Under a new light: validation of eddy covariance flux with light response functions of assimilation and estimates of heterotrophic soil respiration

Abstract. Estimation of the basal or heterotrophic soil respiration is crucial for determination of whether an ecosystem is emitting or sequestering carbon. A severe bushfire in January 2014 at the Calperum flux tower, operational since August 2010, provided variation in ecosystem respiration and leaf area index as the ecosystem recovered. We propose ecosystem respiration is a function of leaf area index and the y-intercept is an estimate of heterotrophic soil respiration. We calculated an assimilation rate from eddy covariance data for light response functions to calculate ecosystem respiration incorporating suppression of the daytime autotrophic respiration. Ecosystem respiration from light response functions correlated with data processing calculations of ecosystem respiration by OzFluxQC (y0 = 0.161x + 0.0085; Adj. r² = 0.698). The relationship between ecosystem respiration and leaf area index (y0 = 1.43x +0.398; Adj. r² = 0.395) was also apparent. When this approach was compared to field measurements of soil respiration and mass balance calculations from destructive leaf area, leaf area index calculations and litter fall, the year of data corresponding to the year of soil respiration measurements, the y-intercept was 0.432 µmol m⁻² s⁻¹ or 163.44 gC m⁻² year⁻¹ (y0 = 1.37x + 0.432, Adj. r² = 0.325). The mass balance approach for the net primary productivity when subtracted from the tower NEE estimated heterotrophic soil respiration of 134.59 gC m⁻² year⁻¹. This is only 28.9 gC different, therefore the y-intercept approach indeed provides an estimate of heterotrophic soil respiration.