You are part of a research team tasked to learn as much as possible about the C

Question

You are part of a research team tasked to learn as much as possible about the C budget of a 300 year old temperate forest that grows on a soil profile only 50 cm in depth. The average d13C value of the forest’s biomass was 27‰, but then the forest was exposed to a source of CO2 that was isotopically labeled with 13C for ten years. The label was 20‰ greater than the average 13C value of atmospheric CO2, 8‰. One of your jobs as a team member is to compute the fraction of soil organic C (SOC) in the soil profile that represents photosynthate formed after the tracer was applied. You sample soils across their 50 cm depth, and observe that their average 13C value is 12‰ (bulk soil). Assume the forest trees exposed to the tracerladen CO2 discriminate against 13C to the same extent as trees exposed to CO2 in ambient air. a) What proportion of the SOC originates from photosynthate formed during the previous ten years, and what proportion of the SOC originates from before that ten year period? b) Would you expect soil volumes supporting dense root networks to exhibit d13C values greater or less than soil volumes with fewer roots, and why? c) A colleague on your team captured CO2 diffusing off the top of the soil surface in year 10 of this experiment, and observed that respired CO2’s 13C value was, on average, 9.5‰. What is the maximum proportion of soilrespired CO2 emanating from root respiration? Assume no isotopic discrimination during either autotrophic or heterotrophic respiration.

Explanation / Answer

Soil respiration refers to the production of carbon dioxide when soil organisms respire. This includes respiration of plant roots, the rhizosphere, microbes and fauna.

Soil respiration is a key ecosystem process that releases carbon from the soil in the form of CO2. CO2 is acquired from the atmosphere and converted into organic compounds in the process of photosynthesis. Plants use these organic compounds to build structural components or respire them to release energy. When plant respiration occurs below-ground in the roots, it adds to soil respiration. Over time, plant structural components are consumed by heterotrophs. This heterotrophic consumption releases CO2 and when this CO2 is released by below-ground organisms, it is considered soil respiration.

The amount of soil respiration that occurs in an ecosystem is controlled by several factors. The temperature, moisture, nutrient content and level of oxygen in the soil can produce extremely disparate rates of respiration. These rates of respiration can be measured in a variety of methods. 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.

Soil respiration rates can be largely affected by human activity. This is because humans have the ability to and have been changing the various controlling factors of soil respiration for numerous years. Global climate change is composed of numerous changing factors including rising atmospheric CO2, increasing temperature and shifting precipitation patterns. All of these factors can affect the rate of global soil respiration. Increased nitrogen fertilization by humans also has the potential to effect rates over the entire Earth.

Soil respiration and its rate across ecosystems is extremely important to understand. This is because soil respiration plays a large role in global carbon cycling as well as other nutrient cycles. The respiration of plant structures releases not only CO2 but also other nutrients in those structures, such as nitrogen. Soil respiration is also associated with positive feedbacks with global climate change. Positive feedbacks are when a change in a system produces response in the same direction of the change. Therefore, soil respiration rates can be effected by climate change and then respond by enhancing climate change.

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