Tropical Forest Soil Carbon Stocks Predicted by Nutrients and Roots, not Aboveground Plant Biomass

Soil base cation availability regulates tropical soil carbon stocks via a negative relationship with fine root biomass.

The Science

Scientists at the University of California, Los Angeles (UCLA), and the Smithsonian Institution conducted an extensive study of predictors of tropical soil carbon stocks to 1 m depth at 48 sites in Panama, including measurements of soil characteristics, plant biomass, and climate. The study revealed a nearly three-fold change in soil carbon stocks across five soil orders, with soil characteristics like fine root biomass, clay content, and nutrient base cations the strongest predictors of soil carbon stocks.

The Impact

Tropical forests are the most carbon rich ecosystems on Earth, containing 25% to 40% of global terrestrial carbon stocks. Quantification of aboveground biomass in tropical forests has improved recently, but soil carbon dynamics remains one of the largest sources of uncertainty in Earth system models. Including soil base cations in carbon cycle models, and thus emphasizing mechanistic links among nutrients, root biomass, and soil carbon stocks, will improve prediction of climate-soil feedbacks in tropical forests.


Overall, soil characteristics were the best predictors of soil carbon stocks, with no relationship to aboveground plant biomass or litterfall. The best fit model for the study’s data suggested that available base cations provide an indirect control over tropical soil carbon stocks via a negative relationship with fine-root biomass. Soil clay content and rainfall also emerged as significant predictors of soil carbon. In addition to the nearly three-fold change in soil carbon stocks, the sites used here covered five soil orders, over 25 geological formations, a two-fold range in rainfall, a 20-fold range in base cations, and a 100-fold range in available phosphorus. Thus, although the data come from a relatively restricted geographic region, the diversity of environmental conditions means that the results are likely to be broadly applicable over much larger geographical ranges.

Principal Investigator(s)

Daniela F Cusack
University of California


Funding was provided by the National Science Foundation (NSF) GSS Grant #BCS-1437591 and the U.S. Department of Energy (DOE) Office of Science Early Career Research Program Grant DE-SC0015898 to D. F. Cusack, and UK Research and Innovation’s National Environment Research Council (NERC) Grant NE/J011169/1 to O. T. Lewis.


Cusack D.F. et al. “Soil carbon stocks across tropical forests of Panama regulated by base cation effects on fine roots.” Biogeochemistry 137, 253–66 (2018). [DOI:10.1007/s10533-017-0416-8]