Linking root traits to soil carbon: model and data gaps

Climate change is expected to alter plant growth and traits, and in turn alter the quantity and quality of plant-derived carbon inputs to soils. Even though root-derived carbon inputs are more likely to be stabilized as soil organic matter (SOM) than aboveground inputs, our models of how climate change influences plant traits and SOM often focus on aboveground plant dynamics. This is in part because our knowledge of root trait linkages to SOM is limited. Recent efforts synthesizing root trait and soil data make it possible to harmonize these data towards an improved representation of roots in terrestrial biosphere models but a conceptual framework to do this is missing. To this end, we review processes that bridge root traits to SOM formation and stabilization and suggest future model improvements. We estimated that 80% of total global soil carbon is in the rooting zone. We then determined that root traits relevant for SOM can be broadly divided into those pertaining to either living or dead roots, within which, the amount, characteristics, and lifespan of living roots, and the decomposability of dead roots and root fragments are particularly important for SOM. Model recommendations included improved allocation regimes, representation of root interactions with microbes and minerals, and incorporation of root-trait variation across the heterogeneous soil matrix. Our review provides a framework necessary for data syntheses and modelling of root trait-SOM linkages to understand future changes in SOM driven by changing plant inputs in a warmer and elevated atmospheric CO₂ world.