Measuring Land Use Carbon Flow

A bipartisan group led by Senators Harkin and Grassley is questioning the EPA's ability to measure net carbon flow from land use changes:

... A bipartisan group of 12 U.S. Senators led by Tom Harkin (D-IA) and Chuck Grassley (R-IA) called on the Environmental Protection Agency (EPA) not to propose regulations assuming that greater U.S. biofuels use would increase carbon dioxide emissions.

The senators argued the data and methods for calculating such “indirect land use changes” such as from forest or grassland to crops are not adequately developed, and thus should not be used in ways making it harder for ethanol and biodiesel to meet requirements of the Energy Independence and Security Act of 2007 for reduced carbon emissions from advanced biofuels under the Renewable Fuel Standard (RFS). ...

I'm confused. As I've understood it, a lot of farmers are pretty keen to sell carbon offsets, and one presumes that these would also be predicated on measuring the net carbon flow of land use changes. Perhaps from crops to grassland, for example.

If government scientists at regulatory agencies, and the suggestion is that it's a problem with the state of the science itself, are incapable of calculating net carbon outflow to a land ecosystem, why should they be capable of calculating net carbon inflow to a land ecosystem?

... [Tom Vilsack] also seemed to be echoing some of the comments by environmental groups and Senate Agriculture Committee Chairman Tom Harkin, his chief sponsor for the secretary’s position, during the 2008 farm bill debate.

Those groups and Harkin have argued that traditional farm program payments should be dismantled and replaced with “greener” conservation program payments that would reward producers for reducing the carbon footprint in the farming practices. Markets for trading “carbon credits” have already sprung up in some areas. ...

I understand that slightly different things are being discussed. However, the underlying bodies of knowledge have a lot of overlap.

If agricultural carbon credits are to be a sound investment and, more importantly, to provide true added value to the goal of reducing global carbon emissions, the scientific models for describing land use changes and the physical/economic implications of their indirect impacts must be sound. If these are in truth poorly understood, then we're nowhere close to being able to credit farmers for increasing carbon sequestration in their farming practices.

Sen. Harkin is one of my favorite Senators and I rarely find myself disagreeing with him, but I don't really get where he's going with this line of reasoning.