Many studies claim one of the benefits of arboriculture is the increased uplift, retention, and recycling of mineral nutrients. However, quantification of this topic tends to be limited to very rudimentary techniques, such as simply quantifying the total nutrients redelivered to the soil surface through litter. I am interested to attempt to quantify the uplift and recycling of nutrients in arboricultural systems. The only way I can think to do it is to define a ratio of isotope fractionation for the tree crop (which will likely change with the ratio on isotopes in the soil), and to assess the isotope ratios in the soil and tree over time. Any thoughts, suggestions, citations, etc. along this topic?
Dear Noa,
we performed several studies in that direction based on isotope labeling, means:
we injected e.g. 15N tracer (ammonia or nitrate - depends on natural N species composition of your soil solution) in different soil depth and quantified the uptake / nutrient uplift from these different soil depth; For mineral nutrients like Ca2+, K+ you can do similar studies based on stable isotopes. I prefer this approach compared to the natrual abundance approach as it directly quantifies the nutrient uplift and is not affect by different fractionation factors of individual plant species.
Noa,
You pose your question with respect to mineral nutrients. Are you working primarily in volcanic ash derived soils? Have the soils you are concerned with been cropped for some time or are you looking at soils recently brought under cultivation, or long time agricultural soils? In many volcanic soils organic nutrients stabilized by aluminum oxyhydroxides are a major, if not the predominant, source of many nutrients, especially in natural or recently cultivated soils. Oxyhydroxide sequestration of organic nutrients can be surprisingly rapid, measurable in a single season, or even less. If you are looking into the potential for benefits of converting soils from conventional cropping to arboriculture, one of the consequences will be restoration of (organic) nutrient storage processes that are more active in natural (agriculturally undisturbed) soils. If natural sequestration processes do resume at nearer natural (undisturbed soils) rates, then you could easily see what would appear to be a loss in recycling efficiency unless you assess the soil organic pool. An isotope approach should be enlightening. I wonder about the appropriateness of injecting labeled mineral nutrients into such systems in that uptake of readily available nutrients in solution does not reflect accessibility of nutrients from actually functional sources in the soil, which, I think, is what you actually want to look at.
Aloha Michaela,
Thank you. How then did you make the quantification based on the tracer? If the soils were at a known label and could only utilize the labeled soils (so basically in a pot) I could understand how the partitioning could be worked out, but in the field I canʻt imagine how that would work. Do you have a citation for that methodology?
Aloha Bryce,
Thank you for your extensive response. For the initial project I am working on young basalt soils in Kona, Hawaii. They likely have a decent amount of tephra contribution to the fines portion of the soils, with also contributions from weathering of primary material. These soils have been cropped in arboriculture in pre-European times, maintained but altered after European contact, and largely fallow in the last 50 years.
Yes the organics are a critical part of the equation, especially in the young soils as the carbon-alluminum form the majority of the nutrient retention and cation exchange capacity. One part I am interested in is how the arboricultural system might enhance the capture of weathing products, and assuming a fractionation of isotopes by plant uptake I think that the looking at the isotopes on the exchangeable nutrient sites (on carbon and alluminum oxyhydroxide) might indicate some of that. If possible in these rocky soils Iʻd also like to analyze the seepage to see what is being lost (what percentage of the loss is primary weathering and what loss is recycled nutrients that have passed through the trees).
One final bit would be about the understory species, and looking at what percentage of their nutrients are from primary weathering and what percentage have been already uptaken by the canopy trees. In this sense Iʻm interested in how the trees enable the production of the understory species.
Any citations or references to some applications of the above methods you talked about would be most helpful. Or more thoughts would also be most welcome. Thank you!
Thank you for your contribution Abhishek, but I really donʻt see how that paper gets at my question. It is a broad overview of root physiology and how it relates to plant growth, but does not get at any techniques for actually measuring uptake and recycling rates nor does it even get into the physiology specifically enough for me to think critically of how those measurements might be made.
To be clear I think what I want to do it really quite novel and cutting edge, and as far as I can envision so far will have to be using isotopes to understand the relative and absolute amounts of nutrients in the cycle. Most interesting to me will be how the use of arboriculture decreases (or does not decrease) the amount of nutrients lost in very young soils, and increases the nutrient capture and use at an ecosystem scale. There is an emerging technique of using isotope fractionation to quantify phosphorous that has been biologically cycled or not that I think might be useful in this context.
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