Photosynthesis, our solar energy converter
Carbon in the air is magically placed into the soil by tiny brilliant elves. Just kidding. It is a complex process of interactions between plants and microbial life that transfers carbon from the atmosphere to plants to soils providing a crucial carbon sink.
Photosynthesis is the first step of the process by which carbon enters the soil. Atmospheric carbon, carbon dioxide ($CO_2$), is converted within plants using solar energy and becomes stored energy in the form of organic carbon (carbon fixation). This fixation process relies on the enzyme Rubisco, the most abundant protein on Earth. Chloroplasts host photosynthesis where a series of reactions convert $CO_2$ into energy-rich carbohydrates, which are used as an energy currency for the plant and organisms within the ecosystem, with oxygen as a byproduct.
Chemical equation for photosynthesis. Learn more about photosynthesis and Rubisco here.
Figure 1. Photosynthesis within the leaf of a plant, producing energy in the form of sugar. View this image here.
Three different metabolic pathways exist for carbon dioxide fixation in plants: C3, C4, and CAM. High temperatures and low precipitation favor C4 and CAM plants, which are adapted to dry conditions by the enzyme, PEP carboxylase, which is more efficient at fixing $CO_2$ than Rubisco. The increased efficiency of carbon fixation allows the plants to reduce the amount of time their stomata are open, thus reducing water loss via transpiration. PEP carboxylase fixes $CO_2$ into a 4 carbon sugar while Rubisco fixes $CO_2$ into a 3 carbon sugar, hence the names C4 and C3 photosynthesis. In C4 plants the 4 carbon sugar is then transferred to bundle sheath cells deeper within the leaves where the carbon is converted into glucose in the Calvin cycle. In CAM plants, the initial carbon fixation step only occurs at night and the 4 carbon sugars are then stored in vacuoles within the cell to await the products of the light reactions during the day in order to complete the Calvin cycle. The increased efficiency of the intial carbon fixation step in C4 and CAM comes at an energy cost. Rubisco is still used at the start of the Calvin cycle, thus the 4 carbon sugar needs to be converted back to $CO_2$, which takes some of the energy produced by the light reactions. Photosynthetic capacity varies through the course of plant development, among species, and among environments. For example, because nitrogen is needed to produce Rubisco, plants in low nitrogen environments often have lower photosynthetic rates.
Larcher, W., 1995. Physiological plant ecology: ecophysiology and stress physiology of functional groups. Springer-Verlag.
Carbon use within plants
Once carbohydrates are created within the plant, they are used both as the physical plant building blocks and substrate to fuel plant metabolism. The organic carbon stored in plants forms the basis of the green food web, including the tasty things for animals to eat. Plants can store this carbon in aboveground and belowground biomass. However, not all plant growth forms and climates are created equal. Globally, in forests and pastures, mean annual temperature affects how plants partition carbon between aboveground and belowground tissues. As we can see in Figure 2, forest and grassland plants store more of their biomass belowground as the climate warms. However, in very cold ecosystems like tundra, most plants store the majority of their biomass belowground rather than aboveground.
Figure 2. Carbon partioning between above and below ground mass in forests and pasture show increase in ratio of below-ground to above-ground mass in warmer regions of the world. View image here from Litton and Giardina, 2008.
Where carbon is allocated within the plant can also be impacted by atypical environmental conditions such as heat and drought. High stress from heat and drought can cause increases in above ground biomass and decreases in below ground mass (Figure 3). In the tundra, where large amounts of the world’s soil carbon is stored, increased temperatures could shift plant carbon allocation above ground and result in less soil carbon storage (Wang et al., 2016).
Figure 3. Heat and drought affect a plant’s carbon allocation, and change depending on the stress condition of the plant. Under stress, heat and drought can shift plant carbon allocation above ground. View this image here from Sevanto and Dickman, 2015.
Iversen, C. M., Sloan, V. L., Sullivan, P. F., Euskirchen, E. S., McGuire, A. D., Norby, R. J., … Wullschleger, S. D. 2015. The unseen iceberg: plant roots in arctic tundra. New Phytologist, 205(1), 34–58. DOI: 10.1111/nph.13003
Litton, C.M., Giardina, C.P., 2008. Below-ground carbon flux and partitioning: global patterns and response to temperature. Functional Ecology 22, 941-954. DOI: 10.1111/j.1365-2435.2008.01479.x
Sevanto, S., Dickman, L.T., 2015. Where does the carbon go?–Plant carbon allocation under climate change. Tree Physiology 35, 581-584. DOI: 10.1093/treephys/tpv059
Wang, P., Heijmans, M.M.P.D., Mommer, L., van Ruijven, J., Maximov, T.C., Berendse, F., 2016. Belowground plant biomass allocation in tundra ecosystems and its relationship with temperature. Environmental Research Letters 11, 055003. DOI: 10.1088/1748-9326/11/5/055003
Interactions of carbon with soil environment
Carbon can enter the soil from plants through multiple pathways. Senesced branches and leaves from aboveground fall to the soil surface, while senesced roots become part of the soil in place. Plant carbon can also leave the plant via root exudates, which consist of organic compounds and enzymes that protect and support the plant within the diverse microorganism community living in the rhizosphere (the region of soil near to and directly influenced by plant roots). These exudates can prime microbes to more efficiently decompose existing soil organic matter, which benefits the plant as necessary nutrients are released. Lastly, plant carbon can be allocated to symbiotic mycorrhizae that provide an increase in water and nutrient absorption capabilities for the plant in exchange.
Figure 4. Interactions of root exuates with the rhizosphere which funtion to attract beneficial microbes and deter pathogenic microbes. View this image here from Coates and Rumpho, 2014.
All of this plant-derived organic carbon then is decomposed by a host of soil fungi and bacteria into microbial products such as biomass and enzymes and respired as $CO_2$. Potatoes are an example of stored plant energy, which we all know and love, and just as we can consume potatoes for energy, microbes can use many forms of plant carbohydrates for energy as well. In some ecosystems with conditions that limit microbial decomposition (cold or dry conditions or more recalcitrant, harder-to-decompose litter, the litter falling from the plants can form an organic horizon atop the mineral soil. The leaching of dissolved decomposition products from the organic horizon delivers carbon from the aboveground litter into the mineral soil. Bioturbation, the physical mixing of the soil by organisms like earthworms, can also bring aboveground plant litter into the mineral soil. In general, the decomposition rate of plant-derived organic carbon is determined by physical mixing via decomposers such as earthworms and microorganisms, mineral-organo associations, and by freeze/thaw cycles in colder climates.
Figure 5. An emerging understanding of how carbon enters the soil from plant matter and exudates, and how it is incorporated into the soil profile. View this image here from Schmidt et al., 2011.
Coats VC, Rumpho ME. The rhizosphere microbiota of plant invaders: an overview of recent advances in the microbiomics of invasive plants. Frontiers in Microbiology. 2014;5:368. DOI: 10.3389/fmicb.2014.00368
Kuzyakov, Y., 2010. Priming effects: Interactions between living and dead organic matter. Soil Biology and Biochemistry 42, 1363–1371. DOI: 10.1016/j.soilbio.2010.04.003
Marschner H (1995) Mineral Nutrition of Higher Plants (Academic Press, London), Ed 2.
Schmidt, M.W.I., Torn, M.S., Abiven, S., Dittmar, T., Guggenberger, G., Janssens, I.A., Kleber, M., Kögel-Knabner, I., Lehmann, J., Manning, D.A.C., Nannipieri, P., Rasse, D.P., Weiner, S., Trumbore, S.E., 2011. Persistence of soil organic matter as an ecosystem property. Nature 478, 49-56. DOI: 10.1038/nature10386
Walker, T.S., 2003. Root Exudation and Rhizosphere Biology. PLANT PHYSIOLOGY 132, 44-51. DOI: 10.1104/pp.102.019661
Carbon sources for carbon sequestration
Vegetation type, management, and biomass location (above ground or below ground biomass) can vary the amount of plant-derived carbon retained in the soil organic matter (SOM). In Figure 6, we see that soybeans, for example, show a high ratio of below ground to above ground carbon inputs retained in the SOM, as well as organic agriculture practices. The dominance of belowground carbon being retained in the SOM shows the importance of root sourced carbon in long term carbon stabilization. It is becoming increasingly clear that most soil carbon is derived from root carbon.
Figure 6. The source of carbon inputs retained in soil organic matter. View image here from Jackson et al., 2017.
Jackson, R.B., Lajtha, K., Crow, S.E., Hugelius, G., Kramer, M.G., Piñeiro, G., 2017. The Ecology of Soil Carbon: Pools, Vulnerabilities, and Biotic and Abiotic Controls. Annual Review of Ecology, Evolution, and Systematics 48. DOI: 10.1146/annurev-ecolsys-112414-054234
Rasse, D.P., Rumpel, C., Dignac, M.-F., 2005. Is soil carbon mostly root carbon? Mechanisms for a specific stabilisation. Plant and Soil 269, 341–356. DOI: 10.1007/s11104-004-0907-y
The big picture
Global climate changes will shift the balance between plant carbon uptake and soil carbon losses, which under steady state conditions (such as unchanging climate) are balanced. By researching how plants react to temperature changes, drought stress, and increased atmospheric $CO_2$ concentrations, we can begin to predict how soil carbon inputs will change in the future. For example, many C3 plants show increased productivity in response to elevated $CO_2$ concentrations, which may increase soil carbon storage. We can also test whether growing plants with a high belowground to aboveground biomass ratio will increase carbon storage. By understanding the patterns of carbon fixation within plants and its relationship to long term soil carbon storage, we may be able to use plants as tools in climate change mitigation.