[Carbon budget estimation based on different methods of CO₂ storage flux in forest ecosystems]

Accurate measurement of CO₂ storage flux (F_s) in forest ecosystems is of great significance for estimating ecosystem carbon budget by eddy covariance (EC). The errors in the estimation of ecosystem carbon budget caused by different methods for calculating F_s has yet not been comprehensively assessed. Using data from an open-path EC system and an eight-level CO₂/H₂O profile system (AP100, Campbell Scientific Inc., USA) in a broadleaved deciduous forest at the Maoer-shan in 2018, we evaluated the methodological effect of F_s[2-min mean profile (P_{2 min}), 30-min mean profile (P_{30 min}) and 30-min mean EC single point (P_s)] on the estimation of net ecosystem exchange (NEE), ecosystem respiration (R_e), and gross primary productivity (GPP). The results showed that the impact of F_s methods on forest carbon flux generally increased with the increases of time scale, indicating that gap-filling of flux data would further amplify the impacts of F_s estimation methods. At the annual scale, NEE based on P_{2 min} and P_s methods were 36.3% and 29.4% lower than that based on P_{30 min}, while R_e based on P_{2 min} was higher than that based on P_{30 min} and P_s by 8.7%. The GPP based on P_{2 min} was 5.4% higher, while that based on P_s was 2.1% lower than that based on P_{30 min}. The traditional P_{30 min} ignored the instantaneous changes in CO₂ concentration, P_s missed the changes of CO₂ concentration within canopy, and thus both underestimated the actual R_e. The approximately instantaneous profile (2-min mean profile) had higher temporal and spatial resolution and could more accurately estimate forest carbon budget with non-flat terrain and complex canopy structure. Our findings had great implications for solving the underestimation of forest R_e and GPP as well as the overestimation of net carbon sink on complex conditions with the EC method.