Soil respiration
The temperature, moisture, nutrient content and level of oxygen in the soil can produce extremely disparate rates of respiration.
Other methods can be used to separate the source components, in this case the type of photosynthetic pathway (C3/C4), of the respired plant structures.
This is an important source of CO2 in soil respiration in waterlogged ecosystems where oxygen is scarce, as in peat bogs and wetlands.
These exudates include sugars, amino acids, vitamins, long chain carbohydrates, enzymes and lysates which are released when roots cells break.
Under high soil moisture conditions, many bacteria take in too much water causing their cell membrane to lyse, or break.
Upper levels of soil moisture will depress root respiration by restricting access to atmospheric oxygen.
[13] It is essential that plants uptake nitrogen from the soil or rely on symbionts to fix it from the atmosphere to assure growth, reproduction and long-term survival.
Soil respiration can be measured alone or with added nutrients and (carbon) substrates that supply food sources to the microorganisms.
With the addition of nutrients (often nitrogen and phosphorus) and substrates (e.g. sugars), it is called the substrate-induced soil respiration (SIR).
[18] Open mode systems are designed to find soil flux rates when measuring chamber equilibrium has been reached.
Air flow in the chamber at the soil surface is designed to minimize boundary layer resistance phenomena.
Survey soil respiration systems can also be used to determine the number of long-term stand-alone temporal instruments that are required to achieve an acceptable level of error.
[19] By analyzing stable carbon isotope data it is possible to determine the source components of respired SOM that was produced by different photosynthetic pathways.
[23] Throughout the past 160 years, humans have changed land use and industrial practices, which have altered the climate and global biogeochemical cycles.
In addition, increasingly frequent extreme climatic events[24] such as heat waves (involving high temperature disturbances and associated intense droughts), followed by intense rainfall, impact on microbial communities and soil physico-chemistry and may induce changes in soil respiration.
Numerous free air CO2 enrichment (FACE) studies have been conducted to test soil respiration under predicted future elevated CO2 conditions.
[27] It is extremely likely that CO2 levels will exceed those used in these FACE experiments by the middle of this century due to increased human use of fossil fuels and land use practices.
This is due to human activities such as forest clearing, soil denuding, and developments that destroy autotrophic processes.
Much of the organic matter swept away in floods caused by forest clearing goes into estuaries, wetlands and eventually into the open ocean.
Increased turbidity of surface waters causes biological oxygen demand and more autotrophic organisms die.
It has been shown that soil respiration in arid ecosystems shows dynamic changes within a raining cycle.
Since the onset of the Green Revolution in the middle of the last century, vast amounts of nitrogen fertilizers have been produced and introduced to almost all agricultural systems.
This has led to increases in plant available nitrogen in ecosystems around the world due to agricultural runoff and wind-driven fertilization.
Soil respiration plays a significant role in the global carbon and nutrient cycles as well as being a driver for changes in climate.
Soil respiration plays a critical role in the regulation of carbon cycling at the ecosystem level and at global scales.
Each year approximately 120 petagrams (Pg) of carbon are taken up by land plants and a similar amount is released to the atmosphere through ecosystem respiration.
A major component of soil respiration is from the decomposition of litter which releases CO2 to the environment while simultaneously immobilizing or mineralizing nutrients.
It is estimated that a rise in temperature by 2 °C will lead to an additional release of 10 Pg carbon per year to the atmosphere from soil respiration.
There also exists a possibility that this increase in temperature will release carbon stored in permanently frozen soils, which are now melting.
Many methods are used to measure soil respiration; however, the closed dynamic chamber and use of stable isotope ratios are two of the most prevalent techniques.
