Rhizodeposition efficiency of pearl millet genotypes assessed on a short growing period by carbon isotopes (δ13C and F14C)

Rhizosheath size varies significantly with crop genotype, and root exudation is one among its driving factors. Unravelling the relationships between rhizosheath formation, root exudation and soil carbon dynamics may bring interesting perspectives in terms of crop breeding towards sustainable agriculture. Here we grew four pearl millet (C4 plant type: delta C-13 of -12.8 parts per thousand, (FC)-C-14 = 1.012) inbred lines showing contrasting rhizosheath sizes in a C3 soil type (organic matter with delta C-13 of -22.3 parts per thousand, (FC)-C-14 = 1.045). We sampled the root-adhering soil (RAS) and bulk soil after 28 d of growth under a semi-controlled condition. The soil organic carbon (SOC) content and delta C-13 and (FC)-C-14 of soil samples were measured and the plant-derived C amount and C-lost / C-new ratio in the RAS were calculated. The results showed a significant increase in delta C-13 in the RAS of the four pearl millet lines compared to the control soil, suggesting that this approach was able to detect plant C input into the soil at an early stage of pearl millet growth. The concentration of plant-derived C in the RAS did not vary significantly between pearl millet lines, but the absolute amount of plant-derived C varied significantly when we considered the RAS mass of these different lines. Using a conceptual model and data from the two carbon isotopes' measurements, we evidenced a priming effect for all pearl millet lines. Importantly, the priming effect amplitude (C-lost / C-new ratio) was higher for the small rhizosheath (low-aggregation) line than for the large rhizosheath (high-aggregation) ones, indicating a better C sequestration potential of the latter.

Automated summary

Rhizosheath size varies significantly with crop genotype, and root exudation is one of the driving factors. Different plant sizes in rhizosheath showed distinct effects on the C input and sequestration in the soil.