DesignGeneralPlant SystemsPlants

Qualifying Dynamic Accumulators: a Sub-Group of the Hyperaccumulators.

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Chickweed (Stellaria media) is a popular Dynamic Accumulator.

To the untrained eye, a lot of scientific language appears superfluous – yet may best describe some detail or process within a broader concept. The scientifically trained eye, likewise, may be suspicious of superfluous language. Robert Kourik first uses the phrase ‘dynamic accumulators’, in 1986 to describe plants considered a valuable addition to composting due to their mineral/nutritional content.

Theoretically, dynamic accumulators take up high concentrations of useful nutrients (from the subsoil) into their biomass; the biomass then drops or is chopped as mulch, or composted. Ultimately nutrients are redistributed from the subsoil to the topsoil. It was highlighted in John Kitsteiner’s article that there is no scientific foundation for the term ‘dynamic accumulator’; and while a lack of evidence is not evidence of a lack, there is much work to be done to properly qualify and quantify these plants.

Here I further the discussion concerning dynamic accumulators. I present ideas to define dynamic accumulators as a group; methods for quickly pre-qualifying dynamic accumulators; and an example using plant-phosphorus concentrations to tentatively qualify dynamic accumulators of phosphorus, and identify shortcomings in the methods. The shortcomings are catch cries of scientists everywhere: “the data is in ill repair” and “we need more data”. Better qualification and some rough quantification are possible with a little bit of work & research as outlined. The post-harvest functions of dynamic accumulator materials are beyond the scope of this article.

Losses of nutrients from the topsoil via leaching, erosion and destructive soil practices are ubiquitous. Strategies to return leached nutrients from the subsoil to the topsoil appear relatively non-existent, except for the concept of deep rooting dynamic accumulators. For this reason alone, spending some time to qualify these plants is warranted. Shallower rooting species are still useful redistributors of nutrients, and are certainly worthy of addition for the original intent – good plants to compost (or mulch) with.

A Definition.

While the phrase dynamic accumulator doesn’t hold up to scientific scrutiny, the concept does. Over 400 hyperaccumulating plants were discovered in the initial search for plants to extract heavy metals. Metal hyperaccumulators qualify according to threshold concentrations of specific metals: 0.1% of dry mass for some, 1% for others. In addition to mining toxic metals with no known biological function; many take up micronutrients of use to plants e.g. nickel, molybdenum, copper, zinc, manganese and iron. Dynamic accumulators might be seen a subgroup of the hyperaccumulating plants. They are distinguished by accumulating useful substances; as opposed to contaminants.

Trace elements useful to plants are contaminants above a certain threshold. Micronutrients in plants are typically < 0.01% of dried weight. Metal hyperaccumulators can have concentrations from 10x to well over 100x these amounts. The wastes of some species must be treated as toxic waste. Dynamic accumulators might begin to serve their purpose well below hyperaccumulator qualification thresholds. Dynamic accumulators require qualifying thresholds with a lower (useful) and upper (safe) limit in some cases; these will need to be determined for each nutrient.

What’s in a Word?

dynamic: (physics) using or relating to energy.

The easiest method for plants to take up nutrients is passive transport. Though some wicking via capillary action occurs, transpiration is responsible for the majority of passive transport. As water evaporates from leaves osmotic pressure in the plants vascular system acts as a siphon to ‘pull’ water from the soil into the roots and up through the plant. Through these processes the concentration of nutrients in plant sap closely matches that of the soil.

Active transport involves the use of proteins in the root membrane (or bacterial or fungal cell membrane) that require energy to fuel the transport of various substances from the soil into the roots4. The active ‘mining’ of a specific thing from soil water creates a deficit in the immediate environment; diffusion replaces the substance where it is depleted so long as it is present in connected soil water. Production/acquisition of molecules for assisted transportation of substances within plants may also be employed. In addition, trade with symbionts for substances has an indirect cost associated with such organisms. Dynamic accumulators take up higher than average concentrations of a substance regardless of soil concentrations (provided the substance is present).

dynamic accumulators: Plants that use active transport to accumulate useful nutrients in reasonable (but non toxic) concentrations.

Qualification.

What are ‘reasonable concentrations’ of nutrients to qualify dynamic accumulators? Active transport depicts they are at least higher than soil concentrations. In science the soil concentration of a substance is called the background level. This is used in conjunction with the concentration of a substance in a plant to determine the bioconcentration factor (BCF)5.

plant concentration/background level = BCF

BCF = 1: passive uptake only

BCF > 1: accumulator

BCF < 1: excluder

e.g. If plant A has 100 mg/kg iron, and the soil 25 mg/kg: BCF = 100/25 = 4. This plant is an accumulator taking on 4 times the iron of background levels. Unfortunately, BCF changes if background levels change. This can wildly distort an accumulator’s status in a polluted or stripped environment.

e.g. If plant A has 100 mg/kg iron, and the soil 100 mg/kg: the BCF = 100/100 = 1. This same plant does not appear to be accumulating in this soil – it doesn’t need to pay to get iron at those concentrations. BCF fails to properly qualify potential accumulators; it is site-specific and subject to change.

We need something similar, but fixed. If we use the average-plant concentration of a specific nutrient from a broad sampling of terrestrial plants – we get a proxy for background level that remains constant.

Kitsteiner’s article pointed out a set of spreadsheets with hundreds of species of plants nutrient contents. These spreadsheets were anonymously deposited on buildasoil.com, but compiled from Dr James A. Duke’s Phytochemical and Ethnobotanical Database. Relatively comprehensive, the spreadsheet data is sufficient to get strong statistics of average-plant concentrations for P, K, S, Ca, Mg, Si, Fe, Mo, Bo, Cu, Na & Mn. Unfortunately, the format ensures you need to work for these.

Coming up with new terms (e.g. dynamic accumulator) is what started all this article writing to begin with; however, the plant concentration/average-plant concentration ratio requires its own term to reflect it is no longer a bioconcentration factor (BCF). It is a biome-concentration factor (Bf). The biome type Bf represents in this instance is a permaculture system. This is well represented by the hundreds of cultivable species in the spreadsheet data we have to derive average-plant concentration.

plant concentration/average-plant concentration = Bf

Let’s Recap.

Dynamic accumulators are a subset of hyperaccumulating plants. They actively mine substances useful to plants, in useful (yet non-toxic) concentrations. Thresholds for concentrations qualifying dynamic accumulators are yet to be determined. BCF is inaccurate for qualifying these plants due to variance in background levels. Average-plant concentrations from large data sets will give proxy ‘background’ statistics to calculate a biome-concentration factor – Bf. Bf can qualify accumulators and excluders regardless of background concentrations.

An example: Dynamic Accumulators of Phosphorus.

Using the phosphate concentration data of aboveground biomass (excluding seed data) from 438 samples gave an average-plant phosphorus concentration of 3706 ppm or 0.37%.

A threshold of 2% phosphorus by dry weight, or 20 000 ppm, reveals only three species out of 438 samples (0.07%) that fit this criteria (Table 1). Lowering the threshold to 1% includes another 15, to give 18 species from 438 samples (4.1%). The Bf immediately shows us each species phosphate uptake ability compared to plant-average, identifying accumulators and excluders (excluded) alike.

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These plants gather phosphorus in useful amounts. 1 in 24 plant samples accumulate from a 1% phosphorus threshold, while only 1 in 142 from 2%. The plant-average phosphorus (3706 ppm) is very useful, statistically strong from such a large sample size (438). The plant species (ppm) data to compare it to is very weak, with only one data point per species.

The large jump seen in Bf values from horsetail, to bitter melon and up, is suspect. Normally a gentle grading is observed with so many samples, as is observed in the Bf values from horsetail down in this and the greater data set. Is someone pulling our Equisetum? The top three plants may have special mechanisms or symbionts that make them stand out; or special reasons to accumulate phosphorus e.g. plant defence. Or the data are not good.

We need more plant concentration data for each high Bf species, to get average ppm values. At least 8 data points each species would be of tremendous value: qualifying dynamic accumulators with some certainty; and beginning the process of quantification.

About the Author:

DC Brown is an ecologist, microbiologist and keen student of permaculture and traditional agriculture. Residing in Auckland, New Zealand, Dean is currently researching and writing a book: Heavy Metal Detox: The Holistic Treatment of Undesirable Elements.

References.

1. Kourik, R. (1986). Designing and Maintaining Your Edible Landscape – Naturally. Chelsea Green Publishing Company. p 269.

2. Kitsteiner, J. (2015). The Facts about Dynamic Accumulators. retrieved from: https://www.permaculturenews.org/2015/04/10/the-facts-about-dynamic-accumulators/ 27/04/2015.

3. Lone, M. I., He, Z. L., Stoffella, P. J., & Yang, X. E. (2008). Phytoremediation of heavy metal polluted soils and water: progresses and perspectives. Journal of Zhejiang University Science B, 9(3), 210-220.

4. Taiz, L., & Zeiger, E. (2006). Plant physiology. Sinauer Asso. Inc., Pub., Sunderland, Massachusetts, USA.

5. Bioconcentration. Retrieved from: https://en.wikipedia.org/wiki/Bioconcentration 27/04/2015.

6. Dynamic Accumulators & Nutrient Contents. Retrieved from: https://cdn.shopify.com/s/files/1/0248/9641/files/Dynamic_Accumulators_and_Nutrient_Contents.xlsx?723 27/04/2015.

7. Dr Duke’s Phytochemical & Ethnobotanical databases. Retrieved from: https://www.ars-grin.gov/duke/plants.html 27/04/2015.

8. https://upload.wikimedia.org/wikipedia/commons/0/05/Kaldari_Stellaria_media_01.jpg

Dean Brown

DC Brown is an ecologist, microbiologist and keen student of permaculture and traditional agriculture. Residing in Auckland, New Zealand, he is still researching and writing a book: Heavy Metal Detox: The Holistic Treatment of Undesirable Elements.

5 Comments

  1. Good question with no easy answer. The rhizosphere beneath a plant can be extended for miles by fungi. Literal miles. I imagine phosphate distribution over time becomes an ecosystem-wide cycle involving x-amount supplied via weathering and biological activity being released on a per unit of time basis. I’ve run experiments with ten successive crops and no nitrogen additions (except a mulch layer). What happens in soils is not what I expected. They get more fertile and efficient with plants in them, not less. If weathering and biogeochemical activity match what you remove you’ll never need phosphate again. But you do need a healthy soil food web.

  2. I have an load of horsetail in my backyard (anchorage area) and interested in the phosphorus content of them. Is there an inexpensive way to test the Bf values?

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