Abstract
Gross primary productivity (GPP), the gross uptake of carbon dioxide (CO2) by plant photosynthesis, is the primary driver of the land carbon sink, which presently removes around one quarter of the anthropogenic CO2 emissions each year. GPP, however, cannot be measured directly and the resulting uncertainty undermines our ability to project the magnitude of the future land carbon sink. Carbonyl sulfide (COS) has been proposed as an independent proxy for GPP as it diffuses into leaves in a fashion very similar to CO2, but in contrast to the latter is generally not emitted. Here we use concurrent ecosystem‐scale flux measurements of CO2 and COS at four European biomes for a joint constraint on CO2 flux partitioning. The resulting GPP estimates generally agree with classical approaches relying exclusively on CO2 fluxes but indicate a systematic underestimation under low light conditions, demonstrating the importance of using multiple approaches for constraining present‐day GPP.
Highlights
The net exchange of CO2 between an ecosystem and the atmosphere consists of two major components of opposite direction, gross primary productivity (GPP), and ecosystem respiration (Reco)
For the contemporary carbon cycle, the single most important source for Gross primary productivity (GPP) estimates has been NEE measurements by means of the eddy covariance (EC) technique from which GPP as well as Reco are inferred in a standardized fashion by applying so‐called flux partitioning (FP) models, which exploit the fact that GPP is zero during nighttime and/or depends on solar irradiation during daytime
Our study is the first to overcome these issues by treating the leaf relative uptake rate (LRU) as an adjustable parameter, which is jointly optimized against both CO2 and Carbonyl sulfide (COS) flux measurements, and explicitly accounts for the soil COS exchange
Summary
The net exchange of CO2 between an ecosystem and the atmosphere (net ecosystem exchange, NEE) consists of two major components of opposite direction, gross primary productivity (GPP), and ecosystem respiration (Reco). For the contemporary carbon cycle, the single most important source for GPP estimates has been NEE measurements by means of the eddy covariance (EC) technique from which GPP as well as Reco are inferred in a standardized fashion by applying so‐called flux partitioning (FP) models (see section 2; Beer et al, 2010; Lasslop et al, 2010; Mahecha et al, 2010; Papale et al, 2006), which exploit the fact that GPP is zero during nighttime and/or depends on solar irradiation during daytime. In contrast to CO2, whose uptake is always accompanied by release through mitochondrial respiration, the uptake of COS is a one‐way flux (but see Gimeno et al, 2017), opening the opportunity to infer GPP at leaf and canopy scale as (Sandoval‐ Soto et al, 2005): GPP 1⁄4 ðFCOS χCO2Þ=ðχCOSLRUÞ (1)
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