When a calorie of food comes from a nut tree in a perennialpolyculture there is a Carbon Sequestration number calculated based on Carbon locked up in soil OM. However, are there calcuations that take into account the carbon emission offset that is created when that calorie replaces a calorie produced in carbon emitting ag, like industrial annual mono-cropping? Or do the various carbon sequestration numbers for various practices take this into account? I know that this is a slippery number, as every calorie has a different carbon number (+ or -), but perhaps an average conventional ag calorie C# can be assumed and utilized?
I feel that without taking this into account we are vastly under-estimating carbon farming's potential climate impact.
Secondly, if we are carbon farming wood for calories AND for heating, the same offset calculation should be utilized, every BTU of wood heat is offsetting a BTU of methane.
I think I see what you are getting at here. Offsetting has a particular meaning in carbon accounting (and has been described as like buying medieval indulgences), but I don't think this is what you are talking about.
Basically, it's simply not how carbon intensity is calculated. There are range estimates and means calculated for production of various foodstuffs under different conditions, typically including all known inputs, plus transport, wastage and so on. There is a discussion on another thread for how to calculate carbon sequestration under various agricultural techniques, including food forestry, but you also need to take inputs into account. I think you would struggle to find a food that is carbon negative, although with food forests we can move in that direction. A nut from your home garden may well be, but we are not at the point of accurate measurement because we don't yet have reliable, comparable means of comparing carbon sequestration in soils.
The same is true for natural gas being burned, natural gas in leaks (although it's starting to look like this has been grossly underestimated, but we know how much is being sold), and estimates for biofuels, including wood burning, which creates its own environmental problems (at a global level chopping of fuel wood is a major source of deforestation, but different issues apply in small-scale coppice; both include issues of soil depletion and so on). Wood heating is not "good": on a small scale it can be less problematic than using fossil fuels, but it should not be considered a renewable resource, especially on a large scale. Burning wood is not a negative on the ledger: it's theoretically neutral in a perfect situation, which you are unlikely to find. It may reduce the amount of fossil fuels being burned, but you have to take other factors into account.
Good question. Most of the studies that were available for me to work with just look at the amount of carbon in the soil and or in aboveground biomass. Another way of doing this is life cycle analysis, which also looks at emissions from transportation, chemical fertilizers, integrated livestock, etc. Lifecycle analysis is a much more complex and much more useful for providing the kinds of comparison you're talking about. I haven't been able to find a whole lot of lifecycle analysis studies of carbon farming systems, though I hope more will be coming. That kind of study does permit a direct comparison between say conventional agriculture and an agroforestry system. Was clear even from the studies I do have is that it is possible to have much much better results than conventional agriculture, and to turn agriculture from a source of emissions to a net source of sequestration and mitigation.