Carbonyl sulfide (COS) as a tracer for canopy photosynthesis, transpiration and stomatal conductance: potential and limitations.

Georg Wohlfahrt,Lukas Hörtnagl,Federico Brilli,Heinz Bingemer,Francesco Loreto,Armin Hansel,Xiaobin Xu

doi:10.1111/j.1365-3040.2011.02451.x

Abstract

The theoretical basis for the link between the leaf exchange of carbonyl sulfide (COS), carbon dioxide (CO2) and water vapour (H2O) and the assumptions that need to be made in order to use COS as a tracer for canopy net photosynthesis, transpiration and stomatal conductance, are reviewed. The ratios of COS to CO2 and H2O deposition velocities used to this end are shown to vary with the ratio of the internal to ambient CO2 and H2O mole fractions and the relative limitations by boundary layer, stomatal and internal conductance for COS. It is suggested that these deposition velocity ratios exhibit considerable variability, a finding that challenges current parameterizations, which treat these as vegetation-specific constants. COS is shown to represent a better tracer for CO2 than H2O. Using COS as a tracer for stomatal conductance is hampered by our present poor understanding of the leaf internal conductance to COS. Estimating canopy level CO2 and H2O fluxes requires disentangling leaf COS exchange from other ecosystem sources/sinks of COS. We conclude that future priorities for COS research should be to improve the quantitative understanding of the variability in the ratios of COS to CO2 and H2O deposition velocities and the controlling factors, and to develop operational methods for disentangling ecosystem COS exchange into contributions by leaves and other sources/sinks. To this end, integrated studies, which concurrently quantify the ecosystem-scale CO2, H2O and COS exchange and the corresponding component fluxes, are urgently needed.We investigate the potential of carbonyl sulfide (COS) for being used as a tracer for canopy net photosynthesis, transpiration and stomatal conductance by examining the theoretical basis of the link between leaf COS, carbon dioxide (CO2) and water vapour (H2O) exchange. Our analysis identifies several limitations that need to be overcome to this end, however at present we lack appropriate ecosystem-scale field measurements for assessing their practical significance. It however appears that COS represents a better tracer for CO2 than H2O. Concurrent measurements of ecosystem scale COS, CO2 and H2O exchange are advocated.

Highlights

IntroductionTranspiration and stomatal conductance are key conceptual terms in most contemporary models of ecosystem carbon and water cycling (Sitch et al 2008)
Canopy net photosynthesis, transpiration and stomatal conductance are key conceptual terms in most contemporary models of ecosystem carbon and water cycling (Sitch et al 2008)
Our study confirmed previous pioneering work highlighting the potential of carbonyl sulfide (COS) as a tracer for canopy net photosynthesis; we have indicated a number of limitations

Summary

Introduction

Transpiration and stomatal conductance are key conceptual terms in most contemporary models of ecosystem carbon and water cycling (Sitch et al 2008). Ecosystem enclosures yield the net ecosystem CO2 and H2O exchange, that is, they are unable to partition between canopy net photosynthesis/transpiration and CO2/H2O fluxes from/to the soil and other ecosystem components. This drawback holds for micrometeorological techniques such as the eddy covariance method, which, has the advantage of being unobtrusive and able to provide near-continuous long-term flux data (Baldocchi, Hicks & Meyers 1988; Baldocchi 2003). Available estimates of canopy net photosynthesis, transpiration and stomatal conductance are inherently uncertain, which in turn translates into uncertain model parameterizations and predictions

Objectives

Findings

Conclusion