Direct and indirect effects of environmental factors on daily CO2 exchange in a rainfed maize cropland—A SEM analysis with 10 year observations

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The carbon budget of agricultural ecosystems is of great importance to the global carbon balance. In this study, the structural equation modeling (SEM) method was employed to quantify the direct and indirect effects of environmental factors on daily net ecosystem CO2 exchange (NEE) in a rainfed maize cropland, northeast China. The results showed that net radiation (Rn) was the most important factor controlling daily NEE, followed by the leaf area index (LAI), air temperature (Ta), vapor pressure deficit (VPD), and clearness index (Kt). The strong effect of Rn was mainly attributed to its direct effect, while Ta and VPD showed comparable or even higher indirect effects than direct effects, indicating that Ta and VPD influenced NEE mainly through regulating canopy development on a daily scale. Moreover, the responses of NEE to individual environmental factors differed greatly among multiyear climatic conditions. Ta and VPD were shown to be more important in warm (WM) years and warm and dry (WD) years than in normal (NM) years and cold and wet (CW) years. LAI was the primary controlling factor of daily NEE in WD years, resulting in large indirect effects of Ta and VPD on NEE. Kt had a large effect on daily NEE only in CW years. SWC had significant effects on daily NEE in WM and WD years but in the opposite direction, i.e., daily NEE increased with SWC in WD years but decreased with SWC in CW years. The partitioning of direct and indirect effects of environmental factors with SEM can greatly enhance the understanding of the controlling mechanism of NEE and remind researchers to consider the distinct effects of environmental factors on NEE among multiyear climatic conditions in future models.