In wet, northern hemispheric soils, there is a strong body of work which suggests that the quantification of soluble N in pore water (or soil solution) is the most appropriate method of assessing point-in-time N availability via lysimeters, rhizon samplers, or centrifugal extraction. More recently, Erich Inselbacher's microdialysis methods also looks promising.
However, in dry soils (such as those covering Australia, my study area, for much of the time) extraction of pore water is impossible. Traditional salt extracts (KCl / K2SO4) probably don't give that accurate a picture of what was actually plant/microbe available, and significant changes in analyte chemistry have been observed over the duration of a water extract. Also, extracts totally lose any effect of soil structure on what is extracted.
So, are there any other methods that may be more analogous to pore water extraction than KCl extracts, but are suitable for dry soils?
I think the type of analysis you do is extremely dependent on the precise question you are trying to answer. Coming from (relatively) wet New Zealand, I'm sort of guessing that you are trying to assess how much N might become available for plant growth after rainfall and/or flooding. Is this right? If so, I think it would be hard to go past doing an N mineralisation rate test X time and soil moisture. This could be done either at typical soil temperature, or higher if you wanted a faster result. Hope this helps.
Thanks for your answer. I do actually plan to also do some basic PMN / net mineralisation assays on these soils also (to be collected from across Australia). It will be interesting to see just how related the results from various different methods turn out to be.
Dear Mark, I never work in dry ecosystems (a real pity) but do you know the work of Kate Lajtha: The use of ion-exchange resin bags for measuring nutrient availability in an arid ecosystem (http://link.springer.com/article/10.1007/BF02371147#). Especially for a comparative methods I would try it.
Best greeting from rainy Germany
Florian
Hi Florian, thanks for the suggestion - I shall definitely look into the use of resin, that is one technique that I hadn't considered. Are you aware of any work investigating how suitable such a method might be to capture DON (bulk, and aminos / peptides)? I'm guessing some of the more polar compounds may be captured by the resin, but not all.
Hope all is well. Currently in a cold Austria... I get to head back to the warmth and dry soils tomorrow.
Cheers,
Mark
We used various extraction methods to examine C and N from a rain fed (1000 mm yr) tillage and cropping experimental field here in Texas. (Carillo-Gonzalez et al 2013 - Geoderma). I found it interesting how much the different extractants differed in their recovery of N species. We recently used a 0.1 M HCl for dryland soils in Ghana and did a subset with DDW extracts - but I need to look at how they compare for N species. Ultimately I agree there are no standard methods for getting at the plant/microbial available N that I have been able to find.
Hi Mark,
I'm not very sure how dry is your region, but we are working in Spain in a semi-arid region and the KCl method works very well in our soils for irrigated summer crops or for rainfed winter ones. If you want to have a look to our publications...
Whether wet or dry zone I think that the hunt for a suitable method to assess available N is still ongoing. Monitoring mineral-N in pore water may not necessarily indicate the extent of N availability in wet areas. Sometimes assessing potentially available N (or mineralisable-N) by incubation may be helpful (e.g. 7 day anaerobically mineralisable-N @400C). Please see my publication (Technical Report: An evaluation of some laboratory methods of assessing availability of nitrogen in agricultural soils) on this area.
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