Nutrients Moderate Root Dynamics

The continuous cultivation of trees in plantations requires nutrient management to maintain soil fertility. The method and extent to which nutrients are increased can alter species interactions in mixed plantations by influencing root growth. Until recently, little was known about these below-ground dynamics, as they were difficult to study. Advanced tools like minirhizotrons, which make non-destructive, in situ measurements possible, are proving to be game-changers in understanding root dynamics over time.

Natural Nutrient Cycling is Disrupted in Tropical Plantations

In natural ecosystems, nutrient cycles involve recycling. The decomposition process of litter—from living trees and dead parts—returns many of the nutrients that are removed from soil by plants. However, in plantations where trees are harvested for timber and other forest products, much of the biomass is lost to the ecosystem, leaving soil depleted of nutrients. Tropical soils are notoriously affected by low fertility since the nutrient turnover rate is fast due to the high growth rate of vegetation in these environments.

This problem is often encountered when growing eucalyptus in commercial tropical plantations. Growing eucalyptus with a nitrogen-fixing species seems an attractive way to enrich soil fertility. Eucalyptus is expected to benefit from the nitrogen; however, it is possible that growing eucalyptus with another species could also lead to competition.

According to Grime’s hypothesis, when resource availability changes, the interaction between species can be expected to change from facilitation in high-stress areas to competition among species when stress is moderate or low. While this hypothesis has been proven among plants in connection with light availability, it is not clear if this rule applies for below-ground interactions, as roots are known to also compete for resources when there is low fertility.

Could Interactions Between Eucalyptus and Acacia Change?

To answer this question, a team of forest and agricultural scientists from Brazil tested the interactions, which occur when Eucalyptus (E. urophylla x E. grandis) is grown with (Acacia) Acacia mangium at different levels of soil nutrient availability. The impact of the interactions was measured by fine root dynamics in both the species when they were grown in fertilized and unfertilized plantations.

The scientists chose Acacia mangium, as previous studies have shown that this fast-growing tree can fix large amounts of atmospheric nitrogen (N) and add the element to the soil through litterfall.   

The current study, which was conducted at the University of São Paulo at the Itatinga experimental station, included four treatments:

• Monocultures of Eucalyptus with and without fertilization.
• Mixed species stands with fertilization (F+) and without fertilization (F-). An equal number of Acacia and Eucalyptus were grown in these stands.

The monoculture Eucalyptus stands were used to compare their above-ground productivity with the tree when grown in mixed plantations. Destructive sampling was used to estimate biomass and leaf nutrient levels.

Root dynamics were studied only in mixed plantations. In one set of trials, after sixteen and thirty-four months of setting up the experiment, the scientists collected soil cores from twelve positions around the Eucalyptus and Acacia trees to check root growth at varying distances from the trees. Cores were taken at four depths in each spot. Living roots were recovered and biomass of fine roots longer than one cm were weighed to estimate root mass density. The length of roots was used to calculate specific root length and root length density. The branching patterns, color, and thickness of roots were also studied to find species characteristics. 

Using the Minirhizotron

The minirhizotrons, or root scanners, were set up sixteen months after the start of the experiment. The scientists used two root tubes in the fertilized and unfertilized blocks of the experiment in mixed plantations, so that one was close to an Acacia tree and the other to a Eucalyptus tree. The 180 cm long acrylic tubes were inserted at an angle of 45 degrees into the soil; parts above the soil were painted white to prevent light from entering the root tube, while the bottom ends were closed with a cap to prevent water entry into the tubes.

The scientists waited for six months after installation to allow time for the roots of trees to grow, undisturbed, around the root tubes before taking root scans. This setup is useful for repeat non-destructive measurements in long-term experiments like this one and the scientists made scans at different depths, once every two weeks for a year, using the CI-600 In-Situ Root Imager.

The Root Imager, manufactured by CID Bio-Science Inc.,  has a scan head along its body that can make high-resolution scans of up to 600 dpi at various depths, using the indexing handle. The head of the root scanner can be rotated 360 degrees to take images of all sides of the transparent root tube. Eight images were taken in each tube and segregated based on soil depths of 0–25 cm, 25–50 cm, 50–75 cm, and 75–100 cm. The accompanying root analysis software, RootSnap!, makes it easy to differentiate roots from the surrounding soil. Using the software WinRihizotron, the scientists traced the length and diameter of roots in each image. Superimposing images allowed scientists to track the growth and mortality of roots. From the images taken by the root scanner, live and dead root length production and cumulative live and dead root length production was calculated.

Effect of Fertilization on Biomass Accumulation

Fertilization changes biomass accumulation in both species. In plots without fertilization, Eucalyptus above-ground biomass remained similar in monocultures and mixed stands. Fertilization increased Eucalyptus above-ground biomass in both monocultures and mixed stands, but its biomass was 19% greater in mixed stands than in monoculture. In fertilized stands, Eucalyptus had 33% more biomass than Acacia, but in unfertilized plots, Acacia biomass was 9% higher than Eucalyptus.

After thirty-four months, foliar N levels were observed to be higher in Eucalyptus grown in mixed plantations than in monocultures. The lower nitrogen levels in Eucalyptus leaves in unfertilized plants compared to fertilized tree leaves also show it is suffering from nutrient deficiency.

The above-ground biomass trends can be explained by higher fine root mass and length densities of Eucalyptus compared to Acacia in fertilized plots, in the topsoil, as shown in Figure 1. There is 50% more Eucalyptus root mass density than Acacia in the topsoil in fertilized plots, compared to only 10% more in unfertilized plots. This helps Eucalyptus trees absorb more nutrients than Acacia in fertilized conditions. It is only in non-fertilized conditions that Acacia trees have more roots closer to them.  Moreover, in unfertilized plots, there was no difference in root mass and length density between the two trees.

In 0-15 cm of the topsoil, both species have more root mass density than in deeper soils. However, Eucalyptus has 21% more specific root length after 16 months in both fertilized and unfertilized conditions and 10% after 34 months in fertilized plots.

Nutrient Influences on Root Dynamics

The interaction that dominates root dynamics depends on nutrient levels and age. In unfertilized soils, Acacia benefits Eucalyptus, but when there are more nutrients, the latter becomes competitive to the detriment of Acacia.

In non-fertilized plots, Acacia, with higher above-ground biomass than Eucalyptus, facilitated the latter’s growth. The presence of 40% more of Eucalyptus specific root length near Acacia in unfertilized plots shows that Eucalyptus extend their root growth to forage for nutrients closer to Acacia. After 34 months, Eucalyptus’ need increases, and there is 94% root mass density near Acacia trees than near its own stem.

There was also 66% more fine root growth in the top soil of the unfertilized plots of Eucalyptus,  recorded around the minirhizotron tubes between 16 and 34 months, as shown in Figure 2. These results also show the extra effort Eucalyptus puts into foraging for nutrients in low fertility soils. On the other hand, Acacia’s cumulative live root length is similar in fertilized and unfertilized conditions.

When fertilized, in the initial stages of growth, higher specific root length and mass densities give Eucalyptus a competitive edge over Acacia. Even though Eucalyptus’ specific root length decreased over time, it still managed to hold its advantage. Due to the competition, Acacia trees had to grow more roots in deeper layers of soil and had more specific root lengths than Eucalyptus in the 50-100 cm soil depth after 16 months of growth. After thirty-four months, the difference in height between Eucalyptus and Acacia increases, indicating that the latter is also suffering from lack of light due to competition from Eucalyptus.

Foresters Must Consider Below-Ground Interactions

Above-ground interactions between eucalyptus and Acacia had been studied prior to this investigation, but the underground processes in any mixed forest were still unknown. So, this study marks an important step in uncovering below-ground interactions, due to nutrient availability, with important applications in precision forestry. The interaction is positive during low nutrient availability and Eucalyptus benefits from the nitrogen fixation by Acacia. But when there are more nutrients, competition is more evident. The study shows that foresters must also consider below-ground interactions when they choose species and how they manipulate soil fertility to make the best of a species mix, in which case root analysis will also become more important.

—

Vijayalaxmi Kinhal
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture

Sources

Bordron, B., Germon, A., Laclau, J.-P., Oliveira, I. R., Robin, A., Jourdan, C., Paula, R. R., Pinheiro, R. C., Guillemot, J., Gonçalves, J. L., & Bouillet, J.-P. (2021). Nutrient supply modulates species interactions belowground: dynamics and traits of fine roots in mixed plantations of Eucalyptus and Acacia mangium. Plant and Soil, 460(1-2), 559–577. https://doi.org/10.1007/s11104-020-04755-2