I worked under an NSF grant studying GW problems @ U of M biostation for two years. There was a researcher there doing experiments on forests as carbon sinks, of course, going in on the premises they would be carbon sinks. You should have seen the squirming and backtracking and making up of false reasons when it turned out forests were net carbon sources.
The discussion section of that paper was sheer fiction because they were trying to make up reasons why their results didn’t match their forgone conclusion, ya don’t want that grant money drying up ya know!
Moreover, they are so caught up in their insanity they can’t even acknowledge fundamental facts. There is only a fixed amount of carbon on the planet. So, any annual carbon budget, or carbon cycle analysis will be dealing with the same amount of carbon. The issue then is where does it go. That poor schlub who found trees were net carbon sources failed to work with the larger picture. Whether a sink or a source, the tree is either putting more carbon back into the soil, or drawing it out of the soil, or out of the air and the water in the soil and air. If he did it properly, the total picture would net out to zero and he would have an honest appraisal of how carbon gets partitioned in the environment by trees.
Even then, if you take a closer look, you will find fascinating differences in carbon utilization among different species of trees, as well as the microorganisms associated with them.