Divide or expand? Implications of root growth physiology for soil carbon inputs

Plants and their ability to capture atmospheric CO2 are indispensable for the buildup of soil organic matter, underscoring their crucial role in terrestrial carbon cycling. Yet, the plant physiological processes regulating soil carbon inputs and their environmental controls remain severely underrepresented in soil carbon research, which limits our understanding of soil carbon sequestration potential across biomes and land uses. Root biomass constitutes a major input of organic matter to soil that is particularly difficult to estimate. Here, we outline a framework for the explicit integration of root growth physiology into soil carbon dynamics. Using data acquired in rice (Oryza sativa, L.), we provide mechanistic evidence that the expansion of cortical cells in growing roots is a key process determining the fate of the carbon plants allocate to their root system. We combined measurements of carbon partitioning between biomass formation and respiration in growing roots with three-dimensional quantifications of root cortical cell size using high resolution (1.8 μm) X-ray Computed Tomography. With increasing cortical cell size, indicating greater contribution of cell expansion over cell division to root growth, more carbon was allocated to root biomass formation and less to root respiration (R2 = 0.83). We then integrated our experimental findings with data obtained from the literature covering different land use types to highlight the fundamental importance of including root physiological processes in estimating soil carbon inputs. The established structural-functional relationships between root cortical cell size and carbon partitioning point out the paramount role of root physiology in improving our understanding and prediction of carbon fluxes and retention in plant-soil systems. We therefore propose that measurements of root cortical anatomy be included when assessing global change impacts on soil carbon inputs and the potential of soils to sequester carbon.