Analysis of changes in water-heat-carbon fluxes at different time scales and their influencing factors in the cold black soil region of Northeast China

The changing characteristics of water-heat-carbon fluxes and their relationship with environmental influencing factors are crucial for understanding food security and achieving 'carbon peak' and 'carbon neutrality' in a changing climate. Utilizing the measured data from the study area, this study incorporated temperature coefficients to improve the Lloyd-Taylor model. Subsequently, the patterns of change in water-heat-carbon fluxes and environmental factors were analyzed across different time scales. The partial least squares structural equation model (PLS-SEM) was employed to calculate the direct and indirect effects of water-heat-carbon fluxes and environmental factors on one another. The findings indicate the following: (1) During the crop growth periods in the maize growing area, the daily mean value of net ecosystem carbon exchange (NEE) was measured at −1.46 μmol/m²/s, signifying a significant carbon sink. In contrast, during the freezing-thawing periods, the daily mean value of NEE was recorded at 0.23 μmol/m²/s, indicating a marginal carbon source. Overall, the annual carbon balance resulted in a carbon sink of 242.2 gC/m²/y, categorizing the entire ecosystem as a carbon sink. (2) Sensible heat flux (H) is negatively correlated with latent heat flux (LE) and evapotranspiration (ET) during the reproductive period at both daily and monthly scales but positively correlated with ET during other time scales. Environmental factors at the full-hourly-scale indirectly influenced ET by directly affecting H. (3) Air temperature (Ta), soil surface temperature (Ts), and the difference in saturated water vapor pressure (VPD) consistently demonstrated significant positive effects on ecosystem respiration (Reco), while Rn consistently exhibited substantial positive effects on ecosystem gross primary production (GPP). (4) Reco consistently exerted a significant direct positive effect on NEE, while GPP consistently demonstrated a significant direct negative effect on NEE. Environmental factors substantially impacted the carbon flux components, indirectly influencing farmland ecosystems' carbon sequestration capacity in the cold black soil region. These findings enhance our understanding of how water-heat-carbon fluxes respond to environmental factors and can provide theoretical support for coupled hydrothermal carbon modeling. This knowledge is crucial for assessing and predicting the responses of cropland ecosystems in the cold black soil Region to ongoing climate change.