C and N isotopes

Sieg suggested we check out the UC Davis Isotope lab which nitrogen and carbon isotope sampling for pretty darn cheap ($8.50). Here’s the website and details: http://stableisotopefacility.ucdavis.edu/13cand15npricing.html.

In theory, we should be able to identify the source of the nitrogen (fertilizer versus from the air via soil biology). As a estimate of the biological activity of the soil, this is an effective and relatively inexpensive addition to our suite.

We would need to moderately grind, weigh, dry, and pack the sample before shipping so there is some minor prep work. It also takes about 10 weeks to get the result, so this will no provide immediate feedback!

I called the lab with some questions and here’s what I got back:

  1. How accurate does the weight need to be?
  • depends on material
  • if we need to calculate absolute %N and C of the sample, weighing down to .001g is suggested.
  • However, if we’re just interested in the ratios of N isotopes and C isotopes, then we only need to get down to .1g. In our case, we’re really just interested in those ratios, not the absolute values.
  1. how mashed up does the sample need to be?
  • again, depends…
  • incomplete combustion isn’t the reason for grinding (that’s good), the reason to grind it is based on ensuring a representative sample, that’s all. In our case for plant material, that won’t require a ton of grinding - we just need to make sure the sample we’re submitting is representative of the object.
  1. Does it need any other prep work?
  • Drying is important for shelf stability. You can freeze it, but you still need to dry to avoid mold growing. The water isn’t an issue from the standpoint of the test itself, it’s just bad because it can cause molding.
  • For us, we could probably prep all samples, put them in the freezer, then once we have a full 96 well plate, dry them all together before shipping.
  1. What other factors do we need to know to reduce error?
  • be care of nitrogen contamination (from nitrile gloves for example).
  • avoid punctures in the capsule as powder and sample can come out.
  • optimal sample weight should be ok. Doing dual C and N the calculator is good. Only N tends to overshoot

@DanT I think you’d enjoy this and I’d be curious for your opinion.

Just to add in here:

This is an interesting study showing direct correlations (not great, but significant) between C and N in plants and presence of microbial/nematode communities in the soil… this is also all in the natural environment - so there may be significantly more variation in fields (till versus no-till).

But I can’t find any compelling articles showing differences between till and no till, or long-term organic versus long-term conventional fields…

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I found this article that I think shows the potential of stable isotopes in determining soil organic matter stability under different agricultural management regimes: http://www.sciencedirect.com.proxy2.cl.msu.edu/science/article/pii/S0038071715001819?via%3Dihub

I’m not sure how important it is to know the source of Nitrogen, but knowing how stabilized soil organic matter is can go directly to the quality of soil organic C (i.e. active C pools may mineralize quickly while more recalcitrant pools have a greater impact on soil structure, water holding capacity, nutrient cycling, etc).

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Interesting - well, it doesn’t cost much more to get both C and N. They are not quick turnaround tests, but if our target is to establish connections between production practices, soil quality, and food quality, seems like this adds useful data to the mix.

@snapp Sieg can you give your thoughts on the utility of the N and C isotope analysis? Especially Dan’s reference as a way to differentiate the C pools… have you all done that in your work?

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this is really interesting - it fits with some work we started to do but the math is a little hairy - definitely worth pursuing i think…also an interesting point is do we want stable C always, no! We want some POM (unstable C and N eg recent residues) to release N and some ‘stable’ C eg starting to stablize and become processed into sequestered C…

Ok - here’s a great example where we can leverage this data flow to answer related interesting questions. To your point, I wonder if we could identify the optimal C or N isotope ratios which produce the best produce, or (with some additional data collection) the healthiest plants. It’s a tall order and would require either 1) a thoughtful sampling set or 2) a crap ton (see here for conversion of crap ton to other units) of data, but it’s possible.

Is the math hairy like don’t do it, or hairy like you can do it but it’s a pain?