The answer, at least as far as 13C abundance goes (expressed as d13C values v. VPDB) depends on two things; (a) C3 or C4 plant; (b) leaf matter, seed or extracted bio-organic compound, e.g. flour, sugar, oil.  While there are stark differences between C3 and C4 plants due to the differences between Calvin-Benson and Hatch-Slack type CO2 fixation, within a given plant there are subtle yet measureable difference between e.g. leaf matter and seed oil or fruit sugar.  Approximate d13C values for C4 plants like maize and sorghum range from -10 to -15 ‰ while for C3 plants like apple, sunflower and beans you are looking at a range from -24 to -30 ‰.

When it comes to 15N abundance the question is not whether the plant belongs to the C3 or C4 family, N content and therefore 15N abundance depends on plant tissue.  While plant tissue such as root, stem and leaf will contain N, plant products such as flour (a.k.a. starch), sugar or oil contain very little (flour) or none at all. The range of d15N values found in plants extends from -10 to +10 ‰ but this can be narrowed down to -8 to +3 ‰ for most terrestrial plants with the exception of N2 fixing plants (e.g. legumes); the typically express an even narrower d15N range from -2  to +2 ‰.

If there don't pop up data for Sorghum, you could consider adding data from a comparable C4 plant (with the same type of photosynthesis that results in a relatively low discrimination against 13C) broomcorn millet (Panicum miliaceum), depending on the error margin you are willing to accept.

For the Netherlands, an initial project included two millet samples (and a total of 22 archaeological samples of charred plant remains), which were measured for delta13C and delta15N after acid-only, acid-base-acid and no pre-treatment. Both samples date to the Iron Age (800 - 12 BC). I can provide you with the isotopic data on request.

Our measurements also indicated an interesting trend through time for the total group of cereals, which we are currently investigating further with more samples. If this remains the case, it will be very hazardous to include data from greater distances (in time, but possibly also in space) into models for particular sites.

With kind regards,

Otto Brinkkemper

Maybe you know it, did you checked the sorghum standard provided by IVA?.

Many thanks to those that responded. Rather than applying an approximate value, I felt an actual published value would be appropriate. The IVA standard value suggested by Jose is one I was not aware of and fits those values predicted for a C4 plant. I recognise the ability of fertilisers to contribute significantly to N isotope signatures in plants and will deal with these values accordingly in my model. Thanks

obviously sorghum is a C4 plant, C13 similar to the values obtained on maize in the same area, but the values N15 depends, of fertilizer ability to contribute significantly to N isotope signatures in plants ... sorghum is not atmospheric N2 fixing plant.

The easiest way to solve your problem is to measure some sorghum directly. The isotope lab at UCT should be able to do that and I believe there is a new lab in Pretoria. One thing to be aware of is the carbon isotope difference between modern plants under the new higher pCO2 fossil-fuel regime today, and that even 50 years ago. So if you want only to address a current research issue modern values are fine but there is up to a -2 per mil difference between C4 plants grown now and those grown pre-fossil fuel (many people now standardise to 1750AD). Also nitrogen isotopes  vary depending mainly on the type of soil and nature of its nitrogen cycle; manuring is a subset of the latter.