An improved process-oriented hydro-biogeochemical model for simulating dynamic fluxes of methane and nitrous oxide in alpine ecosystems with seasonally frozen soils

Abstract. To evaluate the sustainability of terrestrial ecosystems, the hydro-biogeochemical model Catchment Nutrient Management Model – DeNitrification-DeComposition (CNMM-DNDC) was established to simultaneously quantify ecosystem productivity and losses of nitrogen and carbon at the site or catchment scale. As a process-oriented model, this model is expected to be universally applied to different climate zones, soils, land uses and field management practices. This study, as one of many efforts to fulfil such an expectation, was performed to improve the CNMM-DNDC by incorporating a physical-based soil thermal module to simulate the soil thermal regime in the presence of freeze-thaw cycles. The modified model was validated with simultaneous field observations in three typical alpine ecosystems (wetlands, meadows and forests) within a catchment located in the seasonally frozen region of the eastern Tibetan Plateau. Then, the model was further applied to evaluate its performance in simulating the effects of alpine wetland degradation on methane (CH₄) and nitrous oxide (N₂O) fluxes. The validation showed that the modified CNMM-DNDC was able to simulate the observed seasonal dynamics of soil temperature, moisture, and fluxes of CH₄ and N₂O in the three typical alpine ecosystems, with index of agreement values of 0.91–1.00, 0.49–0.83, 0.57–0.88 and 0.26–0.47, respectively. Consistent with the emissions determined from the field observations, the simulated aggregate emissions of CH₄ and N₂O were significantly reduced due to wetland degradation and were dominated by a reduction in CH₄ emissions. This study indicates the potential for utilizing the process-oriented model CNMM-DNDC to predict hydro-biogeochemical processes, as well as related gas emissions, in seasonally frozen regions. As the original CNMM-DNDC was previously validated in some unfrozen regions, the modified CNMM-DNDC could be applied to evaluate the sustainability of various ecosystems under different climates, soils and field management practices at the site or catchment scale.