison of monthly simulated results is better than that of daily results. Between 2011 and 2040, if local climate becomes wetter, both steppes would become C sinks, leading to a great net primary productivity (NPP) of 821 and 828 kg C ha(-1) year(-1) sequestered in soil organic C (SOC), respectively. If local climate becomes drier, they would become C-sources, having the lowest NPP with 166 and 121 kg C ha(-1) year(-1) lost from the soil C pool, respectively.

The validated DNDC model can be used to predict C dynamics in two semiarid grassland ecosystems. For the upcoming 30 years, if the local climate became wetter, the meadow and desert steppes would act as C sink, absorbing CO2 from the atmosphere and increase the C sequestration potential. However, both steppes would act as C source, releasing CO2 to the atmosphere, and decrease the C sequestration when the local climate becomes drier.