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

Carbon dioxide is released from the soil through soil respiration, which includes three biological processes, namely microbial respiration, root respiration and faunal respiration primarily at the soil surface or within a thin upper layer where the bulk of plant residue is concen- trated 12

14, and one non - biological process, i.e. chemical oxidati on which could be pronounced at higher tempera - tures

15. Processes affecting dynamics of soil carbon are presented. Soil microflora contributes 99% of the CO

2 arising as a result of decomposition of organic matter

16, while the contribution of soil fauna is much less

17 . Root respiration, however, contributes 50% of the total soil respiration

18.Several studies have shown that factors such as soil texture, temperature, moisture, pH, available C (labile and non labile components of soil organic matter), and N content of the soil influence CO 2 production and emission from the soil 19,20.

For root respiration, the source of C is photosyn thates and its translocation to the root; while litter fall, root mortality, application of manures and crop residues provide carbon for microbial respiration in the soil. Soil organic matter includes a wide variety of orga -nic substances ranging from freshly added leaves or manures to substances at varying stages of decomposition

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