Most studies carried out in the Amazon and other tropical ecosystems to investigate roots are limited to depths between 10 and 30 centimeters below the surface. Although the surface layer of the soil houses most of the tree roots, technical/methodological limitations and the availability of financial resources prevented deeper regions of the soil from being properly investigated.
In the article recently published in the magazine Plant-Interactions Environment, the authors show that the biomass of fine roots – those up to 2 mm in diameter – differs relatively little between the superficial layer (with 54% of the biomass) and the deep layer (with 46% of the biomass) of the Amazon Forest soil . The work also reveals that fine roots in the deep soil layer contribute a significant portion of productivity, around 41% of total root productivity. Fine roots are responsible for most of the absorption of nutrients and water contained in the soil by plants, being fundamental for productivity and the accumulation of biomass by the plant as a whole, that is, above and below the soil.
The results of this new work reveal that the carbon stock stored in the roots of Amazonian trees is underestimated in the studies available in the literature to date, since the biomass of roots in the deep layers of the soil was unknown. This aspect has implications for understanding the Amazon Forest's response to climate change due to the forest's role in the carbon cycle and global climate regulation. In general, studies that investigate ecosystem processes in the Amazon Rainforest are focused on above-ground components, such as productivity and biomass accumulation in tree trunks and branches. In this sense, little is known about the components below the soil surface, and even less about the components in the deeper layers of the soil, that is, below 30 cm from the surface.
Another novelty of the study is the dynamics of fine roots throughout the dry and rainy seasons of the year. Research shows that root productivity is about 5,4 times greater during the rainy season compared to the dry season. Precipitation also had a greater effect on the productivity and mortality of fine roots in the surface layer compared to that in the deep soil layer, so that in the dry season root mortality in the deep layer was lower than in the surface layer. This results in a greater relative contribution of the roots in the deep layer to maintain the productivity and physiological processes of the plant as a whole in the period without rain.
The published work is the result of Amanda Cordeiro's master's degree, being part of the results that form part of the first phase of the research program AmazonFACE, a large-scale and long-term experiment that seeks to understand how the increase in carbon dioxide affects the functioning and resilience of the Amazon Rainforest, under the coordination of David Montenegro Lapola, researcher at the Center for Meteorological and Climate Research Applied to Agriculture (Cepagri) at Unicamp and Carlos Alberto Quesada, researcher at INPA. Both collaborated on the study, as did researchers from seven other international research institutions.
Contribution of technological innovation to the study
The study was conducted in an area of preserved mature forest, 70 km north of Manaus, in the Central Amazon. The results obtained were possible due to a methodological innovation, which was the use of a small-sized photographic camera called a minirhizotron. The camera was inserted into transparent acrylic tubes measuring 5 cm in diameter, buried in the soil, thus allowing images of fine roots to be obtained along the soil profile, from the superficial part to a depth of 90 cm.
With the mechanism, the dynamics of forest roots began to be known in detail, which includes growth, birth, biomass accumulation, lifespan and death of roots. Amanda Cordeiro, first author of the study, highlights the relevance of the study as it concerns an unknown underground world. "The work developed is interesting because you have images of what happens", analyzes Amanda Cordeiro.
Photos on the left (bottom): growth of fine roots inside the soil, over a period of one month, between 24/6 and 23/7, 2017
Photo on the right (top): minirhizotron camera and rod attached to the camera to obtain images inside the soil
Photo on the right (bottom): researchers moving along the road towards the study area where the plots of the AmazonFACE (credit: João M. Rosa)
The study managed to understand in detail the dynamics of fine roots in the superficial (up to 30 cm) and deep soil layers (between 30 cm and 90 cm). Most studies to date, in addition to reaching a few centimeters below the ground, were based on invasive methodologies, which involve the destruction and removal of roots to carry out measurements. Amanda explains that these methodologies involve going to the study site, digging holes, and thus evaluating the root stock – taking measurements of the roots, removing them and weighing them to find out their biomass. And this could be done repeatedly throughout the year, digging new holes and evaluating the roots of the site. However, this methodology used until now does not allow direct knowledge of the growth and turnover (average lifespan of the roots and replacement by new roots) of the roots over time, as they were killed or removed for the measurements to be carried out.
Using the minirhizotron camera, it became possible to monitor and evaluate the dynamics of the fine roots of trees. Thicker roots, with a diameter above 2 mm, mainly perform the function of transporting water and nutrients, and were not considered in the study. To use the camera, the roots that were in the ground were destroyed only when the holes in the ground were made to install the tubes. After “installation” in the ground, the pipes remained in place throughout the study. The collection of root image data began only 24 months after the tubes were implanted in the soil, a time that was considered adequate for the roots to stabilize again.
The study area comprised two portions of the AmazonFACE, which consists of circular sites 30 m in diameter in the forest, and which are approximately 90 m apart. Five tubes were installed in each plot, 1 to 2 meters apart in the same plot. The study monitored root dynamics over 12 months, thus covering the dry and wet seasons. For each tube, 100 images were taken along the length of the tube, one image every 1,6 cm from the soil surface to a depth of 90 cm, which took 30 to 40 minutes per tube sampled. But as in addition to the photos there was the logistics of preparing and placing the minirhizotron in each tube, in addition to the time to disassemble and store the equipment to move to the points/tubes of the second plot sampled, Amanda says that data collection on the day of Fieldwork took about 7 hours.
Collections were carried out every 15 or 30 days, totaling 18 field trips in 12 months. Due to technical problems, the images collected for one of the tubes had to be disregarded by the authors. In total, around 14.600 images were obtained. As in the sampled area the roots could be almost the same color as the soil, and the minirhizotron images were taken in low resolution, it was not possible to use programs with machine learning algorithm for analyses. Thus, for the analysis, the roots in each image needed to be recognized individually, visually and manually on the computer, differentiating what was a root from what was soil, as is done when outlining a drawing that is underneath a sheet of paper. vegetable.
Amanda Cordeiro explains that the images have low resolution. "Working in the Amazon is very difficult because while you are collecting your data, out of nowhere the rain comes. As you have a lot of expensive equipment, you have to run out to protect it. And sometimes the rain lasts several days, and it's not possible to resume data collection”, he comments. Therefore, the time available for fieldwork was a very limiting factor for data collection, which made it necessary to use a lower resolution to obtain the images, with a higher resolution. 96 dpi. For comparison purposes, the image resolution required to be published in an article is at least 600 dpi.
From the image analysis, measurements of the length and diameter of the fine roots were obtained, thus allowing the biomass of the roots to be estimated, as well as the accumulation of biomass over time.
Current and future perspectives of AmazonFACE
In addition to the importance of the study in accurately revealing the productivity and activity of superficial and deep fine roots, this new data will be fundamental for evaluating the dynamics of roots in the Amazon when the Program AmazonFACE start your next stage of research. The project will advance towards the carbon enrichment of the outdoor forest in the delimited areas of the plots, which will make it possible to answer how the increase in CO2 will affect the productivity and accumulation of root biomass, taking as a reference the data from the study now published in the journal Plant-Interactions Environment. The study will also contribute to understanding root productivity during dry seasons – which have become more intense and longer in recent years in the Amazon due to climate change and deforestation – and to understanding how forest productivity will respond in relation to to the increase in CO2 in the future. Coordinator David Lapola says that the new work shows how research at the AmazonFACE are being carried out in a very integrated way, following the dynamics of the forest both above and below the ground.
The new results on the biomass allocated to the fine roots of the deep soil layer could also significantly affect estimates of the carbon stock allocated to the soil of the Amazon Basin. Lapola recalls that previously it was understood that most of the roots were only in the very superficial layer of the soil, in the first 30 cm.
The first phase of the AmazonFACE It was initially expected to last two years, but due to the cut in resources and budget that research in Brazil has suffered in recent years, the period of this phase was extended, resulting in around five years of data on the dynamics of the forest before from the beginning of the experimental conditions. These data include continuous measurements throughout the 24 hours of the day, for a large set of atmospheric and biological variables, both above and below ground.
The results obtained also add another piece to the puzzle of how the Amazon Forest will respond to climate change, which in a previous study by the program AmazonFACE revealed that the low availability of phosphorus in most soils in the Amazon basin, including the study area, is expected to be an important limiting factor for forest productivity in a “future atmosphere”, with higher carbon levels.
Other studies related to the published work are in progress, and are part of a second phase of the AmazonFACE, which consists of the use of open top chambers (OTCs), like small 3 m high greenhouses open to the ceiling. These OTCs are enriched with CO2, at carbon dioxide concentration levels 50% above that currently found in the atmosphere (400 ppmv, parts per million by volume), simulating atmospheric conditions that will be observed on the planet in the coming decades to evaluate the effect of increase in CO2 in the dynamics and productivity of young trees, which are part of the forest's undergrowth. Thus, the productivity of fine roots found in the published study will also serve as a basis for comparing the productivity that will be found for the roots of young trees within the OTCs.
In the future, the same enrichment of carbon dioxide inside the OTCs will be carried out for the entire portion of the AmazonFACE, when the research will enter the experimental phase, in which three plots will be enriched by CO2, and another three plots will remain at ambient atmospheric concentration, thus allowing comparison with the experimental condition.
In a scenario of climate change and considering the role of the Amazon Forest in the storage and cycling of carbon and water, fundamental in regulating the global climate, the authors also highlight that the new data will allow us to refine the estimates of computational ecological models that predict how Tropical forests must respond to climate change.
Read the article published in Plant-Interactions Environment : Fine‐root dynamics vary with soil depth and precipitation in a low‐nutrient tropical forest in the Central Amazonia
To learn more about the study and research AmazonFace visit the LabTerra website (Earth System Science Laboratory), under the coordination of professor David M. Lapola.
