Root Systems’ Role in Ecosystems

• Root systems make several vital contributions to ecosystems through their structure and functioning, influencing their environments and encouraging microbial growth and activities.
• Roots influence biogeochemistry through their involvement in carbon, nitrogen, and phosphorus nutrient cycling. They are also significant contributors to the soil carbon sink.
• Roots build soil structure, stability, and water-holding capacity and promote the hydraulic uplift of water. They also help in soil formation through bedrock weathering.

Roots influence ecosystems, the atmosphere, and geospheres. Understanding how roots alter and shape ecosystem processes can have practical applications in designing management systems for agriculture and forestry. More information has been generated recently about root systems and their functioning through new technology, allowing underground processes to be studied. Find out what we know so far about roots’ role in ecosystems.

Important Root Systems Traits

Several structural and physiological root traits influence the functioning of plants and the ecosystem.

Figure 1: “Drawings of the above- and below-ground extension of the species Pinus sylvestris, Pimpinella saxifrage, Zygophullum xanthoxylo, and Convolvulus tragacanthoides. H/D is the ratio of the above-ground plant height divided by the maximum rooting depth (MRD) ” Pierret et al. 2016. (Image credits: https://academic.oup.com/aob/article/118/4/621/2196536)

The crucial structural traits are discussed below:

• Root type: Large roots >1 cm diameter comprise 56%, and fine roots <2 mm diameter comprise 10% of the total root biomass, according to the Hubbard Brook Ecosystem study.
• Root length and mass fraction: Most root biomass is found in shallower soil depths from 0-20 cm; for example, 83% in temperate forests, though deeper roots (>10 cm deeper) become common in larger trees. Though the average soil depth is considered to be 0-30 cm and where root studies are focused, soils can be deeper than 1 to 1.5 meters, see Figure 1.
• Root branching or architecture: Higher localized branching is good for soil aggregation and fighting competition. Lower branching density favors the exploration of resources.
• Age: As forests grow older, root biomass, the proportion of larger roots, and the root: shoot (R:S) ratio increase.

Physiological traits of importance are:

• Root exudation composition and rate
• Root respiration
• Nutrient absorption
• Root enzymatic action

Other essential traits include biotic interactions with mycorrhizae and bacteria and anatomical aspects like root persistence and turnover; on average, the fine roots last for 1.5 years. However, 20% of roots can live over three years.

Studying root systems’ structure, physiology, and functions earlier relied on tedious methods. However, more efficient, non-destructive, precise, and rapid measurements of many root traits are possible with minirhizotrons, such as CI-600 In-Situ Root Imager and CI-602 Narrow Gauge Root Imager.

Roots’ traits affect its functions of acquiring nutrients and water, anchoring plants, and storing excess nutrients and carbohydrates. This will regulate the roots’ influence on their surroundings, as discussed below.

Resource Acquisition for Plants

Figure 2: “Average deep rooting and nutrient uptake in annual crops (n = 4). (A) Root intensity; (B) plant ¹⁵N for carrot and cabbage,” Pierret et al. 2016. (Image credits: https://academic.oup.com/aob/article/118/4/621/2196536)

Root length is a crucial trait for resource acquisition than root biomass. In soils with low fertility, drainage, or higher sand portions, roots are longer, and the R:S ratio becomes higher.

However, nutrient acquisition reduces with soil depth; for example, in alpine grasslands, nitrogen absorption falls from 67% from the top 5 cm of soil to 24% in the 60-120 cm range in Cambisol soils. Patterns of nutrient acquisition can vary based on soil type and species; for example, maize removes only 44% of its nitrogen from the top 5 cm. However, annual crops like winter wheat and autumn cabbage grow 2.5 m deep roots within 3-4 months and are significant even though root biomass is less at these depths, see Figure 2.

Deeper roots increase access to water and nutrients and help trees and crops tide over drought. When faced with summer dryness, water absorption increases 2.6-fold from spring values in trees. Deep roots keep leaf water status constant in tropical forests during dry months. On a landscape scale, the percolation of nutrients due to excessive fertilization is not entirely lost but is retrieved by deeper plant roots.

Knowledge of nutrient acquisition is valuable in root trait selection for developing new cultivars with efficient nutrient and water use.

Biogeochemistry

Processes that add, retain, transform, or lose nutrients become crucial in biogeochemistry. The root systems substantially influence carbon, nitrogen, and phosphorus nutrient cycling. The roots, in their entirety, determine the interface between soil and roots. The extent of the interface will affect the carbon inputs to the soil in the form of litter and exudates. The amount of root litter is also influenced by turnover rates of small roots and fine roots. These root life traits and microbial activity are involved in cycling the three nutrients. Knowing the exact mechanisms can help in crop management practices.

Carbon Cycling

Root occupancy, respiration, lifespan, turnover, litter formation, and biological interactions influence carbon cycling. These root traits will influence the creation of organic material in the soil and the release of carbon dioxide (CO2) into the soil. Symbiotic biological interactions are also crucial and can be enhanced by root exudates. The ecosystems that are dominated by arbuscular mycorrhizal (AM), ericoid mycorrhizal (ERM), and ectomycorrhizal (ECM) fungi will moderate roots influence due to differences in enzymatic activities of the symbionts. For example, ectomycorrhizal plants produce 70% more carbon in soils than arbuscular mycorrhizal plants.

Nitrogen Cycling

For nitrogen cycling, symbiosis by nitrogen-fixing bacteria with the roots influences the availability of the dominant plant-limiting nutrient. Therefore, species that can associate with nitrogen-fixing bacteria, like legumes, will increase nitrogen levels in the soil.

The amount of nitrogen in roots influences the association of microbes and the type of nitrogen cycling processes that will occur; for example, ammonia oxidizers are common in the rhizosphere of roots with high nitrogen. Root exudates will increase the breakdown of organic nitrogen but can also inhibit nitrification and denitrification.

ECM and ERM fungi can mobilize nitrogen by breaking down the organic polymers that make nitrogen unavailable. AM plants use inorganic nitrogen and are associated with fast nitrogen cycling, nitrification, low C: N (carbon: nitrogen) ratios, and quicker nitrogen loss. In comparison, ECM plants that use organic nitrogen compounds are slow nitrogen cyclers and produce a high C: N ratio in the soil. Nitrous oxide (N2O), a greenhouse gas, is produced more by AM in oxygen-less conditions than by ECM plants.

Roots and the association they promote affect nitrogen cycling speed and phases.

Phosphorus Cycling

Most soils contain large amounts of organic and inorganic phosphorus, and more is added through fertilizers, but only around 1% is available to plants. Mycorrhizal symbiosis with roots is crucial for plants’ phosphorus mobilization and nutrient uptake. AM plants recruit phosphate-solubilizing bacteria to break down organic phosphate into soluble phosphorus that plants can use. AM acids also break down rock phosphate in fertilizers into soluble forms. Therefore, root associations can make phosphorus available by helping in the elements’ cycle.

Roots Stabilize Soil

The ability of roots to stabilize shallow and deep soil is crucial for natural ecosystems and people by reducing soil erosion and landslides.

Roots Reduce Soil Erosion

Root hairs increase the surface area where soil particles can attach to roots and form stable aggregates. Root branching and mycorrhizal hypha also increase soil enmeshment. More root tips increase root exudates like polysaccharides, cations, and hemicellulose containing pentoses and uronic acids, which bind smaller soil aggregates into stabilizer large ones. Also, the soil wetting and drying cycles strengthen the binding agents’ effects. Root exudates and hydrophobins by ECM make soils water-repellant.

Roots reduce erosion by holding together soil and making it hydrophobic. Deforestation and extensive grazing that leaves grounds bare do not have this natural protection and risk having the topsoil washed away by rain.

Roots Prevent Landslides

Vertical deep roots enhance root anchorage and prevent landslides by anchoring the soil to deeper, stabler bedrock. Root branching forms a network minimizing tension cracks and ties planes of potential slip surfaces. Root branching of coarse and fine roots crossing and holding potential failure zone improve soil reinforcement. Roots increase soil tensile strength, crossing across the unstable soil mass.

Roots can be elastic; thick roots act like nails, continue anchoring the trees, and prevent or limit soil collapse. Water absorption by plants keeps soil drier that is more resistant to landslides.

Water Holding Capacity and Drainage

Root death and decay leave empty capillaries for more water retention. Smaller pores in the topsoil help in rainfall percolation. Larger roots leave large pores or macropores and create potential soil flow paths for subsurface water movement. Macropores formed from dead and live roots comprise 70-100% of surface soil macropores and 35% of the total soil volume.

Root architecture and branching direction can influence water drainage from the surface to deeper layers and can be beneficial or detrimental. For example, roots directed downhill on a slope will improve drainage, while roots growing uphill cause water stagnation near stems.

Also, fibrous roots only increase water in shallow soils, and roots in deeper soils will increase hydraulic lift.

Hydraulic Redistribution

The presence of live roots connects different soil horizons. Deep roots absorb water from deeper and moister soils and bring them to shallower and drier layers by releasing water through exudates. As a result, other shallow-root species and seedlings also manage to grow during drought. Soil moistening also maintains microbial activity to increase nutrient uptake by plants. The roots also provide a channel for the drainage of precipitation into deeper soils where there is no evaporation.

Hydraulic lift improves water relations for plants and ecosystems. It occurs worldwide in many ecosystems and tree species. The extent of hydraulic lift varies by magnitude from 0.04 mm H2O d^-1 in savannas to 1.3 mm H 2O d^-1  in temperate sugar maples. Hydraulic lift accounts for 15% of water lost through transpiration.

Soil Formation

Root length, fine root mass, and exudates can affect bedrock weathering and soil formation.

• Deep roots could reach bedrock, and their elongation exerts growth pressure that extends and expands rock cracks.
• Lateral growth of roots will also cause radial pressure and widen rock cracks.
• Root exudation containing organic acids and enzymes increase bedrock weathering.
• Rhizosphere respiration releasing carbonic acid also weathers primary minerals, releasing soluble nutrients absorbed by deep roots.
• Carbon addition to the rhizosphere increases microbial activity, especially of AM and ECM, whose radial and axial growth also leads to bedrock weathering. Rock weathering will depend on the mycorrhizal species, which determines hyphae length and exudates.

Carbon Sink

Roots contribute significantly to soil carbon sinks. Root biomass, respiration, and mycorrhizal associations are vital traits involved in the process.

• Roots comprise 20-25% of plant biomass but contribute 75% of carbon from plants to soils. The contribution is in the form of biomass or root exudates. Around 30-50% of the carbon contribution is from exudates that contain organic acids, amino acids, mucilage, and other soluble compounds.
• Microbes use soluble root exudates that can contribute to 25-30% of their carbon biomass.
• CO2 is released daily to the soils through root respiration, which accounts for 40-50% of soil CO2 flux, even though variations exist among species.
• Mycorrhizal fungi synthesize chitin and melanin, which are less biodegradable and accumulate in soils, especially in cold climates.

Carbon from root tissue has a long residence time as it takes longer to degrade than topsoil litter. As a result of the long root turnover time compared to vegetative parts, soil provides stable carbon sinks as they are not prone to fire, erosion, or annual variations in deeper soils. However, carbon use by microbes, decomposition, nutrient cycling, and plant uptake of nutrients will cause the depletion of soil organic carbon, see Figure 3.

Figure 3: Roots soil organic carbon (SOC) stabilization and destabilization, Dijkstra et al. 2020. (Image credits: https://nph.onlinelibrary.wiley.com/doi/full/10.1111/nph.17082)

Carbon contribution to soils will vary based on tree species and diversity. Agroforestry results in deeper roots due to avoiding competition from annuals. Higher carbon assimilation in multiple species systems results in more carbon contribution to the soils.

Climate Change And Root systems

Roots connect the pedosphere, biosphere, and atmosphere. Roots and microbes will be affected by climate change as much as above-ground plant parts. Root exudates and their influence on nutrient cycling can increase carbon sequestration. However, climate change effects on soil carbon flux must be considered. After a drought, the root exudates can also help the microbes to recover and maintain their ecosystem functioning.

Deep roots could also help trees and other shallow-rooted species reach more water. Deep root water acquisition maintains photosynthesis and transpiration even in dry periods and preserves plant productivity.

Roots play a role in the influence that plant canopy has on rainfall patterns at a regional scale. Therefore, root traits and functioning alterations due to climate change will be crucial.

More Information is Needed

Upscaling observations from smaller scales to landscape and regional scales may be inaccurate, as small-scale variations exist in root interactions with soil. Moreover, effects last long and may not reflect current root traits. Therefore, spatial and temporal variations studied over long periods from different depths will be necessary to better understand root influences on the ecosystem.

Sources

Cislaghi, A. (2021). Exploring the variability in elastic properties of roots in alpine tree species. Journal of Forest Science, 67(7), 338–356. https://doi.org/10.17221/4/2021-jfs

Dijkstra, F. A., Zhu, B., & Cheng, W. (2020). Root effects on soil organic carbon: A double‐edged sword. New Phytologist, 230(1), 60–65. https://doi.org/10.1111/nph.17082

Fahey, T. (2016). Roots. Retrieved https://hubbardbrook.org/online-book-chapter/roots/

Fall, A. F., Nakabonge, G., Ssekandi, J., et al. (2022). Roles of arbuscular mycorrhizal fungi on soil fertility: Contribution in the improvement of physical, chemical, and biological properties of the Soil. Frontiers in Fungal Biology, 3. https://doi.org/10.3389/ffunb.2022.723892

Freschet, G. T., Roumet, C., Comas, L. H., et al. (2021). Root traits as drivers of plant and ecosystem functioning: Current understanding, pitfalls and Future Research Needs. New Phytologist, 232(3), 1123–1158. https://doi.org/10.1111/nph.17072

Mushinski, R. M., Payne, Z. C., Raff, J. D., et al. (2020). Nitrogen cycling microbiomes are structured by plant mycorrhizal associations with consequences for nitrogen oxide fluxes in forests. Global change biology, 27(5), 1068–1082. Advance online publication. https://doi.org/10.1111/gcb.15439

Neumann, R. B., & Cardon, Z. G. (2012). The magnitude of hydraulic redistribution by Plant Roots: A review and synthesis of empirical and modeling studies. New Phytologist, 194(2), 337–352. https://doi.org/10.1111/j.1469-8137.2012.04088.x

Pierret, A., Maeght, J.-L., Clément, C., et al. (2016). Understanding deep roots and their functions in ecosystems: An advocacy for more unconventional research. Annals of Botany, 118(4), 621–635. https://doi.org/10.1093/aob/mcw130

Quigley, M. Y., & Kravchenko, A. N. (2022). Inputs of root-derived carbon into soil and its losses are associated with pore-size distributions. Geoderma, 410, 115667. https://doi.org/10.1016/j.geoderma.2021.115667