Nov. 30, 2021
Nov. 16, 2021
Root respiration has long been studied by plant scientists. However, a complete and clear picture of the influence various factors have on this crucial process is still missing, as scientific results don't always agree. Better understanding is needed to adequately predict root respiration's impact on ecosystems and the global carbon cycle.
Plant respiration happens not only in the leaves, but also through the root system. In root systems, oxygen (O2) in soil pores diffuses into the root hairs and gets transported to all parts of the roots. Plant roots use photosynthates during respiration for their various functions, like nutrient uptake and maintaining and growing their tissues. The carbon dioxide (CO2) formed is then released into the soil, diffusing through the pores into the atmosphere; see Figure 1, terrestrial soils.
Figure 1: "Root respiration involves the uptake of oxygen entering the soil mainly through diffusion from the atmosphere into the soil air, and the release of CO2 ", Blume et al. 2016. ( Image credits: https://www.researchgate.net/publication/326069042_Plant_Nutrition_and_Advanced_Soil_Sciences_in_Arabic)
Root respiration (Rr) has three components, maintenance, growth, and rhizospheric respiration.
Though root respiration helps in plant growth and productivity, it is also resource-intensive, and up to 75% of the assimilated carbon can be used up in many species. As a result, root respiration rates can affect above-ground growth.
Figure 2: Pneumatophores or aerial breathing roots in mangrove tree species. (Image credits: Image by Bishnu Sarangi from Pixabay)
The oxygen requirements of plants will vary across species, where some need more than others. Most species cannot function in oxygen deficit soils.
Waterlogged soils are a common cause of oxygen deficiency. When water clogs soil pores, it hinders oxygen diffusion, causing an oxygen deficit. The result is that root water permeability is reduced. The entire plant will suffer from water deficiency, showing symptoms like leaf yellowing and wilt. Agronomists are familiar with this problem.
Most plants can survive a brief spell of waterlogging. When soils are waterlogged for a long duration, the roots and soil microbes use all the available oxygen. Under prolonged submergence in water, most plants' roots are attacked by anaerobic pathogens, which replace aerobic bacteria in the soil or rot. The tissues get decomposed by anaerobic microbes and produce methane and carbon dioxide; see Figure 1, flooded soils.
Since plants respire less during the cold winter months, waterlogging causes less harm in this season. However, because roots respire more during summer, waterlogging quickly leads to oxygen deficit.
Some species whose roots grow in soggy conditions have evolved different adaptations to get oxygen.
At times even species adapted to waterlogging can suffer in oxygen-deficient conditions, resulting in anoxia in roots, especially if the whole plant is submerged during floods. In these cases, anaerobic respiration begins within roots, and the plant goes into survival mode, carrying on only the processes necessary to live. If anoxia continues, anaerobic respiration will lead to plant decay.
Root respiration’s importance cannot be overstated. The process contributes to an average of 50% of soil respiration, an essential component of the global carbon cycle.
The actual, case-by-case contribution can vary between 10 and 90% of the soil respiration, based on differences in ecosystem and climate. For example, annual root respiration in cool temperate forests is 4.8 t C ha−1, around 45% of the total yearly soil respiration (10.6 t C ha−1 year−1).
Soil respiration transfers carbon from the soil to the atmosphere and is the second-largest carbon flux (50-75 Pg C/yr), following gross primary production (100-120 Pg C/yr). Carbon flux through soil respiration is higher than the annual global terrestrial net primary productivity (NPP) of 50-60 Pg C/yr. See Figure 3 for global variations of soil respiration.
Hence, even a tiny change in root respiration can be consequential for the global carbon cycle. Both rising levels of CO2 and temperature can increase root respiration, which already exceeds NPP. So any gain in carbon sequestration through rising CO2 can be reversed by even small increases in root respiration.
Figure 3: "A global map of predicted annual soil respiration (Rs) at 1 km spatial resolution created by applying the QRF model to gridded covariates. Right is a plot of the latitudinal mean predicted annual Rs," Warner et al. 2019. (Image credits:https://doi.org/10.3334/ORNLDAAC/1736)
Root respiration rates are crucial because respiration involves the loss of carbohydrates, which plants use for growth and biomass accumulation. While root respiration is vital, very high rates can impact plant productivity. Therefore, various intrinsic and environmental factors can impact plants by affecting root respiration.
Some of the critical environmental factors are:
Soil structure: A good soil texture that holds enough water but has proper drainage is necessary for root respiration. Heavy clay or compacted soil that stores water and has less drainage will impact root respiration by reducing soil oxygen availability.
Soil temperature: An increase in soil temperature raises total root respiration and specific root respiration (rate per unit of mass of roots). Species and cultivars that can tolerate prolonged heat without raising their root respiration can instead use their resources for root elongation, nutrient uptake, and membrane stability. Also, as soil temperature increases, the oxygen concentration in soil water decreases.
Soil nitrogen content: Increasing nitrogen in the soil lowers root respiration, but the form nitrogen takes in the soil can be critical. Overall, there is less bacteria when nitrates are present, so rhizosphere respiration—including bacterial respiration—falls. However, nitrogen as ammonium attracts more bacteria and increases rhizosphere respiration.
Increasing soil nitrogen can enhance fine root growth, which results in higher specific root respiration. As a result, carbon allocation to roots is lower, but fine root turnover can rise and increase carbon flux to the soil; however, the impact of nitrogen on fine root turnover is not completely clear.
Soil Potassium: The presence of soil potassium is crucial for root respiration rate and its effects on the roots. Absence of potassium can lead to increased root respiration and soils high in potassium, but low in oxygen can hinder roots’ ability to absorb water. Low potassium, however, has not been seen to affect water permeability.
Atmospheric CO2: As CO2 levels rise in the atmosphere, the CO2 in the soil also increases. Scientists accept that new root growth is increased with higher levels of soil CO2, which can be seen in higher total root respiration.
Seasons: Temperature is the main reason behind the seasonal variation in root respiration, with higher rates in summer and lower rates in winter. However, the extent of seasonal influence can vary depending on the ecosystem.
Intrinsic factors in the root, which can influence root respiration rates are:
Root nitrogen: All studies agree that respiration rate increases as nitrogen content in root tissue increases.
Root density: Across 245 plant species, both woody and non-woody, respiration rates decreased as the density of the root tissue increased.
Root types: Historically, scientists thought that all parts of the roots did not respire at the same rate. Coarse and medium roots had the same respiration rates, but thinner roots with higher nitrogen content would respire faster. However, a recent metastudy of 52 studies covering 254 woody and non-woody species, found that specific root length and root diameter did not influence root respiration rates. The study did not include many tropical ecosystems, so the conclusion is not global.
Growth form: Broad-leaved trees have higher respiration rates than conifers. The scientists reasoned that broadleaved species have higher leaf area and, therefore, photosynthesis and more carbon. This higher carbon supports increased respiration. There is no difference between root respiration between wood and non-woody species in their respiring rates.
The study of root respiration has many applications:
Many scientists have noted that different estimation methods give varying results. The Infra-red Gas Analyzer in combination with closed chamber systems is popular but not the only method used. Measuring factors like leaf area or impact on root growth are more standard and non-destructive, using tools from CID Bio-Science Inc. It is easier to measure biomass accumulation and predict the effect of the environment on crops. However, we need to measure the process accurately to estimate its global impact due to climate change.
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture
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