Ever wonder what the relative capture of CO2 is from an acre of mature forest vs regrowth vs an acre of pasture or corn? For forests it goes like this:
After harvest, for a few decades, forests are net sources of Carbon (C) to the atmosphere. That is because the C capture from the atmosphere is low (no trees, low net primary productivity, NPP) and the C loss from the ecosystem to the atmosphere stays the same or increases after harvest because soil microorganisms keep eating soil organic matter and respiring it as CO2.
Then, at some point a few decades after harvest, things switch because tree C capture from the atmosphere increases to the point where NPP is greater than C losses from soil microbial decomposition. After this switch, for hundreds of years, forests can be net sinks for atmospheric C.
In theory, at some point, an old growth forest should have net C capture of zero as inputs from NPP balance outputs from microbial respiration. In reality, there haven’t been many observations of this theoretical steady state – most forests have net C capture for centuries, but it becomes low in the oldest forests.
Rain fed agriculture, row crop or pasture, almost always has lower annual NPP than the adjacent mature forest vegetation. However, whether an agricultural system has negative or positive net atmospheric C capture (the balance of NPP inputs and microbial respiration losses) depends a lot on how management practices have changed soil practices recently.
Under some circumstances, reducing tillage can briefly turn ag systems into a C sink that accrues atmospheric C for a decade or so. Conversion from annual to perennial vegetation, row crops to pasture, is even better – creating a large sink for atmospheric C for several decades by increasing storage of soil C.
Like forests, row crops and pastures that have been under the same management regime for long times capture zero atmospheric C because NPP inputs to soil are balanced by microbial outputs. (This assumes all of the harvested product returns to the atmosphere as CO2 quickly when humans eat them or burn them for fuel). The time scales are shorter for these ag systems. While forests might take centuries to reach this steady state, ag systems might take less than a century. (Jason Kaye, Professor of Soil Biogeochemistry, Penn State University)
For a comparison of carbon storage and accumulation in US Forests based on region and forest type check out the following paper by the US Forest Service: CarbonStorage and Accumulation in United States Forest Ecosystems
A 40 acre woodlot of relatively mature (50-year old) deciduous trees absorbs, or sequesters, approximately 30,000 pounds of carbon dioxide per acre in a typical year. And would be emitting about 22,000 pounds of oxygen per acre. The contribution varies with the age of the forest and species involved. (Timothy J. Fahey, Ecology Professor, Department of Natural Resources, Cornell University)
The EPA has calculated the amount of carbon dioxide emitted by the average car at about five metric tons, more than 11,000 pounds, so a single acre of woods would be countering the emissions of about 2.7 cars. For 40 acres, that would be about 109 cars.