## 4 Discussion  
Quantifying carbon fluxes and understanding their variation characteristics are of great interest for estimating the carbon fixation potential of kiwifruit ecosystems and comprehend the carbon cycle. The diurnal GPP, NEE, and R\({}_{\rm e}\) followed a unimodal pattern,  
Figure 7: Path diagrams of biophysical factors on carbon fluxes (GPP, gross primary productivity; R\({}_{\rm e}\), ecosystem respiration; NEE, net ecosystem exchange) during the **(a)** vigorous growth period and **(b)** non-vigorous growth period. The biophysical factors include the vapor pressure deficit (VPD), air carbon dioxide concentration (CCO\({}_{2}\)), atmospheric water vapor concentration (CH\({}_{2}\)O), air temperature (T\({}_{\rm a}\)), air relative humidity (RH), photosynthetically active radiation (PAR), soil temperature (T\({}_{\rm s}\)) at 5 cm depth, soil water content (SWC) at 20 cm depth, and leaf area index (LAI). The solid red lines denote the direct paths, the blue dashed lines denote the indirect paths, the values on the arrows denote the path coefficients, and the black dashed lines represent the absence of a pathway. Note: \({}^{\star}\) and \({}^{\star\star}\) represent a significance level of \(p<\) 0.05 and \(p<\) 0.01, respectively. with their peaks occurring at 12:30-15:30, coinciding with the daily maxima of solar radiation and temperature. The carbon sequestration capacity of kiwifruit ecosystems was lower during the early and late growth stages and peaked during the vigorous growth period due to the canopy development and favorable meteorological conditions conducive to growth. The peak monthly average daily NEE was \(-4.61\) g C m\({}^{-2}\) d\({}^{-1}\) and \(-2.96\) g C m\({}^{-2}\) d\({}^{-1}\) in summer (June/August), which suggests that suitable radiation and hydrothermal conditions are beneficial for carbon capture (Table 2 and Figure 2). The seasonal variations patterns were similar to the previous studies conducted in a multi-ecosystem (Table 3). The total NEE measured at our study site in 2018 and 2019 was \(-444.25\) and \(-285.77\) g C m\({}^{-2}\) yr\({}^{-1}\), with an efficiency ratio (defined as the ratio of CO\({}_{2}\) sequestration to CO\({}_{2}\) emissions) of 1.36 and 1.23, respectively, indicating that the kiwifruit orchard functioned as a carbon sink at a moderate carbon sequestration efficiency during the growing seasons. The efficiency ratio of a 10-year-old kiwifruit orchard ecosystem in Italy was 1.15 and the NEE in 2012 was \(-493.30\) g C m\({}^{-2}\) yr\({}^{-1}\), which was consistent with ours, despite the two research projects being conducted under different climatic conditions, soil characteristics, and management practices [44]. As shown in Table 3, the comparison analysis showed that the carbon absorption capability of kiwifruit ecosystems was relatively lower compared to other perennial fruit tree ecosystems, such as apple, pear, vineyard, citrus, and peach ecosystems [15; 45; 46; 47; 48]. This may be attributed to differences in the photosynthetic capacity among various fruit tree species and the shedding of kiwifruit leaves during the non-growing season. For other agricultural ecosystems, the annual NEE of winter wheat and agroforest ecosystems on the Loess Plateau in southern China were \(-437.35\) and \(-545.42\) g C m\({}^{-2}\) yr\({}^{-1}\), respectively, which were slightly higher than our findings [49]. Additionally, based on a yield of 1000 g m\({}^{-2}\) and a carbon content of 9% in the fruit dry matter, we estimated the carbon stored in the fruit to be about 90 g C m\({}^{-2}\). It can be inferred that the carbon stored in the soil and trees accounted for 69-80%. The lack of soil carbon monitoring limits our understanding of the below-ground carbon storage. Future studies should include soil organic carbon, woody biomass, and continuous carbon flux measurements to better characterize lifecycle dynamics and source-sink transitions. Optimizing agricultural practices could further enhance the orchard's carbon sink potential.