## 4 Discussion
### Influence of Nutrient Availability on Growth and Carbon Sequestration of P. australis  
Our results show that the constant of the nutrient availability (\(K_{NP}\)) was one of the key environmental factors of the growth dynamics model (Table 6), which can further influence the carbon sequestration capacity. As eutrophication poses potential threat in the HWNR [74], we further explored how it influences the growth and carbon sequestration capacity of _P. australis_ by setting five scenarios with different constant of the nutrient availability. Assuming the calibrated \(K_{NP}\) in Section 3.2 as the base value, Scenario 1 was set to the base scenario, while Scenarios 2-5 were set by varying \(K_{NP}\) at intervals (\(-50\%\), \(-25\%\), \(+25\%\), \(+50\%\)). Figure 8a,b shows simulated results of the AGB and BGB in all five scenarios.  
With higher values of \(K_{NP}\), both the maximum AGB and BGB in Scenarios 4-5 were significantly higher than in Scenario 1. Conversely, they were lower in Scenarios 2-3 than in Scenario 1. This pattern indicated the facilitating effect of \(K_{NP}\) on the growth of _P. australis_, which was consistent with the findings in several other cases (e.g., Burullus Lake, Egypt; Swazaro, Poland) [71; 72]. Subject to varying \(K_{NP}\), new rhizomes began to grow at different  
Figure 8: Biomass variation in _P. australis_ subject to scenarios with varying nutrient availability: (**a**) in the above-ground organs; (**b**) in the below-ground organs. \(t_{s}\): the date when shoots start senescence; \(t_{p}\): the date when pedunces and new rhizomes start forming.  
rates after \(t_{p}\) (the date when peduncles and new rhizes start forming) because of different amounts of current photosynthesized material transferred to the below-ground organs, leading to contrasting biomass in the below-ground part. After \(t_{s}\) (the date when shoots start senescence), the senescence of the above-ground organs led to abrupt decline in the AGB and also diminished the contrast among the various scenarios. At the same time, the transport of photosynthetic materials from the above-ground part to the below-ground part widened the gap in the BGB. This is also consistent with the finding reported by [PERSON] et al. [59] that increased nutrient availability leads to higher living biomass as well as more photosynthetic materials transported to the below-ground part.  
As a result of the biomass change, our results show that the annual carbon sequestration capacity increases by 12% when \(K_{NP}\) increases by 50%, and decreases by 23% when \(K_{NP}\) decreases by 50% (Figure 9); i.e., annual carbon sequestration capacity of _P. australis_ is enhanced by a higher \(K_{NP}\)[75; 76; 71; 72]. Hence, comprehensive analysis of the ecological consequences of nutrient removal and carbon sequestration capacity should be performed in future wetland management to optimize the trade-off between eutrophication control and carbon sequestration. Additionally, as shown in Section 2.5, carbon sequestration was estimated based on the contribution of both living parts (i.e., above- and below-ground part) and the mortality part in the present study. Our results show that the magnitude of carbon sequestration capacity of each part had a positive correlation with increasing \(K_{NP}\) (Figure 9), which was consistent with reported findings from other studies [77]. However, the effect of the decomposition of mortality part was not considered in our carbon sequestration analysis. In fact, this process can trigger the carbon release and cause loss of carbon sequestration capacity of _P. australis_[77; 78]. As the \(K_{NP}\) increases, enhanced carbon sequestration capacity from the mortality part can cause larger loss in carbon sequestration capacity if not treated properly. As such, we recommend that more frequent harvest and regular collecting of mortality organs should be carried out [77; 79] after the senescence begins so that carbon sequestration loss due to decomposition can be reduced.