###### Abstract  
Estimating the biomass of _Phragmites australis_ (Cav.) Trin. ex Steud., i.e., a common wetland macrophtye, and the associated carbon sequestration capacity has attracted increasing attention. Hanshiqiao Wetland Nature Reserve (HWNR) is a large _P. australis_ wetland in Beijing, China, and provides an ideal case study site for such purpose in an urban setting. In this study, an existing _P. australis_ growth dynamics model was adapted to estimate the plant biomass, which was in turn converted to the associated carbon sequestration capacity in the HWNR throughout a typical year. To account for local differences, the modeling parameters were calibrated against the above-ground biomass (AGB) of _P. australis_ retrieved from hyperspectral images of the study site. We also analyzed the sensitivity of the modeling parameters and the influence of environmental factors, particularly the nutrient availability, on the growth dynamics and carbon sequestration capacity of _P. australis_. Our results show that the maximum AGB and below-ground biomass (BGB) of _P. australis_ in the HWNR are \(2.93\times 10^{3}\) and \(2.49\times 10^{3}\) g m\({}^{-2}\), respectively, which are higher than the reported level from nearby sites with similar latitudes, presumably due to the relatively high nutrient availability and more suitable inundation conditions in the HWNR. The annual carbon sequestration capacity of _P. australis_ in the HWNR was estimated to be 2040.73 gC m\({}^{-2}\) yr\({}^{-1}\), which was also found to be highly dependent on nutrient availability, with a 50% increase (decrease) in the constant of the nutrient availability \(K_{NP}\), resulting in a 12% increase (23% decrease) in the annual carbon sequestration capacity. This implies that a comprehensive management of urban wetlands that often encounter eutrophication problems to synergize the effects of nutrient control and carbon sequestration is worth considering in future practices. P 2022 2022 2022 2022 2022 20222 20222 20222 20222 20222 20222 20222 20222 2022 20222 222 20222 2022 202  
_P. australis_ can assimilate considerable carbon through photosynthesis during its growth cycle, which can effectively mitigate carbon emission and support carbon neutrality [7]. Biomass, i.e., the dry weight of the living organic component, of _P. australis_ is a key predictor of its carbon sequestration capacity [8]. Under growing concern of climate change due to excessive carbon emission, estimation of the biomass and carbon sequestration capacity of wetland plants, including _P. australis_, has received increasing attention to support carbon management and achieve the goal of carbon neutrality [9; 10; 11]. Plant biomass is traditionally measured through field sampling on plot scale, which is both time-consuming and labor-intensive, and is often prohibited due to site accessibility issues [12]. In addition to field sampling, a vegetation growth dynamics model has also been developed to estimate the biomass. Growth dynamics models are established based on the relevant eco-physiological processes of the modelled plants, such as photosynthesis, respiration, and mortality, which allow measuring the plant biomass with higher temporal resolution (e.g., daily) and thus understanding the variation trend of biomass and carbon sequestration capacity in greater detail [13; 14; 15]. Specifically, [PERSON] and [PERSON] [14] were among the pioneers who first established a growth dynamics model of _P. australis_ by selecting five state variables (biomass of shoots, inflorescence, roots, old rhizomes, and new rhizomes) to simulate the plant growth cycle. Plant growth was quantified using mathematical relationships based on the integral effects of photosynthesis, respiration, mortality, and translocation of materials between different organs (e.g., above- and below-ground parts) to calculate the biomass change, which can well reflect the growth dynamics of _P. australis_[7; 14]. By specific calibration and validation, the model has been applied to explore the growth dynamics of _P.