I found a couple references regarding carbon isotopes.
"The studies are based on the ratio of the three different carbon isotopes in atmospheric CO2. Carbon has three possible isotopes: C-12, C-13 and C-14. C-12, which has 6 neutrons, is by far the most prevalent carbon isotope and is a stable isotope. Carbon 13 is also a stable isotope, but plants prefer Carbon 12 and therefore photosynthetic CO2 (fossil fuel or wood fuels) is much lower in C-13 than CO2 that comes from other sources (e.g.: animal respiration) Carbon-14 is radioactive. Studies of carbon isotopes in CO2 has resulted in the following findings "

* There has been a decline in the 14C/12C ratio in CO2 that parallels the increase in CO2. In 1950 a scientist named Suess discovered that fossils do not contain 14C because they are much older than 10 half lives of 14C.
* There has been a parallel decline in 13C/12C ratio of atmospheric CO2. This has been linked to the fact that fossil fuels, forests and soil carbon come from photosynthetic carbon which is low in 13C. If the increased CO2 was due to warming of the oceans, there should not be a reduction in the ratios of C-13 and C-14 to C-12.
http://environmentalchemistry.com/yogi/environmental/200611CO2globalwarming.html

Another reference:
Carbon-14, which has a radioactive half-life of 5,730 years, can be used to determine the rate of carbon exchange between the atmosphere and the oceans and land. Carbon-14 is produced naturally in the atmosphere when cosmic-ray neutrons hit nitrogen-14. The concentration of carbon-14 in atmospheric carbon dioxide remained fairly constant for thousands of years. However, between 1954 and 1963, when atmospheric nuclear tests were conducted, the carbon-14 in the atmosphere doubled. After the tests were halted, the elevated concentrations began to decrease as atmospheric carbon dioxide—which includes the elevated carbon-14 fraction—exchanged carbon into other not-yet-elevated carbon reservoirs in the oceans and on land.
This infiltration of nuclear-test carbon-14 into carbon reservoirs provides scientists with valuable information on carbon exchange. Once they know the rate at which nuclear-testing carbon-14 moves through the carbon cycle, scientists can determine the rate at which human-induced carbon dioxide is absorbed and released, because the same physical rules govern the transfer processes.
According to Brown, the CAMS spectrometer is one of only a few in the world that can measure radiocarbon with the precision and throughput required for a project such as the carbon-aging study....."

"In the Amazon basin study, the project team determined a sample’s age by comparing its carbon-14 concentration to atmospheric concentrations over the last 40 years. The team’s comparisons showed that in most of the Amazon samples the carbon-14 was only slightly higher than the atmospheric concentration at the time the samples were taken. That is, the level of carbon-14 in most Amazon samples corresponded to the atmospheric level about 5 years prior to the sampling.
To clarify the data on carbon aging, the project team also analyzed the carbon-13 abundance in the Amazon samples. The basin’s rivers receive significant drainage from high-elevation areas, which are formed of very old carbonate rock. The amount of dissolved carbon dioxide from carbonate rock may affect the measurements because the carbon-14 concentration in these old formations is essentially zero."
....
"For instance, carbonate rocks generally have carbon-13 concentrations about 25 parts per thousand higher than trees, and most grass plants have carbon-13 concentrations about 10 parts per thousand higher than trees. By comparing the abundance of carbon-14 and carbon-13 in the Amazon basin samples, the project team could account for these influences on the apparent carbon-14 ages of affected samples. These measurements indicated that carbon from the carbonate rock was flushed from the water into the atmosphere as it flowed downstream. By the time the water reached the lowlands, most of the carbon had returned to the atmosphere.

It concludes with:
Balancing the Carbon Budget
“This detailed study of the Amazon basin gives us a clearer picture of the role played by streams and rivers in the carbon cycles of tropical regions,” says Brown. Most of the atmospheric carbon dioxide taken up by the Amazon basin in a given year is not sequestered for decades to centuries but, rather, is returned to the atmosphere as carbon dioxide on a time scale of 5 years. The study also provides information for researching how land use may affect climate. By more precisely measuring the carbon cycle in the Amazon basin, the project team has taken another step in helping scientists understand how atmospheric carbon dioxide concentrations affect the global carbon cycle and the world."
http://www.llnl.gov/str/March06/Brown.html

I don't draw a lot conclusion from these reference, other than to note the first reference didn't mention the atmospheric nuclear tests, which obviously answers the "There has been a decline in the 14C/12C ratio in CO2"
And this seems to show quite a degree of ignorance, especially consider how important this would be in determining human emissions.
And second it seems that not much testing is done, and that for precision there is lack of these "CAMS spectrometers" that are required. And that one can hope considering the significant of such studies more facilities will be made available to allow more of this kind of study to take place.