# A Typical Small Watershed in Southwestern China Is Demonstrated as a Significant Carbon Sink
## 4 Discussion
### Uncertainty in Carbon Sink Estimates  
The fragmented land use in Southwestern China increases the difficulty of studying carbon sinks in small watersheds. In addition to forest, cropland, and non-timber forest, the remaining land use types in the Reshuihe River watershed consist mainly of grassland ecosystems (16.6%) and aquatic ecosystems (2.2%), and the carbon fluxes of these two ecosystems are the main sources of uncertainty in carbon-balance estimations. Previous studies have shown that subtropical alpine meadow ecosystems in Southwestern China are carbon sinks [70]. On the other hand, the carbon balance of terrestrial ecosystems could be affected by the export and emission of different types of carbon through aquatic systems [71]. A study by [PERSON] et al. in subtropical rivers in Southwestern China showed that rivers are a source of carbon for the atmosphere (0.032 \(\pm\) 0.047 Tg C a\({}^{-1}\)) [72]. Nevertheless, an analysis by [PERSON] et al. emphasized that aquatic carbon offsets an average of 9% of NEP in forest ecosystems, and that aquatic carbon was important in ecosystems with small NEP [73]. In a study on the impact of aquatic carbon on carbon sinks in watersheds, [PERSON] et al. calculated the terrestrial and aquatic carbon fluxes in a 117 km\({}^{2}\) watershed dominated by grassland ecosystems [69]. They found that, despite the presence of aquatic carbon emissions, the carbon sink function of the region remained unchanged, and the watershed continued to maintain a net carbon uptake (\(-\)27.7 gC m\({}^{-2}\) a\({}^{-1}\)). Based on previous findings as such, we concluded that carbon uptake by grassland ecosystems and carbon release by aquatic ecosystems may not have a large impact on the carbon sink function in our study area. In addition, for small watersheds that serve as both natural and social units, direct and indirect management activities may have impacts on ecosystem carbon exchange [74]. In the Reshuihe River watershed, we observed that forest ecosystems possess a strong capacity to absorb carbon, but this sink's sustainability is affected by various factors. For example,fires cause direct carbon loss and reduce post-fire carbon sequestration capacity [75], while deforestation directly weakens ecosystem carbon sinks [74]. Fortunately, the government has invested significant resources in reducing the incidence of fires and deforestation [76; 77], which has helped to enhance the ecosystem's ability to fix carbon. Additionally, widely implemented reforestation policies have compensated for carbon loss through logging and fires by promoting new growth [78]. Another concern is the age of the trees. The forests in our study area are currently experiencing rapid growth and development, resulting in a high capacity for carbon sequestration. Traditional views suggested that as forests aged, their carbon sink would cease to accumulate carbon [79]. Nevertheless, numerous studies have demonstrated that old-growth forests remain important carbon sinks [80; 81]. In Southwestern China, [PERSON] et al. discovered that subalpine primeval Abies fabri forests over 150 years old were significant carbon sinks [82], and [PERSON] et al. found through long-term monitoring that evergreen old-growth forests were still strongly absorbing carbon [17]. Therefore, the Reshuihe River watershed forests are expected to maintain strong net carbon sequestration for an extended period of time in the future. Similarly, biomass removal from croplands and non-timber forests may eventually release some of the CO\({}_{2}\) back into atmosphere as it is consumed or decomposed, which may offset some of the observed net carbon sink [25; 61]. This process occurs not only within watersheds, but also outside watersheds due to the marketing and distribution of harvested fruits. Therefore, a harmonized framework should be developed in the future to study the trans-regional transfer of carbon in detail.