Hi Matthew,

we extract chl-a in the dark with methanol and measure it with the photometer at 663 nm, so I can't really give an answer to your question. Anyways, I would like to add one:

I've found that in many publications on biological soil crusts where they measured chlorophyll, people use many different referrence values. Some write µg g-1 soil [e. g. Belnap 2008], some mg m-² [e. g. Kidron and Tal 2012], others µg L-1 [e. g. Zaady et al. 2013]. My question is if there are any good arguments for using the latter? (since most people don't consider the bulk density of the crust in their calculations and in that way add an error by referring to the area rather than the weight of the sample.)

All the best,

Vincent

Do you double extract? Hand grind in mortar & pestle? Use a tissue grinder?

Regarding your question I'll offer some opinions, but with the major caveat that I have never undertaken a study on the relative quality of these different expressions of chl content. These are just based on thinking through the problem.

IF one had a good bulk density measurement, a volumetric expression of chl a makes sense to me.....better than by mass. Per mass expressions will change depending on the characteristics of different soils. 1 g of one soil could sample a greater volume than another soil, which could lead to either a relative over- or underestimations of chl a. But if BD is not available or can only be estimated approximately (as is often the case since it takes time to get an in-field high quality BD estimate that goes along with each chl a sample), then going to volume seems like a source of error. At least one can measure mass to a high precision quite easily. So, both have strengths and weaknesses.

Areal expressions are interesting, since they have a very ecological meaning. One could infer that the results are most directly related to the photosynthetic capacity of the soil surface. Much like the area of a photovoltaic array largely determines the amount of electricity it will deliver. A problem I see here is the difference between flat crusts and bumpy crusts. There is quite alot more soil surface area packed into a 1m2 quadrat of bumpy crust than an equal quadrat of flat crust. Actual measurements are made at a much smaller scale, then extrapolated up to per m2. This would be fine in a flat crust, but underestimate chl a in a bumpy crust.

Yes, we do double extract and grind in a ball mill.

I totally agree with what you said, but my concern is that often, people do not have good BD values and just assume it to be, for example, 1.4 g/cm³. The problem is that the BD of BSCs is highly variable. Our results from the Negev show a range of approx. 1.2 to 1.6 g/cm³ and a study from China (Lan et al. 2012 in Env. Earth. Sci.) could detect an even higher range of 1.04 to 1.81 g/cm³. This is why I am very careful when it comes to "just assuming" that BD is this or that.

I get the importance of the ecological meaning of areal expressions, and I was asking myself if the problem you mentioned with the flat vs. bumpy crust could be solved when the g chl/m² value is related to the relative surface area, but of course this would require the quantification of m² total soil surface/m² soil (for example with a profilemeter, similar to the one used by Kidron 2007 in Catena) or, as new techniques are more and more available, with laser scanning methods (see https://www.dropbox.com/s/942us418t5m77ry/screenshot3.jpg and https://www.dropbox.com/s/tzqo4dca0jyfc7s/screenshot2.bmp).

just found this...