Increasing crop yield and resource use efficiency via root-zone CO2 enrichment

Summary: CO2 enrichment of the aerial environment is widely used to increase yields of protected crops. The annual cost of CO2 production (which can be as high as £200,000 for a 5 Ha glasshouse) together with an industry-wide drive to reduce agricultural greenhouse gas emissions (to which CO2 contributes 9%) requires urgent improvements to the efficiency of resource use, the cost-effectiveness, and the environmental performance of these production systems. A specific problem of current practice is that the high humidity in the greenhouse environment requires frequent ventilation to prevent the occurrence of foliar diseases, yet this results in CO2 losses from the greenhouse to the atmosphere. This studentship will adopt a multi-scale approach to investigate the potential for using localised enrichment of the plant root-zone with low concentrations of CO2 as an alternative to bulk CO2 enrichment of the crop’s aerial environment. Small–scale pilot studies will assess the viability of different approaches for root-zone CO2 enrichment. Glasshouse studies will examine the physiological mechanisms by which plants respond to root-zone CO2 and how these responses can be optimized to maximize crop growth and economic productivity. In large-scale studies conducted in a commercial setting, current best practice in aerial CO2 enrichment will be directly compared with root-zone CO2 enrichment under comparable conditions. A full cost-benefit analysis will establish the potential of root-zone CO2 enrichment to reduce the cost and environmental impact, and improve the productivity, of protected crops.

Aims and Objectives: This studentship addresses key research and development priorities of HDC’s Protected Edibles panel, which identifies four key objectives for future profit enhancing research and development in the protected edibles sector. These include:

• Increasing productivity through resource efficiency.
This continues to be a top priority in a sector where efficiencies are required throughout the production system (e.g. inputs of water and CO2) to keep pace with competition.
• Improving environmental performance.

The protected crops sector needs a dynamic approach to the challenges of emissions reduction in the face of legislative pressures to reduce greenhouse gas (GHG) emissions. Specifically, the government’s low carbon transition plan will affect all of agriculture and growers will need assistance from research into the development of sustainable agricultural systems to meet the requirements of the industry-led voluntary action plan.
The goal to increase global agriculture production by up to 70% by 2050 appears increasing challenging in an environment where resources are becoming limited and in the face of increasing societal and legislative pressures to lessen the impacts of agriculture on the environment. CO2 enrichment of the aerial environment (typically to 1000-2000 ppm) is widely used to increase the yield of protected crops, but the costs are considerable. The annual cost for a 5 Ha glasshouse of CO2 (17,580 tonne) produced from natural gas where heat is not required is approx. £200,000 (HDC 2011; http://www.hdc.org.uk/sites/default/files/research_papers/PE%20003%20Final%202011_0.pdf). In addition, there is a drive to reduce GHG emissions from UK agriculture, of which 9% is from CO2 (Defra 2012; https://www.gov.uk/government/publications/2012-review-of-progress-in-reducing-greenhouse-gas-emissions-from-english-agriculture). This project will adopt a multi-scale approach to investigate the potential for using localised enrichment of discrete zones within the growing environment with CO2 as an alternative to large-scale CO2 enrichment of the aerial environment, thereby contributing to a reduction in both costs of CO2 and agricultural GHG emissions. As such, this studentship sits centrally within Objective 2 of the Protected Edibles Panel’s research and development priorities “Improving the cost-effectiveness and sustainability of production”, Target 1 “Increasing efficient use of resources”, Initiative C “To maximise the efficient use of all growing inputs – CO2 (production & usage)”.
The plant root-zone may be ideal for the localised enrichment of the growing environment with CO2. Previous studies have highlighted the potential of this approach for increasing crop growth and productivity. Elevated rhizosphere dissolved inorganic carbon (DIC) increased the biomass of hydroponically-grown tomato by up to 200% (Cramer et al 1999 JExpBot 50:79-87) whilst increasing the concentration of CO2 ([CO2]) in the root-zone to supra-optimal levels benefits plant growth and productivity. Enriching the [CO2] to 45% (45,000 ppm) in the root-zone of hydroponically-grown potato substantially increased tuberization, and increased stolon length, number of tubers per stolon, and overall dry weight (Arteca et al 1979 Science 205:1279-80). CO2 enrichment (2,000-50,000 ppm) of the root-zone of aeroponically-grown lettuce resulted in a 15-31% increase in fresh weight and a ~1.6-fold and~1.8-fold increase in shoot and root dry weight, respectively (He et al 2010 JExpBot 61:3959-3969). However, to our knowledge, the effects of enriching the root-zone to the optimal [CO2] currently used for the large-scale CO2 enrichment of the aerial environment (or to levels considered sub-optimal with respect to the aerial environment) has not been considered.
Our proof-of-concept study determined whether enriching the root-zone with low (≤1,500 ppm) [CO2] affected plant growth and productivity. Fig.1 (data collected within HDC Studentship PO17) shows that elevating root-zone [CO2] by ~700 ppm (relative to control values) significantly increased shoot fresh weight (~10%) of drip-irrigated semi-hydroponically-grown lettuce – plants were significantly heavier (15.6±6.6 g more) due to an increase (~7%) in dry weight.
This studentship will apply localised root-zone CO2 enrichment to protected crops with the aim of improving the efficiency of resource use, the cost-effectiveness, and the environmental performance of production systems. As such, it compliments current HDC-funded research (PE021) examining the relationship between atmospheric CO2 management and crop yield and quality. The project will:
• Compare the effects of DIC and root-zone CO2 enrichment on aeroponic- and hydroponic-grown plants, and direct CO2 enrichment of the root-zone of soil-grown plants, on plant growth and yield.
• Determine the optimum [CO2] range for root-zone CO2 enrichment.
• Determine the physiological mechanisms by which plants respond to root-zone [CO2].
• Examine how plant responses to root-zone [CO2] can be manipulated to optimize productivity gains.
• Analyze the costs and benefits of root-zone CO2 enrichment relative to current best practice for CO2 enrichment of the aerial environment.
Access to the commercial facilities required for larger-scale trials of root-zone CO2 enrichment will be provided through a collaboration between the Lancaster Environment Centre (LEC) and Cornerways Nursery. The project will also seek to collaborate more widely with a range of industry partners on larger-scale trials through the Tomato Growers Association (TGA) and the TGA’s technical committee. This collaboration(s) will, in addition to supporting an essential part of the study, provide important industry-focused training for the student and accelerate knowledge transfer to the industry.
Studies will focus on three crops: lettuce, tomato and pepper. Experiments will be performed across three different scales: (1) small–scale studies assessing different approaches for root-zone CO2 enrichment, (2) glasshouse studies examining how plants respond to root-zone CO2 enrichment, and (3) larger-scale trials of root-zone CO2 enrichment. The latter will occur in a commercial setting to compare the costs and benefits of this approach relative to current best practice of aerial CO2 enrichment whilst (1) and (2) will occur at LEC:
(1) The suitability of root-zone CO2 enrichment within aeroponic, hydroponic and drip-irrigated semi-hydroponic production systems will be assessed in small-scale experiments through enrichment of the airspace, aeration of the fertigation media with CO2, or via an increase in DIC through the addition of CaCO3 and KHCO3. In addition, the feasibility of direct CO2 enrichment of the root-zone of soil-grown plants, via injection of CO2 into the soil or using porous pipes within the soil, will also be assessed. The effects of enriching the root-zone with optimal and sub-optimal [CO2] (750-1,500 ppm), with respect to those currently used for the large-scale CO2 enrichment of the aerial environment, on plant growth and productivity will be determined through plant morphological and physiological measurements (e.g. leaf number, leaf area, relative growth rate, the fractions of biomass allocated to different plant organs).
(2) The question of how plants respond to root-zone CO2, in terms of explaining the observed growth response (Fig.1) will be investigated in glasshouse experiments under conditions informed by the results from (1). Plant physiological and morphological measurements will be combined with measurements of leaf gas exchange and photosynthesis (IRGA – specifically to plot photosynthesis against leaf internal [CO2] to determine if acclimation has occurred in enriched plants), stomatal behaviour (thermal imaging and porometry) and chlorophyll content (solvent extraction and spectrophotometry). These measurement will identify any photosynthetic limitation to root-zone CO2 enrichment – the results will be compared with those of HDC project PE021 evaluating links between standard commercial CO2 enrichment and plant photosynthetic demand. The role of hydraulic and chemical signals will be determined by measuring leaf water potential (psychrometry) and whole-leaf and/or xylem sap ionic (ICP-OES) and plant hormone (radioimmunoassay/GC-MS) composition. Leaf carbon isotope discrimination will be measured as a proxy for water use efficiency. Leaf carbohydrate status will be determined via standard biochemical techniques. These studies will be used to optimize growth and productivity gains delivered through root-zone CO2 enrichment in an iterative process.
(3) Larger-scale trials will initially be undertaken at Cornerways Nursery. The precise timing of these will depend on the outcomes from (1) and (2). Parallel “field-based” physiological and morphological measurements will be taken from plants subjected to root-zone CO2 enrichment and standard enrichment of the aerial environment with CO2 under comparable conditions allowing photosynthetic responses to root-zone CO2 enrichment, and the associated changes in crop productivity, to be assessed in a commercial context. Samples will be taken from both sets of plants for subsequent ionic and hormonal analysis at LEC. A detailed cost-benefit analysis of root-zone CO2 enrichment versus conventional CO2enrichment will be performed. This will take account of directly incurred expenses (CO¬2 consumption in particular) only and the benefits accrued through increases in the output-yield values of crops.