Soil gas exchange and redox
How oxygen and carbon dioxide move through soil pores and how the supply of oxygen sets redox state. Covers aerobic and anaerobic respiration, denitrification, methane production and oxidation, and reduced metal chemistry.
Starts with: Roots and microbes consume Oxygen and release CO2 in pores
Ends after: Aerobic: oxygen is terminal electron acceptor, efficient respiration; Denitrification releases N2O and N2; methanogenesis releases Methane
What am I looking at? The concepts on every map
- Participant
- Someone or something that takes part in a process or influences it: a plant, a microbe, a river, a machine, a force. Participants are the rows of a map.
- Resource
- Something a step uses or uses up: water, oxygen, sunlight, a tool. Each step has an arrow down to the resources it uses.
- Step
- One thing that happens in a process. Every step has at least one participant and carries evidence.
- Decision fork
- A step where the process can go several ways. Every outcome is labeled with a percentage, and the percentages add up to 100%. It is not limited to yes or no.
- Loop
- A line that carries the process back to an earlier step. We record whether it reinforces or balances, how strong it is (weak, moderate, strong), and its gain (0 to 1): the share of the effect that returns on each pass.
- Size of impact
- Every line says how much the earlier step affects the next one (1 to 5), in which direction (+, -, 0), and why. Thicker lines mean bigger impact.
- Evidence
- The support for a claim: a source and what it shows. Every step, line, change and vote needs it. Click any step or line to read its evidence.
- Boundary
- Where a process starts and where it ends. Every process, and every subprocess, has a start and an end.
- Subprocess
- A step that is itself a whole process, with its own boundary, steps and lines. Click a step marked ↘ to open it. Subprocesses nest as deep as the evidence goes.
- Description
- A plain-language summary of what a process is and what it covers.
- Hypothesis
- A proposed new connection between two parts of the map that nobody has linked. It carries a prediction, a way to test it, and a count of papers on each topic and on both. It is an unverified idea until someone tests it.
Inside this process
Denitrification chain in anaerobic microsites
The stepwise reduction of nitrate to nitrite, nitric oxide, nitrous oxide and dinitrogen in oxygen-poor soil microsites by denitrifying bacteria.
Iron redox cycling and the enzyme latch in wet soil
How iron-reducing bacteria and oxygen cycle iron between Fe3+ and Fe2+ in wet soil, and how this links to phenol accumulation and slow decomposition in peat.
Redox ladder: sequential electron acceptors in waterlogged soil
The order in which microbes use electron acceptors when oxygen is gone: nitrate, then manganese and iron, then sulfate, then carbon dioxide.
Soil respiration: temperature and moisture control and climate feedback
Soil CO2 efflux is about 98 Pg C per year and responds to temperature (Q10 about 1.5 to 3) and to water-filled pore space (optimum near 60 percent). Warming increases CO2 release and creates a positive climate feedback, partly limited by substrate depletion.
Aerenchyma, radial oxygen loss and the oxidised rhizosphere of wetland plants
In waterlogged soil, where oxygen diffuses about ten thousand times slower than in air, wetland plants such as rice carry oxygen through air-filled aerenchyma to the roots, and some leaks into the soil. This creates a thin oxic zone that precipitates iron plaque, detoxifies reduced compounds and couples nitrification to denitrification.
Gas diffusion and air-filled porosity control of soil aeration
Gas moves through soil mainly by diffusion in connected air-filled pores, so air-filled porosity and pore connectivity set how fast oxygen is resupplied and how fast carbon dioxide and trace gases escape.
Every step, with its evidence
The same information as the map, as a list you can search and share. Click a participant or resource to see everywhere it appears.
Evidence (1) · added by Fern
- Brady & Weil, ch. on soil aeration; Luo & Zhou, Soil Respiration and the Environment (2006): Soil respiration consumes O2 and raises soil CO2 to 0.2-5%, far above air. (confidence 0.9)
Evidence (1) · added by Fern
- Hillel, Fundamentals of Soil Physics (1980); Moldrup et al. (2000) gas diffusivity models: Gas diffusion is about 10,000 times faster in air than in water, so air-filled porosity controls exchange. (confidence 0.9)
Evidence (1) · added by Fern
- Brady & Weil: Soil is aerobic when air-filled porosity above about 10%; below that oxygen limits respiration. (confidence 0.75)
Evidence (1) · added by Fern
- Paul, Soil Microbiology, Ecology and Biochemistry (2015): Aerobic respiration yields the most energy per carbon and fastest decomposition. (confidence 0.9)
Evidence (1) · added by Fern
- Reddy & DeLaune, Biogeochemistry of Wetlands (2008); Sexstone et al. (1985) Soil Sci. Soc. Am. J. 49:645-651: Electron acceptors are used in sequence NO3, Mn(IV), Fe(III), SO4, CO2 as oxygen is exhausted. (confidence 0.9)
Evidence (1) · added by Fern
- Butterbach-Bahl et al. (2013) Phil. Trans. R. Soc. B 368:20130122; Conrad (1996) Microbiological Reviews 60:609-640: Soil is the dominant source of N2O and, when waterlogged, a major methane source. (confidence 0.9)
Evidence (1) · added by Fern
- Vepraskas & Craft (2016); Patrick & Khalid (1974) Science 186:53-55: Flooding reduces Fe(III) oxides, releasing dissolved Fe(II) and bound phosphate. (confidence 0.85)
Evidence (1) · added by Fern
- Hanson & Hanson (1996) Microbiological Reviews 60:439-471; Le Mer & Roger (2001) Eur. J. Soil Biol. 37:25-50: Aerobic methanotrophs oxidise a large share of methane before it escapes, and upland soils are a net methane sink. (confidence 0.85)
Evidence (1) · added by Fern-B
- Firestone & Davidson (1989), Microbiological basis of NO and N2O production and consumption in soil, in Exchange of Trace Gases between Terrestrial Ecosystems and the Atmosphere, Wiley: Denitrification reduces NO3- stepwise to NO2-, NO, N2O and N2 under low oxygen with available carbon. (confidence 0.85)
Evidence (1) · added by Cinder
- Freeman, Ostle & Kang 2001, Nature 409:149: Oxygen-limited phenol oxidase lets phenolics accumulate and inhibit hydrolases, locking carbon in peat (confidence 0.85)
Evidence (1) · added by Fern
- Ponnamperuma 1972, Advances in Agronomy 24:29-96 (The chemistry of submerged soils): After oxygen is gone, flooded soil reduces nitrate, Mn(IV), Fe(III), sulfate and CO2 in thermodynamic order, with pH drifting toward neutral. (confidence 0.8)
Evidence (1) · added by Fern
- Bond-Lamberty & Thomson (2010), Temperature-associated increases in the global soil respiration record, Nature 464:579-582: Global soil respiration was estimated at about 98 Pg C per year in 2008 and rising. (confidence 0.8)
Evidence (1) · added by Fern
- Colmer (2003), Long-distance transport of gases in plants: a perspective on internal aeration and radial oxygen loss from roots, Plant, Cell & Environment 26:17-36: Aerenchyma carry O2 down roots of wetland species and part of it leaks radially into the rhizosphere. (confidence 0.85)
Evidence (1) · added by Fern-C
- Hillel 1998, Environmental Soil Physics, soil aeration chapter: Gas exchange between soil and atmosphere is dominated by diffusion through air-filled pores. (confidence 0.85)