Atmosphere Level

Canopy Level

Leaf Level

Soil Level

Mass spec.

References

Though the locale of the missing carbon sink is currently being sought, it is known that the soil organic carbon pools maintain twice as much carbon than the atmosphere, and three times the carbon hoarded by vegetation. (Andrews and Schlesinger 2001). This does not mean that all of terrestrial earth is a sink. The difference between carbon sequestration and soil respiration determines whether a particular area is a source or a sink. In fact, soil microbial respiration "constitutes a carbon flux 10 times greater than that from fossil fuel combustion and 2.5 times greater than the delivery of litter to the soil surface (Andrews and Schlesinger 2001)." A positive difference between sequestration and respiration indicates a source while a negative difference implicates a sink. To complicate things further, climate variability can differentially affect rates of sequestration and respiration rendering a source a sink and vice versa.

In light of post-Industrial Revolution atmospheric CO₂ levels and deforestation, potential changes in soil processes, including source-sink behavior, are of interest. Carbon allocation to soils greatly depends on plant processes both above and belowground. Experimental evidence points toward elevated CO2 levels enhancing photosynthesis and stem, leaf, root mass in various species (SERC 1999) This, coupled with the fact that North American temperate forests currently exhibit rapid growth rates compared to twenty years earlier, translates into large amounts of carbon sequestration. Root and microbial respiration are also up.

This enhanced rate of growth has been attributed though not exclusively limited to elevated atmospheric level. Other factors influencing this growth trend include forest re-growth, and a warmer Earth. (Ramanujan 2002) Most recently, precipitation has been cited as a driving force. According to a NASA modeling study, the United States saw and 8 percent increase in average annual precipitation from 1950 to 1993. The study linked the increased precipitation and humidity levels to a 14 percent increase in plant growth. (Ramanujan 2002)

Plant partitioning under elevated carbon dioxide may result in a greater percentage of carbon stored in roots belowground (Norby 1993). Zak et. al 1993 contend fine root production and overall subterranean biomass may result. This biomass increase coupled with addition of above and belowground detritus could potentially increase the C soil inputs. The question then becomes what is the effect of these carbon inputs on belowground carbon cycling? How is the soil organic carbon pool affected? ¹³C Stable isotope analysis of soil carbon inputs proves useful in these determination.

Under conditions of deforestation, soil organic matter (SOM) simultaneously changes in two ways:

1. Mineralization of native humus continues and is not renewed upon deforestation

2. Input of C into SOM from crop remains coupled with the native C loss from deforestation

(Cerri et. al 1990)

If the deforested vegetation are of the C₃ photsynthetic pathway, reforestation with C₄ plants yields a different ¹³C signature. This is because C₃ plants have a d¹³C = -28‰ vs C₄ = -13‰. Cerri et. al 1990 describe such a system in which a forest has been replaced with sugar cane crop:

Figure 1:

(Cerri et. al 1990)

Gerzabeck et.al 1990 use ¹³C to characterize soil humus dynamnics. They found that high molecular weight fractions of humus in the Ol and Ah horizons exhibited less negative d¹³C in two locations, with the low molecular weight in the Ol horizon exhibiting a more positive d¹³C. Non-humic substances in both horizons "were isotopically heavier than the humic substances of the second maximum," suggesting greater microbial action and hence enhanced ¹³C (Gerzabeck et.al 1990).

Figure 2:

Andrews and Schlesinger 2001 examined soil dynamics over a three year study of Free-Air CO₂ Enrichment (FACE) in sections of the Duke Forest. The researchers subjected three of six 30m diameter plots populated with 15-year-old Loblolly pine to elevated CO₂ levels relative to the other 3 ambient CO₂ plots. 13C label in the experimental plots appeared belowground from enhanced root and microbial respiration. In fact the 55% increase in atmospheric carbon translated into a maximum daily soil respiration increase up to 131%. Overall Schlesinger and Andrews observed a 27% increase in soil respiration (Andrews and Schlesinger 2001). They also found that the greater soil CO2 concentration enhances both soil weathering and acidification by increasing mineral cations by 271% and alkalinity by 162%. That soils respired 27% more implies soils may not be an effective global warming carbon sink despite enhanced belowground biomass storage.