Rhizosphere priming of isogenic wheat lines varying in citrate efflux

Roots don’t simply take up nutrients from the soil. They also exude chemicals. The exudates released from living roots play an important role in plant nutrient acquisition. and biochemical processes in the rhizosphere. Root exudates can stimulate microbial activity to accelerate mineralization of soil organic matter, a phenomenon termed rhizosphere priming effect (RPE). The RPE is a vital process which regulates soil carbon and nutrient dynamics and hence soil fertility and productivity. From the plant’s point of view, release of these chemicals can spur microbes into mining for nitrogen. The plant is then able to make use of that.

roots in soil
Image: Canva.

Xu et al. introduced a chemical process that could also contribute to RPE: root exudates (organic acid ligands) could liberate mineral-protected carbon (C) in soil for microbial degradation.

To test this proposal, wheat (Triticum aestivum L.) near-isogenic lines varying in citrate efflux were grown for six weeks in a C4 soil supplied with either low (10 µg g-1) or high P (40 µg g-1). Total below-ground CO2 was trapped and partitioned for determination of soil organic C decomposition and RPE using a stable isotopic tracing technique. Mineral dissolution was examined by incubating soil with citric ligand at a series of concentrations.

While high P supply increased microbial growth and RPE possibly due to higher total root exudation, citrate efflux from the root might have facilitated the liberation of mineral-bounded C, leading to the higher RPE under Egret-Burke TaMATE1B. Mineral dissolution may be an important process that regulates RPE and should be considered in future RPE research.