How so? A net negative compared to what? Any facts to back it up or are we just using "emotional science"?
Say you have 1 unit of carbon dioxide absorbed by a plant over its lifetime.
The atmosphere gets about 0.5 units back in the same time period.
delta = final – initial = 0.5 – 1 = –0.5 units.
In source/sink terminology, plants are sinks, not sources, of carbon dioxide.