Rice rhizodeposition promotes the build-up of organic carbon in soil via fungal necromass

Rice rhizodeposition plays an important role in carbon sequestration in paddy soils. However, the pathways through which rice rhizodeposits contribute to soil organic C (SOC) formation are poorly understood because of specific paddy soil conditions. Furthermore, microbial necromass has been largely ignored in studies examining the contribution of rhizodeposits to C sequestration during plant growth. To evaluate the contribution of microbial necromass to SOC formation via rhizodeposition, rice (Oryza sativa L.) plants were continuously labeled with 13CO2 for 38 days under ambient (aCO2, 400 μL L−1) or elevated CO2 (eCO2, 800 μL L−1) in a paddy field at two levels of N fertilization. The distributions of photosynthetic-13C in the shoots and roots, microbial communities, and SOC fractions were quantified. eCO2 increased plant growth and, consequently, the total 13C incorporated into the shoots, roots, and SOC compared to aCO2, while N fertilization (100 kg N ha−1) decreased root biomass and rhizodeposits in the soil and microbial pools, including living biomass (phospholipid fatty acids, PLFA) and microbial necromass (amino sugars). Rhizodeposits were initially immobilized mainly by bacteria and preferentially recovered in fungal necromass (glucosamine). While 13C incorporation into PLFAs was slightly increased during plant growth, 13C in microbial necromass increased greatly between the tillering and booting stages. Fungal necromass, which is less decomposable compared to bacterial residues, was the largest contributor to C sequestration with rhizodeposits via the mineral-associated SOC fraction, particularly under elevated CO2 without N fertilization. This study reveals the significance of the C pathways from rhizodeposits through fungal necromass and organo-mineral associations for the build up of SOC in paddy fields.