Estimating C:N ratios of different woods

Hi everyone!

I have had this idea cooking around for a while but essentially I have always been bugged by the huge ranges given for the C:N ratio of sawdust and other older wood chips. 500 - 100:1 does give me some idea but I wanted to refine it down further. I had the idea to try and use mBTU values as an estimate for the C:N ratio of mature hardwood heartwood, but the data is pretty lacking. Wood Density would be the main portion of this connection, having positive correlations to both C:N ratios and mBTU values. I had to do a lot of extrapolating to get some answers but I did come up with a rough equation, given C:N 750 - 400:1 and mBTU between 31.7 and 13.0.

C:N = 18.7(mBTU) + 157

The value outputted would be the Carbon value with the Nitrogen value at 1. I have an entire spreadsheet for my math and I also wrote up a fact sheet with a reference table for this, but I wanted to hear some preliminary thoughts here. Do you guys think there is any merit here? I know it may not be very useful for every situation but that's okay.

Also if anyone has any data on more specific C:N ratios of different tree species? If I could find species specific data for that then I could refine this equation. With more data, I could also add in a constant for conifers and nitrogen-fixing trees. I also believe this pattern may be able to extrapolate out to smaller diameter woods like ramial, but again more data would be needed for that.

Eric S.

I would bet that the C:N ratio varies more within species, by part of the tree, than it does between species. Smaller the stick, the lower the C:N ratio. Nitrogen is much higher closer to growing tissues (vs structural wood). Cambium under bark, and growing tips in proximity to leaves will have much higher N than the trunk in any tree.

On a complementary note, a higher than 25:1 C:N ratio only creates N deficits in soil directly in contact with wood in a bubble a fraction of a MM thick. The interior high C wood of a log provides a sponge for water and excess water soluble nutrients that will be released back into soil over time, but does not actively suck out N from soil beyond that little bubble.

So I am more and more inclined to use woody debris in as close to its living form as possible, as twigs are close to 30:1, and big stuff is an excellent spongy habitat for Fungi, insects, and other decomposers, as well as their predators that actually release most nutrients in plant available forms. This seems much more in line with what natural forests do, and likely supports more biodiversity while gassing off fewer nutrients. I am also sick of fixing wood chippers multiple times per use. This is not an answer to your question;)!

Hi Ben,

Yes, that is also correct! The branches below 7 cm in diameter are considered to be ramial wood chips and those generally have a C:N of 170 - 30:1. A much lower carbon content and a much higher nutrient content when compared to older wood chips or sawdust. New growing tissues will have the highest levels of nitrogen as it is a mobile nutrient and critical for vegetative growth.

I'm not too worried about Nitrogen tie-up and I am a big fan of using all sorts of coarse woody debris whenever I can!