Tree roots use a complex chemical language that varies with root size

A team of researchers has detailed the varied ways tree roots of different sizes interact with their surrounding soil, revealing a structured system of chemical communication. The study, led by scientists at the Institute of Soil and Water Conservation, Northwest A&F University and Nanjing Normal University, examined the Chinese red pine and found that roots secrete different carbon-based chemicals depending on their diameter. This finding extends the concept of "hierarchical traits"—where different parts of a system have specialized functions—from the physical structure of roots to their chemical influence on soil ecosystems.

A Hierarchy of Secretions

The investigation showed a clear division of labor among roots of different sizes. The finest roots, measuring less than 0.5 millimeters in diameter, were found to exude high levels of fatty acids and organic acids. These compounds are known to help make soil nutrients available and defend against harmful microbes. In contrast, thicker roots, between 1 and 2 millimeters, released more carbohydrates and alcohols, which serve as a direct energy source for soil microorganisms. This demonstrates that even within a single tree's root system, different parts play distinct roles in managing the soil environment.

Shaping Microbial Neighborhoods

These differences in chemical output create unique microbial habitats along the root system. Researchers discovered that as root diameter increased, the number of bacteria in the surrounding soil decreased, while the number of fungi increased. The diversity of both bacteria and fungi also rose in the soil around thicker roots. This suggests that the acidic compounds from finer roots create a more selective environment, while the energy-rich substances from thicker roots support a wider variety of microbial life.

The Effect of Added Nitrogen

To understand how environmental factors affect this system, the team applied different levels of nitrogen, simulating the effects of agricultural fertilizers and atmospheric pollution. The addition of nitrogen generally prompted the roots to release a greater abundance of carbon metabolites. This altered the soil's microbial balance, leading to an increase in bacterial numbers and diversity but a decrease in the overall quantity of fungi.

A Complicated Chain of Events

The study identified that nitrogen's influence on soil microbes is primarily driven by the changes it causes in root exudates. Nitrogen addition promoted microbial populations near fine roots by causing a reduction in their acidic secretions, making the environment more hospitable. For thicker roots, nitrogen prompted an increase in the release of beneficial carbohydrates and alcohols, which further fed the local microbial communities. This shows a complex interaction where an environmental input alters plant behavior, which in turn reshapes the soil ecosystem.

Identifying Key Chemical Signals

The research was able to pinpoint specific chemicals that act as key communicators between roots and microbes. Hippuric acid and cytidine-5´-monophosphate from the finest roots, and glucose and spermidine from thicker roots, were identified as functional compounds affecting major soil microbial phyla like Actinobacteria, Acidobacteria, Basidiomycota, and Ascomycota. These findings clarify the mechanisms that connect plant physiology to the composition of the soil microbiome. The work provides a more detailed picture of the processes governing nutrient cycling and soil health in forest ecosystems.

Corresponding Author:

Guoliang Wang

Original Source:

https://doi.org/10.1007/s44246-023-00081-1

Contributions:

All authors contributed to the study conception and design. Guoliang Wang and Guobin Liu conceived and designed the research. Hang Jing collected data and wrote the initial paper. Yi Cheng and Huiling Wang revised the paper. All authors read and approved the final manuscript.