Light saturation and temperature jointly dominate the diurnal variation of net ecosystem exchange in grassland ecosystems

Dynamic changes in daily climatic features may induce a lag in the responses of ecosystems, representing a crucial mechanism in the climate-ecosystem interactions. However, the underlying mechanisms are not fully understood. With help of long term half-hourly data series (from 2006 to 2022) of carbon fluxes and climate from the Inner Mongolia no-grazing grassland ecosystem eddy covariance station, we conducted the following studies: (i) investigation of the lagged mechanisms between carbon flux and climatic factors, (ii) characterization of diurnal variation patterns that demonstrated the hysteresis between carbon flux and climatic factors, and (iii) assessment of the potential mechanisms underlying the diurnal variation in carbon flux. The results indicated that (i) there is a pronounced asynchrony between the carbon flux and climatic factors. Light saturation is crucial in the lagged relationship between net ecosystem exchange (NEE) and climatic factors. The lag times between NEE and climatic factors were separately explained by moisture and temperature conditions. (ii) Non-linear response relationships were observed between carbon flux and climatic factors. Under similar climatic conditions, the increasing stage of NEE consistently remained lower than the decreasing stage, presenting a distinctive hysteresis loop. (iii) Further investigations revealed the collaborative role of gross primary productivity (GPP) and ecosystem respiration (RECO) in the diurnal variation patterns of NEE. Specifically, the difference in light-use efficiency between the increasing and decreasing stages dominated the difference in GPP for both stages, whereas the difference in air temperature between the increasing and decreasing stages dominated the difference in RECO for both stages. The diurnal variation patterns of NEE were similar across the different parts of growing season. Results furthermore allowed to optimize the light-ues efficiency model, which exhibited remarkable accuracy in daily GPP estimation and effectively simulated the diurnal variation patterns between GPP and climatic factors. In summary, this study provides a theoretical basis for understanding lagged effects and diurnal variations in grassland ecosystems in response to climatic factors. These research findings are of great importance for predicting the future ecosystem carbon sink potential.