I’ve read that soil samples for soil enzyme assays should be kept cool and at or near field moisture levels. But I cannot find information on how long these samples are viable for under these conditions. Is a month or 2 in the fridge too long to wait to perform the assay? Also, is freezing the samples until they are all ready to be analyzed acceptable, or would this compromise the sample? Reason being I plan on having samples collected from different sites over a 2-3 month time period and need to coordinate sample collection, storage, and shipment. And this leads to my next question…
I’ve also read that comparing soil enzyme assay data from different sites and times is not recommended due to differences in site soil conditions (pH, temp, moisture, etc.) as well as changing conditions over time (rain events, etc.). Is there a way to overcome this, maybe by controlling moisture content? I am essentially trying to compare a treatment effect (+ vs -) on soils under 2 plant types, grown in 3 different climates, and over time following treatment. Any advice here would be appreciated.
Hello Michael
Although the ideal is to use the samples immediately, it is possible to refrigerate them for two months without substantially affecting the activity. Another option you can do is perform the enzymatic extraction and freeze these extracts. I have analyzed frozen extracts (in Bis-Tris buffer) after 1 year and they continue with activity values similar to the original ones.
Regarding your second question, you must report the normalized enzyme activity, for example U / gDW. This eliminates the moisture effect of the soil. You could enter the other variables in your statistical model to know how much your multivariables are affecting the enzyme activity. A multifactorial ANOVA or even a multivariate analysis could work.
Good luck!
Hi Michael. Take a look at this paper:
Pritsch, K., Courty, P. E., Churin, J. L., Cloutier-Hurteau, B., Ali, M. A., Damon, C., ... & Garbaye, J. (2011). Optimized assay and storage conditions for enzyme activity profiling of ectomycorrhizae. Mycorrhiza, 21(7), 589-600.
best
Francisco
Hi Michael,
Fresh samples are always best analysed. However, experience shows that freezing is less harmful than drying samples. Although freezing also gives different results than storing at 4°C. However, the effect of the storage method itself is different depending on the soil type.
According to Dadenko et al. the greatest changes occur during the 12 weeks of sample storage. After that they are less pronounced.
So to be able to compare, samples should be stored under the same conditions and for the same amount of time before analysis. Or they should be analysed freshly just after collection.
See:
https://pubmed.ncbi.nlm.nih.gov/20138194/
https://link.springer.com/article/10.1134/S1064229309120084
https://www.sciencedirect.com/science/article/abs/pii/S0929139312001837
and a similar question:
https://www.researchgate.net/post/What_is_the_maximum_time_period_after_sampling_for_soils_to_carryout_enzyme_activity_and_soil_microbial_biomass_determination
And as far as the changing conditions of the soil environment are concerned, the influence of factors other than fertilisation etc. on enzyme activity should always be taken into account when analysing the results. That is, compare it with the parameters of each soil and, of course, include moisture and temperature in the statistical analysis. Ideally, the controls should be the same at each site without any additional factors, i.e.: without fertilisation and without plant; with fertilisation and with plant; without fertilisation and with plant. Then you would get the result of the activity of this soil under these climatic conditions and then you can compare the different soils with each other.
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