• Energy Research

AbstractThe rising concentration of atmospheric carbon dioxide (CO2) is known to increase the total aboveground biomass of several C3 crops, whereas C4 crops are reported to be hardly affected when water supply is sufficient. However, a free‐air carbon enrichment (FACE) experiment in Braunschweig, Germany, in 2007 and 2008 resulted in a 25% increased biomass of the C4 crop maize under restricted water conditions and elevated CO2 (550 ppm). To project future yields of maize under climate change, an accurate representation of the effects of eCO2 and drought on biomass and soil water conditions is essential. Current crop growth models reveal limitations in simulations of maize biomass under eCO2 and limited water supply. We use the coupled process‐based hydrological‐plant growth model Catchment Modeling Framework‐Plant growth Modeling Framework to overcome this limitation. We apply the coupled model to the maize‐based FACE experiment in Braunschweig that provides robust data for the investigation of combined CO2 and drought effects. We approve hypothesis I that CO2 enrichment has a small direct‐fertilizing effect with regard to the total aboveground biomass of maize and hypothesis II that CO2 enrichment decreases water stress and leads to higher yields of maize under restricted water conditions. Hypothesis III could partly be approved showing that CO2 enrichment decreases the transpiration of maize, but does not raise soil moisture, while increasing evaporation. We emphasize the importance of plant‐specific CO2 response factors derived by use of comprehensive FACE data. By now, only one FACE experiment on maize is accomplished applying different water levels. For the rigorous testing of plant growth models and their applicability in climate change studies, we call for datasets that go beyond single criteria (only yield response) and single effects (only elevated CO2).

This study assesses the ability of 21 crop models to capture the impact of elevated CO2 concentration ([CO2]) on maize yield and water use as measured in a 2-year Free Air Carbon dioxide Enrichment experiment conducted at the Thunen Institute in Braunschweig, Germany (Manderscheid et al., 2014). Data for ambient [CO2] and irrigated treatments were provided to the 21 models for calibrating plant traits, including weather, soil and management data as well as yield, grain number, above ground biomass, leaf area index, nitrogen concentration in biomass and grain, water use and soil water content. Models differed in their representation of carbon assimilation and evapotranspiration processes. The models reproduced the absence of yield response to elevated [CO2] under well-watered conditions, as well as the impact of water deficit at ambient [CO2], with 50% of models within a range of +/−1 Mg ha−1 around the mean. The bias of the median of the 21 models was less than 1 Mg ha−1. However under water deficit in one of the two years, the models captured only 30% of the exceptionally high [CO2] enhancement on yield observed. Furthermore the ensemble of models was unable to simulate the very low soil water content at anthesis and the increase of soil water and grain number brought about by the elevated [CO2] under dry conditions. Overall, we found models with explicit stomatal control on transpiration tended to perform better. Our results highlight the need for model improvement with respect to simulating transpirational water use and its impact on water status during the kernel-set phase.

Climate scenarios show that atmospheric CO2 concentrations will continue to increase. As a consequence, more frequent and severe drought periods are expected. Drought will thus modify plant growth. Although maize is a major crop globally, little information is available on how atmospheric and climatic changes will change maize quality. Here, in a field experiment, maize was grown in 2007 and 2008 under ambient (380 ppm) and elevated CO2 (550 ppm) using free-air CO2 enrichment. In 2007, maize was grown under well-watered conditions only. In 2008, we applied a drought stress treatment in which the plants received only half the amount of water of the well-watered treatment. We measured the concentrations of minerals and quality-related traits in aboveground biomass and kernels at the end of each growing season. Results show first the absence of effect of elevated CO2 under well-watered conditions. By contrast, drought stress modified several traits and interactions under elevated CO2. These results support the hypothesis that the C4 plant maize does not react to an increase in atmospheric CO2 as long as no drought stress is prominent. This finding contrasts with the impact of elevated CO2 on C3 plants. Several drought stress effects found in our study will have important implications for food and feed use. However, the effects of drought stress on the traits were less pronounced under elevated CO2 than under ambient CO2 level. Hence, an elevated CO2 concentration mitigates the drought stress impacts on elemental composition and quality traits of maize.

ABSTRACTA rising global population and demand for protein‐rich diets are increasing pressure to maximize agricultural productivity. Rising atmospheric [CO2] is altering global temperature and precipitation patterns, which challenges agricultural productivity. While rising [CO2] provides a unique opportunity to increase the productivity of C3 crops, average yield stimulation observed to date is well below potential gains. Thus, there is room for improving productivity. However, only a fraction of available germplasm of crops has been tested for CO2 responsiveness. Yield is a complex phenotypic trait determined by the interactions of a genotype with the environment. Selection of promising genotypes and characterization of response mechanisms will only be effective if crop improvement and systems biology approaches are closely linked to production environments, that is, on the farm within major growing regions. Free air CO2 enrichment (FACE) experiments can provide the platform upon which to conduct genetic screening and elucidate the inheritance and mechanisms that underlie genotypic differences in productivity under elevated [CO2]. We propose a new generation of large‐scale, low‐cost per unit area FACE experiments to identify the most CO2‐responsive genotypes and provide starting lines for future breeding programmes. This is necessary if we are to realize the potential for yield gains in the future.