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.

How would one go about calculating the answers to questions a and c?

Explanation / Answer

ANSWERS:

CO2 can act as a resource by increasing carbon fixation rates in some photosynthetic organisms. The degree to which this occurs is dependent on the carbon capture strategies and the degree to which carbon is limiting .The relationship between aqueous CO2, photosynthesis and growth is not simple, because not all photosynthesizing species require environmental CO2 for their source of carbon (C). The majority of marine algae have carbon concentrating mechanisms (CCMs) that facilitate the active influx of CO2 and/or bicarbonate ions (HCO 3 ) and elevate C concentrations at the site of C fixation with few algae being CO2-only users. Despite the prevalence of CCMs, evidence suggests that many algae do respond positively to increasing CO2. Indeed, the ability of some algae with CCMs to benefit from enriched CO2 lends insights into the potential mechanisms for CO2 effects. Species with CCMs can shift away from HCO 3 towards aqueous CO2 when CO2 levels are high. As human activities modify environmental conditions, and therefore resource availability, some species of algae may be released from carbon limitations while others are not. This mismatch has the potential to affect competitive abilities and alter community structure. Moreover, the effects of these shifts would be particularly profound if key functional groups, whose interactions structure entire communities, experience contrasting resource limitations. Indeed, there are a large number of resources that constrain the abundance of marine and terrestrial plants [36] and determine the composition of space-holding species. Combinations of two or, sometimes, three of these limiting factors are often incorporated into models to account for the diversity and composition of plant communities. The hypothesis that phase-shifts towards mat forming algae are likely to be more common under conditions of high [CO2] is therefore of particular interest. If this model has validity, then enhanced CO2 should cause mats to increase their extent and productivity in both temperate and tropical systems. Supporting evidence requires field observations of natural variation in [CO2] to provide insights into ocean acidification effects at the ecosystem level.

grass has hard, sturdy, hollow stems that may reach 3 meters in height. They grow from a network of woody rhizomes and tough roots that form a sod. The roots penetrate over 3 meters deep into the soil .The leaves have sharp, serrated edges The panicle may be up to 50 centimetres long and may have many branches. Each spikelet is up to 2.5 centimetres in length. This grass can spread via its rhizome, producing large monotypic stands. This plant can grow in a variety of habitat types, but it is a facultative wetland species, most often found in wet habitats. These include fens, wet prairies, rivers ,floodplains , ponds ,moraines , and marshes. The grass is tolerant of water, but it does not tolerate prolonged flooding. Its dense root network stabilizes soil, even in areas where it would be eroded by flowing water. This species has been investigated as a possible source of biofuel.

An emerging paradigm is that root traits that reduce the metabolic costs of soil exploration improve the acquisition of limiting soil resources. Here we test the hypothesis that reduced lateral root branching density will improve drought tolerance in maize by reducing the metabolic costs of soil exploration, permitting greater axial root elongation, greater rooting depth, and thereby greater water acquisition from drying soil. Maize recombinant inbred lines with contrasting lateral root number and length (FL: few but long; MS: many but short) were grown under water stress in greenhouse microcosms, in field rainout shelters, and in a second field environment with natural drought. Under water stress in microcosms, lines with the FL phenotype had substantially less lateral root respiration per unit axial root length, deeper rooting, greater leaf relative water content, greater stomatal conductance, and 50% greater shoot biomass than lines with the MS phenotype. Under water stress in the two field sites, lines with the FL phenotype had deeper rooting, much lighter stem water 18 34 O signature signifying deeper water capture, 51 to 67% greater shoot biomass at flowering, and 144% greater yield than lines with the MS phenotype. These results entirely support the hypothesis that reduced lateral root branching density improves drought tolerance. The FL lateral root phenotype merits consideration as a selection target to improve the drought tolerance of maize and possibly other cereal crops.

. Soil respiration, which is the flux of CO2 from soils to the atmosphere, is thus an important component of the ecosystem C budgets and is a major source of CO2 released by terrestrial ecosystems. Soil respiration is the result of the production of CO2 from the biological activity of roots and associated microorganisms and the activity of heterotrophic bacteria and fungi living on litter and in the root-free soil. Different sources of soil CO2 efflux are known to experience high spatial and temporal variation with different controlling factors involved on different time-scales. However, up to now not so many studies have deal with the internal variability of soil respiration and its components and only few of them were performed in grassland ecosystems despite the fact that it is one of the world’s most widespread vegetation types which comprises 32% of the earth’s area of natural vegetation. Methodological approach based on pulse labelling of plants in artificial 13CO2 or 14CO2 atmosphere was used to found out the speed of the cycling of C in grassland ecosystem (in situ) as well as to study the effect of different plant species, plant growing stages, and different nutrient supply on the magnitude of root respiration and on the speed of translocation and respiration of recently assimilated C through roots.

Soil temperature and soil water content exerted a significant effect on microbial component of soil respiration. Being a larger part of total CO2 efflux from soil at Ampler ( 70%), these factors influenced also total soil respiration dynamic on different time scales. Introduction of a management regime have modified however the activity of microbial community by an increase of the quantity of easily available C substrates from the rhizome deposition process, resulting in a general suppression of microbial enzymatic activity and further decrease C mineralization rates.

Related Questions

Navigate

• Browse (All)
• Browse Y
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.