Fine-Root Dynamics Vary with Soil Depth and Precipitation in a Low-Nutrient Tropical Forest in the Central Amazonia

New direct estimates of fine‐root productivity and turnover in a Central Amazonian plateau tropical forest

The Science

A common assumption in tropical ecology is that root systems respond rapidly to climatic cues but that most of that response is limited to the uppermost layer of the soil with relatively limited changes in deeper layers. However, this assumption has not been tested directly, preventing models from accurately predicting the response of tropical forests to environmental change.

The Impact

This study presents new direct estimates of fine‐root productivity and turnover in a Central Amazonian plateau tropical forest, as well as the factors controlling their dynamics, which are crucial to the understanding of above‐ versus below-ground trade‐offs and linkages determining forest function. The findings demonstrate a relationship between fine‐root dynamics and precipitation regimes and emphasize the importance of deeper roots for accurate estimates of primary productivity and the interaction between roots and carbon, water, and nutrients.

Summary

The objective of this study was to quantify the patterns and controls of fine-root productivity, standing stock, and mortality and fine-root population turnover across the vertical soil profile in an Amazonian plateau tropical forest. The team measured seasonal dynamics of fine roots with high spatial and temporal resolution using minirhizotrons to see below the surface in a mature forest in Central Amazonia. Minirhizotron measurements were calibrated with fine roots extracted from soil cores. Direct observations of fine‐root dynamics to a depth of 90 cm enabled researchers to reach three important advances in understanding fine‐root dynamics in this site. First, although the largest fraction of fine‐root biomass and productivity is in the top 10 cm of the soil profile, a substantial fraction is deeper than 30 cm (46.1% and 40.6%, respectively). Second, as is often assumed but rarely observed, fine‐root turnover declined with depth. Third, seasonal variation in precipitation drives root dynamics, but the direction and strength of the influence of precipitation varies with depth. Fine‐root productivity and mortality in surface layers were positively related to precipitation. Fine‐root stock was greater in dry periods in the deepest layer where water is likely more available at that time. Results from this study extend the quantification of root dynamics to deeper in the soil profile than previous studies in tropical forests, contributing to our understanding of ecosystem NPP, carbon cycling, and environmental controls on fine‐root dynamics.

Principal Investigator

Richard J. Norby
University of Tennessee-Knoxville, Department of Ecology & Evolutionary Biology
rnorby@utk.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

This work was supported in part by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research Program, Climate and Environmental Sciences Division through the NGEE-Tropics program at Oak Ridge National Laboratory (ORNL). ORNL is managed by University of Tennessee (UT)-Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725.

References

Cordeiro, A. L., et al. "Fine-root Dynamics Vary with Soil Depth and Precipitation in a Low-nutrient Tropical Forest in the Central Amazonia." Plant-Environment Interactions 1 (1), 3–16   (2020). https://doi.org/10.1002/pei3.10010.

Cordeiro, A. L., et al. "Fine‐root production, mortality, and standing stock from minirhizotron measurements, AmazonFACE site, Brazil." 1.0. NGEE Tropics Data Collection (dataset).  (2019). http://dx.doi.org/10.15486/ngt/1523508.