Be advised....that some of the commentary indicates some flaws...ie...see # 13 just above and Palmer's comments.
Ok, I figured I should read the paper, rather than one sentence. So I did and it can be easily disproven. Vegetation takes up lighter C12 in preference to the C13 isotope. The annual cycle of the C12/C13 ratio shows the same rise and fall as CO2 overall. That means vegetation in the northern hemisphere is absorbing CO2 in the spring and summer and releasing it in the winter.
The thesis of the paper is that warmer oceans are releasing CO2 in our winter (southern hemisphere summer, we will be closest to the sun on Jan 3rd). Unlike plants, the ocean takes up carbon irrespective of isotope. So the ocean is not involved to any great extent.