Have ozone effects on carbon sequestration been overestimated? A new biomass response function for wheat

H Pleijel,D Simpson,G Mills,H Danielsson

doi:10.5194/bg-11-4521-2014

Abstract

Abstract. Elevated levels of tropospheric ozone can significantly impair the growth of crops. The reduced removal of CO2 by plants leads to higher atmospheric concentrations of CO2, enhancing radiative forcing. Ozone effects on economic yield, e.g. the grain yield of wheat (Triticum aestivum L.), are currently used to model effects on radiative forcing. However, changes in grain yield do not necessarily reflect changes in total biomass. Based on an analysis of 22 ozone exposure experiments with field-grown wheat, we investigated whether the use of effects on grain yield as a proxy for effects on biomass under- or overestimates effects on biomass. First, we confirmed that effects on partitioning and biomass loss are both of significant importance for wheat yield loss. Then we derived ozone dose response functions for biomass loss and for harvest index (the proportion of above-ground biomass converted to grain) based on 12 experiments and recently developed ozone uptake modelling for wheat. Finally, we used a European-scale chemical transport model (EMEP MSC-West) to assess the effect of ozone on biomass (−9%) and grain yield (−14%) loss over Europe. Based on yield data per grid square, we estimated above-ground biomass losses due to ozone in 2000 in Europe, totalling 22.2 million tonnes. Incorrectly applying the grain yield response function to model effects on biomass instead of the biomass response function of this paper would have indicated total above-ground biomass losses totalling 38.1 million (i.e. overestimating effects by 15.9 million tonnes). A key conclusion from our study is that future assessments of ozone-induced loss of agroecosystem carbon storage should use response functions for biomass, such as that provided in this paper, not grain yield, to avoid overestimation of the indirect radiative forcing from ozone effects on crop biomass accumulation.

Highlights

Tropospheric ozone is well known to significantly impair the growth of a wide variety of plants (Ainsworth et al, 2012), including forest trees, semi-natural vegetation and crops, e.g. ozone-sensitive species such as wheat (Feng and Kobayashi, 2009)
A key conclusion from our study is that future assessments of ozone-induced loss of agroecosystem carbon storage should use response functions for biomass, such as that provided in this paper, not grain yield, to avoid overestimation of the indirect radiative forcing from ozone effects on crop biomass accumulation
By reducing plant photosynthesis and growth, elevated tropospheric ozone will result in decreased carbon storage in vegetation and in an indirect radiative forcing as a consequence of the CO2 that remains in the atmosphere due to impaired ecosystem carbon storage (Sitch et al, 2007; Collins et al, 2010; Ainsworth et al, 2012)

Summary

Introduction

Tropospheric ozone is well known to significantly impair the growth of a wide variety of plants (Ainsworth et al, 2012), including forest trees, semi-natural vegetation and crops, e.g. ozone-sensitive species such as wheat (Feng and Kobayashi, 2009). The reduction in net photosynthesis and biomass accumulation caused by ozone is inevitably linked to a decreased uptake of CO2 and storage of organic carbon, leading to an enhanced radiative forcing (Ainsworth et al, 2012). Assessments of this indirect contribution to global warming by elevated tropospheric ozone have incorrectly assumed that the grain yield of wheat would represent an estimate of the biomass effect of an ozone-sensitive crop represented by wheat (Sitch et al, 2007; Collins et al, 2010). This neglects the fact that a large fraction of the ozone effect on economic yield, e.g. grain yield in wheat, depends

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