Crops for Carbon Farming

Abstract

Agricultural cropping systems and pasture comprise one third of the world's arable land and have the potential to draw down a considerable amount of atmospheric CO₂ for storage as soil organic carbon (SOC) and improving the soil carbon budget. An improved soil carbon budget serves the dual purpose of promoting soil health, which supports crop productivity, and constituting a pool from which carbon can be converted to recalcitrant forms for long-term storage as a mitigation measure for global warming. In this perspective, we propose the design of crop ideotypes with the dual functionality of being highly productive for the purposes of food, feed, and fuel, while at the same time being able to facilitate higher contribution to soil carbon and improve the below ground ecology. We advocate a holistic approach of the integrated plant-microbe-soil system and suggest that significant improvements in soil carbon storage can be achieved by a three-pronged approach: (1) design plants with an increased root strength to further allocation of carbon belowground; (2) balance the increase in belowground carbon allocation with increased source strength for enhanced photosynthesis and biomass accumulation; and (3) design soil microbial consortia for increased rhizosphere sink strength and plant growth-promoting (PGP) properties.

Keywords: PGPB (plant growth-promoting bacteria); carbon budget; carbon farming; plant-microbe interactions; rhizosphere; rhizosphere microbiome; sustainable agriculture.

FIGURE 1

The terrestrial carbon (C) cycle. Carbon stocks (boxes) are shown as gigatons (GT), and fluxes (arrows) are shown in GT per year. Respiration refers to accumulated plant and microbial respiration.

FIGURE 2

Transfer of atmospheric CO₂ into biotic and pedologic carbon (C) pools the plant ecosystem. Carbon enters the soil as root exudates or via decomposition of root or aboveground biomass. In the soil, C exists in root or microbial biomass, as bioavailable labile organic C, or as more recalcitrant C. Carbon exits the soil as direct emissions, or via root or microbial respiration, with microbial-mediated soil respiration being the major source of CO₂ from terrestrial ecosystems. Carbon is also lost from the ecosystem as volatile organic compounds (VOCs) and methane (CH₄). Modified from Jansson et al. (2018).

FIGURE 3

Source–sink interactions of photosynthate production and utilization. Source-sink interactions link carbon sources such as mature leaves to sinks such as roots and seeds and mediates feedback inhibition of photosynthesis via perceived sink demand. Sink strength of the rhizosphere is contributed by the root biomass and associated microbial communities, including arbuscular mycorrhiza. Reprinted from Fan et al. (2008).

FIGURE 4

Rationale for designing an integrated plant-microbe-soil system with the dual goal of improving the soil carbon budget while maintaining crop yield, showing current crop (left panel) and desired crop ideotype (right panel). Larger root biomass confers increased sink strength that funnels more carbon to the soil, and deeper roots increase the likelihood for long-term soil carbon storage. Custom-made rhizosphere microbiomes are designed to further augment the demand for belowground carbon, thereby increasing the rhizosphere sink strength, while also providing PGP properties. To complement promotion of plant productivity from improved soil health, PGP microbes, and enhanced photosynthesis by increased sink demand, plants are also designed for increased source strength to further enhance photosynthesis and biomass accumulation.