Variation and Controlling Factors of Carbon Flux over a Humid Region Kiwifruit Orchard in Southwest China

Investigating the carbon flux in orchard ecosystems is crucial for assessing agroecosystem productivity and optimizing management practices. We measured and estimated carbon fluxes (gross primary productivity, GPP; ecosystem respiration, R e ; and net ecosystem exchange, NEE) and environmental variables in a seven-year-old kiwifruit orchard over two years. Our results showed that diurnal carbon fluxes exhibited bell-shaped patterns, peaking between 12:30 and 15:30. Daily carbon fluxes exhibited a seasonal trend, characterized by an increase followed by a decrease. The average daily GPP, R e , and NEE values were 6.77, 4.99, and −1.79 g C m −2 d −1 in 2018, and 5.88, 4.78, and −1.10 g C m −2 d −1 in 2019, respectively. The orchard sequestered −444.25 g C m −2 in 2018 and −285.77 g C m −2 in 2019, which accounted for 26.4% and 18.6% of GPP, respectively. Diurnal GPP and NEE were significantly influenced by photosynthetically active radiation (PAR), with direct path coefficients of 0.75 and 0.88 ( p < 0.01), while air temperature (T a ) significantly affected GPP and NEE through PAR, with an indirect path coefficient of 1.12 for both. PAR had a similar effect on daily GPP and NEE, while both were indirectly influenced by soil temperature (T s ) at a 5 cm depth and vapor pressure deficit (VPD). R e was primarily impacted by VPD, with a direct path coefficient of 0.64 ( p < 0.01), while T a and the concentration of air carbon dioxide (CCO 2 ) significantly affected GPP through VPD, with indirect path coefficients of 0.82 and −0.80. The leaf area index (LAI) and soil water content (SWC) at a 20 cm depth exhibited a significant correlation with carbon fluxes during the vigorous growing period.