Legume crop production has many benefits for agricultural systems. Through the rhizodeposition process, they release a significant amount of C and N into the soil, increasing soil organic C and reducing the use of N fertilizer. Rhizodeposition is known as a dynamic process influenced by many factors.

The aim of this study was to study the contribution of root exudation and root senescence to the rhizodeposition of atmospheric C and N during vegetative and reproductive growth in annual and perennial legumes and to understand how this is linked to the fixation capacities of C and N and root functional traits.

An original approach that combined ¹³CO₂ labeling and the ¹⁵N dilution method was developed to measure the rhizodeposition of atmospheric C and N throughout plant growth by two annual grain legumes (pea and faba bean) and two perennial forage legumes (white and crimson clovers).

C rhizodeposition was found to increase proportionally with N rhizodeposition during reproductive development and the differences observed between species were related to the C and N fixation abilities. The use of root traits such as specific root length, root tissue density, and root dry matter content suggests a strong contribution of root exudation to C rhizodeposition at vegetative growth and a strong contribution of root senescence to both C and N rhizodeposition during reproductive growth.

Synthesis: Both C and N rhizodeposition appeared to be controlled by traits indicative of resource acquisition and root development.

Two annual grain legumes and two perennial forage legumes were grown and labeled with ¹³CO₂ to measure fixation (Total fixed C, g/m²) and rhizodeposition (CdfR, g/m²) of photosynthetically fixed C. The ¹⁵N dilution method was used to measure biological N fixation (Ndfa, g/m²) and transfer of N (NdfR, g/m²) from the atmosphere to the soil. Root traits were also measured with the winrhizo method: (1) total shoot and root dry biomass (g), (2) root mass ratio (RMR ;  g/g), the ratio of root dry mass to total plant dry mass, (3) root dry matter content (RDMC; mg/g), the ratio of total root dry biomass to total root fresh biomass, (4) root tissue density (RTD ; mg/cm³), the ratio of root dry biomass to total root volume, (5) root diameter corresponding to the mean diameter of the total root system, (6) specific root length (SRL; m/g), the ratio between total root length and dry biomass, (7) root C content (RCC ; mg/g), the ratio of amount of root C to root dry mass, (8) root N content (RNC ; mg/g), the ratio of root N amount to root dry mass, (9) shoot and root C:N ratio, (10) root mass density (RMD ; mg/cm³), the ratio of root dry mass to the cultivation pot volume (2L), (11) root length density (RLD ; cm/cm³), the ratio of total root length to the cultivation pot volume. Root samples were also used to determine the contents of root soluble C and root soluble N.  Root samples were also used to determine the contents of root soluble C and root soluble N. Extractions were carried out by successive treatment of dry root tissue powders with 50 % ethanol and ultrapure water.

All data are expressed here in a csv format