October 13, 2020

Root and Soil Carbon Depth Distributions Are Related Across Fertility and Rainfall Gradients in Lowland Tropical Forests

Surface roots predict deep soil carbon.

In infertile tropical forest soils, large surface root biomass corresponded to large subsurface soil carbon (C) and extractable dissolved organic C (DOC, left). In contrast, more fertile soils had less surface root biomass and less deep soil C.

[Reprinted with permission from Cusack, D. F., and Turner, B. L. "Fine Root and Soil Organic Carbon Depth Distributions Are Inversely Related Across Fertility and Rainfall Gradients in Lowland Tropical Forests." Ecosystems 24 1075–92 (2020). DOI:10.1007/s10021-020-00569-6.]

The Science

Root depth distributions in 43 tropical forests were predicted by pH and exchangeable potassium, with more surface roots in acidic, nutrient-poor soils. Similarly, soil carbon stocks in subsoils were greatest in infertile, base cation-poor soils. Root and soil carbon depth distributions were inversely related across sites, such that large stocks of surface root biomass were correlated with large stocks of subsurface soil carbon (deeper than 50 cm).

The Impact

These data show that large surface root biomass stocks are associated with large subsoil carbon (C) stocks in strongly weathered tropical soils. Further studies are required to evaluate why this occurs and whether changes in surface root biomass, as may occur with global change, could in turn influence soil organic carbon (SOC) storage in tropical forest subsoils.

Summary

Overall, results from 43 lowland seasonal tropical forests showed that depth distribution index numbers (Root β and SOC β) were inversely related, suggesting that concentration of fine root biomass in surface soils may be linked to large subsoil C storage (50–100 cm). Soil acidity and nutrient scarcity, in particular lack of potassium, appear to drive proliferation of fine roots in surface soils, while subsoil properties appear to drive retention of SOC in these sites. Further mechanistic studies are needed to elucidate the observed patterns, including measurements of fine root turnover and exudation rates, organic matter in leachate and macropore flow, microbial recycling, contribution of coarse roots to deep SOC, and influence of mineralogy and other physiochemical subsoil properties in retaining C in subsoil. The short- and longer-term sensitivity of subsoil C storage to changes in surface root dynamics could improve prediction of future climate-forest feedbacks for the humid tropics.

Principal Investigator

Daniela Cusack
Colorado State University
daniela.cusack@colostate.edu

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

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.

References

Cusack, D. F., and Turner, B. L. "Fine Root and Soil Organic Carbon Depth Distributions Are Inversely Related Across Fertility and Rainfall Gradients in Lowland Tropical Forests." Ecosystems 24 1075–92 (2020). https://doi.org/10.1007/s10021-020-00569-6.