Sitor Ndour P. M., Hatté C., Achouak W., Heulin T., Cournac L.

This study contributes to the international 4‰ initiative to manage soils for food security and climate. It highlights that carbon storage must be understood over time and not just as a short-term increase in soil carbon content. The loss of carbon initially present in soil through the addition of new energetic carbon (priming effect) must be minimized. This priming effect, which is not visible by simply monitoring soil carbon content, can easily be assessed by measuring ¹⁴C and ¹³C.

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: d¹³C of -12.8 ‰, F¹⁴C=1.012) inbred lines showing contrasting rhizosheath sizes in a C3 soil type (organic matter with d¹³C of -22.3 ‰, F¹⁴C=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 d¹³C and F¹⁴C of soil samples were measured and the plant-derived C amount and Clost = Cnew ratio in the RAS were calculated.

The results showed a significant increase in d¹³C 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 (Clost =Cnew 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.

Reference: Sitor Ndour P. M., Hatté C., Achouak W., Heulin T., Cournac L., 2022. Rhizodeposition efficiency of pearl millet genotypes assessed on a short growing period by carbon isotopes (d13C, F14C). Soil 8, 49-57 - doi: 10.5194/soil-8-49-2022

Financed by CEA/DRF-IMPULSION CarbonSeq project and a MOPGA grant for the first author