Okay, I was a bureaucrat so when there's talk of paying farmers for carbon offsets, I immediately think of how it might be handled by USDA/FSA. And, because I'm stuck in a rut, I'm likely to think of structures FSA has used in the past.
Back in the 1970's there was something which strikes me as a parallel. Under the program then, ASCS (FSA's predecessor) established a "conserving base" for each farm. This was the acreage in "conserving uses" (think of it as hay fields, pastures, grassland). Under the program,as a condition of receiving benefits farmers might be required to increase the acreage in conserving uses for the year (and not break out any new cropland). That seems to me to be a valid model for any future payments for carbon offsets. FSA/NRCS looks at the farming operation, documents what's being done already which impacts carbon sequestration. Call it a "carbon offset base". Then you could pay taxpayer dollars for changes to the operation which increases sequestration. But you'd also have to assess charges if and when a farmer changes her operation and reduces the carbon offset base. Or, assess the operation yearly and make yearly payments.
No doubt I'll have more to say as the subject continues to heat up.