One key piece in the puzzle of predicting future climate is how soil responds to changing environmental conditions, especially at the interface where living plants contribute to soil carbon storage. Root-derived carbon is much likelier to contribute to long-term soil carbon pools than carbon from aboveground plant parts. However, soil carbon models are disproportionately focused on aboveground plant dynamics. Historically, this focus has been because of limited data on root traits and limited understanding of their relationship with soil properties.

Recent efforts to gather root and soil observations from across the globe into large-scale databases (e.g., the Fine-Root Ecology Database (FRED) and the International Soil Carbon Network (ISCN) database) make it possible to test broad-scale hypotheses regarding mechanistic links between roots and soil carbon. However, we lack a common framework to harmonize these disparate databases.

Which root traits are most important for soil organic matter storage at a global scale? To this end, a workshop earlier this year addressed the following questions:

Which root traits are most important for soil organic matter storage at a global scale?

How are these traits currently represented in Earth system models (ESMs)?

How can we improve representation of root traits in soil carbon models?

The workshop brought together diverse perspectives of empiricists and modelers, soil and rhizosphere scientists, and early-career to senior scientists, who all had to speak the same “belowground” language.

Attendees prioritized and organized root traits along a continuum of carbon allocation to living roots, their life span and turnover, and what happens to roots after they die. Along this continuum, we outlined model representations for each trait, the state of empirical data, and recommendations for model improvement.

Of these traits, dynamic allocation belowground to fine roots, especially distal fine roots responsible for resource acquisition, presented the most promising avenue for incorporation into current model structures. Conversely, complex modifications would be needed to include water and nutrient uptake as a function of root traits, a relationship that is currently lacking in many ESMs but that could be a key feedback in ecosystem carbon cycling.

Root trait data coverage lags far behind that for aboveground traits. Workshop activities also highlighted that root trait data coverage lags far behind that for aboveground traits. While aboveground sampling methods have advanced exponentially and harness Earth observation capabilities, belowground sampling methods remain mostly restricted to shovels and labor-intensive techniques. Workshop discussions emphasized the value in linking belowground to aboveground traits to make use of data-rich aboveground proxies for belowground processes.

This workshop developed a blueprint for harmonizing root trait and soil carbon data from across the globe to improve our understanding and modeling of the fate of root carbon in the soil. Although hurdles remain—including methodological discrepancies, limitations of a trait-based approach, and the prioritization of global organizing principles—a happy marriage between FRED and ISCN is helping to fill gaps in the puzzle of future climate, one piece at time.

Stan Wullschleger and the Climate Change Science Institute at Oak Ridge National Laboratory provided workshop funding. More information on the workshop is available here.

—Avni Malhotra ([email protected]), Debjani Sihi, and Colleen M. Iversen, Environmental Sciences Division and Climate Change Science Institute, Oak Ridge National Laboratory, Oak Ridge, Tenn.