Plant roots may be out of sight, but they are not out of mind for McGill researchers. While it is known that fine roots—those less than two mm in diametre—possess highly variable physiological and morphological properties, the reasons behind this variation remain unknown.
Caroline Dallstream, a PhD student in McGill’s Department of Biology, hypothesized that the heterogeneous nature of soil was a key driver of fine-root trait variation at small spatial scales. To test this, Dallstream and her collaborators investigated the potential drivers of fine root variation of the Handroanthus ochraceus tree across spatial scales—from individual roots to entire forest sites more than 10 km apart from one another in Costa Rica’s dry tropical forests.
Dallstream measured a number of soil variables, including magnesium, ammonium, and nitrate levels. She compared these against different fine root traits, such as the roots’ overall morphology, nitrogen concentration, arbuscular mycorrhizal fungi (AMF) colonization—symbiotic fungi that enhance nutrient uptake—and phosphatase activity—an enzyme that removes phosphate groups from molecules.
Although the research focused on a single tree species, Dallstream and her collaborators collected a wide range of root trait data from the same sample. This approach allowed them to find correlations between several different traits simultaneously: A key strength of the study. Dallstream also noted that later research could apply their experimental approach to other species, which could allow scientists to generalize the findings to better understand how soil influences fine roots.
When discussing their findings, she explained how fine root traits interact in complex ways across different scales.
“[The studied] fine root traits tend to coordinate and trade off in complex ways, and these trade-offs arise across many scales,” Dallstream said in an interview with The Tribune. “But we saw that within [Handroanthus ochraceus], […] the two dominant finite trait coordinations and trade-offs were both driven by soil, but at different spatial scales.”
In other words, different soil properties influenced different root traits.
“Fine root respiration and morphology were being driven by soil nitrogen, and fine root arbuscular mycorrhizal colonization and enzyme activity for organic phosphorus were being driven by soil magnesium and bulk density,” Dallstream explained.
The former data points varied at the individual root level, as soil nitrogen was highly variable within sampling sites. In contrast, AMF colonization and phosphatase activity varied across a larger spatial scale, reflecting large-scale heterogeneity in soil magnesium and bulk density among sites.
Dallstream was surprised to find a correlation between root phosphatase activity and AMF colonization.
“It is possible that the [AMFs] are producing that enzyme, which means that they could be contributing to plant nutrient uptake even more substantially than we already think that they are, because the prevailing idea is that they mostly just absorb [nutrients that are] already available, rather than breaking down more complex compounds for plant uptake,” Dallstream said.
These findings may help scientists find a new perspective on how plants and their roots interact with soil nutrients.
Since magnesium seemed to influence AMF colonization, Dallstream suggested that future studies on plant-soil interactions focus on a wider array of essential plant nutrients. Currently, most studies concentrate on nitrogen and phosphorus, which are thought to be the most limiting nutrients in temperate and tropical areas, respectively.
Dallstream was intrigued by the positive correlation she found between soil magnesium content and AMF colonization; this discovery has since inspired her ongoing work.
“I am currently doing a greenhouse experiment with some tropical tree species to look at how magnesium influences [AMF] colonization and testing a few of the potential mechanisms that could be underlying it,” Dallstream elaborated.
One mechanism she hypothesizes could explain this relationship is that magnesium—a key component of chlorophyll that activates carbon fixation in plants—may increase photosynthesis, allowing plants to send extra carbon to their AMFs to help them acquire even more nutrients.
For Dallstream, the motivation for this research goes beyond academic curiosity.
“I think this research is important because plants literally underlie all life on Earth, and despite that, we still know very little about their basic biology,” Dallstream said.
Looking at the bigger picture, Dallstream’s findings help to better understand ecology in the tropics, addressing a gap in plant literature. Further, they inform how plants coordinate their nutrient acquisition, which usually consists of the fundamental limitation of plant growth, and, therefore, carbon sequestration. These findings could thus allow scientists to improve ecosystem models used to predict ecosystems’ response to climate change.
