Revisiting the choice of the driving temperature for eddy covariance CO2 flux partitioning

Georg Wohlfahrt,Marta Galvagno

doi:10.1016/j.agrformet.2017.02.012

Abstract

So-called CO2 flux partitioning algorithms are widely used to partition the net ecosystem CO2 exchange into the two component fluxes, gross primary productivity and ecosystem respiration. Common CO2 flux partitioning algorithms conceptualise ecosystem respiration to originate from a single source, requiring the choice of a corresponding driving temperature. Using a conceptual dual-source respiration model, consisting of an above- and a below-ground respiration source each driven by a corresponding temperature, we demonstrate that the typical phase shift between air and soil temperature gives rise to a hysteresis relationship between ecosystem respiration and temperature. The hysteresis proceeds in a clockwise fashion if soil temperature is used to drive ecosystem respiration, while a counter-clockwise response is observed when ecosystem respiration is related to air temperature. As a consequence, nighttime ecosystem respiration is smaller than daytime ecosystem respiration when referenced to soil temperature, while the reverse is true for air temperature. We confirm these qualitative modelling results using measurements of day and night ecosystem respiration made with opaque chambers in a short-statured mountain grassland. Inferring daytime from nighttime ecosystem respiration or vice versa, as attempted by CO2 flux partitioning algorithms, using a single-source respiration model is thus an oversimplification resulting in biased estimates of ecosystem respiration. We discuss the likely magnitude of the bias, options for minimizing it and conclude by emphasizing that the systematic uncertainty of gross primary productivity and ecosystem respiration inferred through CO2 flux partitioning needs to be better quantified and reported.

Highlights

Gross primary productivity (GPP; for a recent discussion on the definition of this term see Wohlfahrt and Gu, 2015) and ecosystem respiration (Reco) are key concepts and terms in carbon cycle science (Chapin et al, 2006) and their magnitude determines the sign of the net Wohlfahrt and Galvagno ecosystem CO2 exchange, i.e. NEE = GPP + Reco
Soil and above-ground respiration strictly follow the diurnal course of the respective driving temperatures (Fig. 1a)
Based on simulations with a conceptual dual-source ecosystem respiration model, we have conclusively shown that the phase shift between the air and soil temperatures driving, respectively, above-ground and below-ground respiration, causes a hysteresis in the response of Reco to each of these driving temperatures (Fig. 1)

Summary

Introduction

CO2 flux partitioning algorithms exploit, in one way or the other, the contrasting sign of nighttime (positive – net CO2 release) and daytime (negative – net CO2 uptake) NEE. The nighttime approach put forward by Reichstein et al (2005) uses nighttime NEE measurements to parametrise a temperature-dependent model of Reco. GPP is inferred by extrapolating Reco to daytime temperatures and by subtracting the latter term from NEE. The daytime approach by Lasslop et al (2010) uses nighttime NEE measurements to parameterise the temperature sensitivity of Reco, but uses a light- and temperaturedriven model to infer both GPP and Reco from daytime data only

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Conclusion