Flux partitioning

NEE is the net ecosystem exchange (it has a negative value in this context, representing net CO₂ fixation by the ecosystem). ER includes both autotrophic and heterotrophic respiration [9]. Autotrophic respiration includes the respiration of both the underground and aerial parts of the maize plant, and heterotrophic respiration refers to the respiration of the soil organisms. GPP represents the amount of CO₂ assimilated by the maize plants during photosynthesis, the GPP value is equal to the difference between ER and NEE.

Crops do not conduct photosynthesis at night, i.e., GPP = 0. Therefore, the NEE measured by the eddy covariance system at night is the ER of the farmland ecosystem [8]. Once the relationship between the nighttime NEE and surface temperature (T_s) was established, the daytime ER was obtained by plugging the daytime T_s data into the equation. The most commonly used method is to use the respiratory model to interpolate the missing data. We used the van’t Hoff model to simulate the nighttime CO₂ flux of the maize with drip irrigation under film mulch as Eq. (2).

The difference between the NEE and the calculated ER is GPP.