I think the people working in the soil hydrology group of the CNR-ISAFOM should have enough experience to help you with such methodologies either if I am not sure if they have ever used them for isotopic applications. One of the contacts you can ask for is Angelo Basile ([email protected]).

I hope this helps.

Fabio

Contact Dror Avisar from TAU at [email protected] . He gained lots of experience in this issue.

You might find it useful to read some papers by Jean Hsieh from about 10 years ago. She did some very nice work using a direct equilibration method, which is quite simple to do experimentally if you have access to a gas bench.

See, for starters: Measurement of soil-water delta O-18 values by direct equilibration with CO2. Author(s): Hsieh, JCC (Hsieh, JCC); Savin, SM (Savin, SM); Kelly, EF (Kelly, EF); Chadwick, OA (Chadwick, OA)

Source: GEODERMA Volume: 82 Issue: 1-3 Pages: 255-268 DOI: 10.1016/S0016-7061(97)00104-3 Published: FEB 1998

As there is no hard distinction between the composition of free pore water, matrix water and bound water, I think you will not find large differences. Nevertheless you can turn your question around: By using more duplicates measure the isotopes left in your sample at different pressures instead of gathering the outflow water

I am not experice this research but research metherdologe can make you clear of pactical. My idear if you separating different pools of water in the soil you will separating different environment around area.

Thank you for all the answers!

Given that we have access in our lab to a Picarro for H2O, the simplest way would be H2O-H2O equilibration. The alternative of measuring the "remaining water" instead of collecting the outflow water seems suitable. However soil in Richard's chamber is attached to a porous ceramic plate, which in normal use is saturated of water, so I am afraid there is some risk of contamination with this water. Certainly water in free pore and matrix follows a continuum, but some studies suggest that the water tightly attached during rehidration after drought might remain unchanged after subsequent, moderate rain events, and thus may have a different isotopic fingerprint. We suspect that this might explain unexpected values in our xylem water, as compared to whole soil water, obtained by cryogenic distillation, so we wanted to check whether all water pools were similar in our soil.

You might try the papers of Renee Brooks (https://www.researchgate.net/profile/J_Renee_Brooks/).

See this paper which may be related to your question: https://www.researchgate.net/publication/201996382_Ecohydrologic_separation_of_water_between_trees_and_streams_in_a_Mediterranean_climate?ev=prf_pub

Article Ecohydrologic separation of water between trees and streams ...

Hi,

If I understood, you want extract water from the soil for successive analyses. It is not easy, I never did it. however, my suggestion is to work for differences. You can saturate the soil and impose a potential (e.g. - 30 cm) to remove all the water at higher pressure heads. hence, you can extract all the water with a centrifuge. The same procedure can be applied to another pressure head value (e.g. -100 cm), and so on. Differences in isotopic value can be so attribute to the water extracts at different pressure heads (i.e. between - 30 and -100). I think you need replicates. In such a way the water you broadly defined as free, bound, matrix is more correctly related to a specific soil potential (i.e. between -30 and -100 cm, in my example).

I hope you will find this fruitful,

good luck

angelo

P.S. taking care about the neglecting of the water mixing

It's a nice challenge and interesting too, also from a soil physical point of view! I would be interested to hear some of the results. Using the pressure plate apparatus (Richard's chamber) extracts water from your sample when starting from saturation. Then the only possible way it can contaminate your sample is via diffusion. Measure the diffusion coefficient prior to the actual measurement on a dummy çlean 'sample' to be able to take account for this. You can also think of pre-wetting the plate with the same solution as your sample.

Thanks again! The use of centrifuge might simplify the work, and allow processing several samples at a time. We'll check whether we can use this. In any case, I think the idea of the "dummy" sample will be always useful to account for potential contaminations.

Hey Juan, have you published the results already? Very interesting discussion.