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BSF Processed Food Scrap Leachate | New Insights

by Terry Green on 02/19/18

Several observations on the properties of food scrap leachate processed by Black Soldier Fly (BSF) larvae, combined with an analysis of its chemical composition,  suggest that its plant growth stimulatory activities (see Black Soldier Fly Processed Food Scrap | Foliant and Soil Applications, Amending Soil with Black Soldier Fly Processed Food Scrap Leachate, Black Soldier Flies & Food Scrap |Putting the Leachate to Good Use,  and Black Soldier Flies & Recycling | Keeping Organic Leachates in Perspective) may be associated with two principal biological phenomenon -  the delivery of inoculums of anaerobic nitrogen-fixing soil organisms, notably of the genus Clostridium, to soil which in consortium with other common soil microbes, creates conditions allowing for the fixation of nitrogen proximal to plant roots receiving leachate inoculums; and to the presence and/or subsequent formation of plant growth hormones such as gibberellic acid,  auxins and cytokinins having intrinsic plant hormone growth stimulatory activities (see Gibberellin A3 and Plant hormone).

Substantial differences in the chemical composition and appearance of food scrap leachate can be seen in comparing that untreated with that processed by larvae (see Fig. 1). Within a matter of a few hours exposure to larvae the liquid fraction transforms into a characteristic dark brown solution rich in polyphenolics quite distinct from that of the starting material. Leachate unexposed to larvae furthermore shows the growth of fungal mats on its surface (Fig. 1, upper panel). That processed by larvae shows no growth of fungal mats (Fig. 1, lower panel).

Images of fountain grass and effect of BSF leachate on growth
Fig. 1. The appearance and transformation of food scrap leachate unexposed and exposed to BSF larvae. Upper Panel, Control samples left over night at 30 C without without larvae; Lower Panel, samples run in parallel with the controls but with larvae inoculated into the solutions.

Leachate ferments unless vigorously aerated due to the low solubility of oxygen in water, the limited diffusion of oxygen across the air-water interface, and a drop in dissolved oxygen content beneath the air-liquid interface driven by aerobic microbes consuming what little dissolved oxygen is present in the liquid. Anaerobic digestion ensues resulting in the formation and accumulation of volatile short chain fatty acids, and a downward shift in the pH of the leachate fraction to values in the range of about 3.5 to 4.0 (see Turnover of Carbohydrate-Rich Vegetal Matter During Microaerobic Composting and After Amendment in Soil).

Differences in the leachate’s chemical composition in contrasting that unprocessed by larvae with that on which larvae have fed are summarized in Table 1.

Table summarizing chemical changes in food scrap leachate processed by BSF

Larvae feeding on the liquid markedly accelerate mineralization of organic solutes and particulates as can be seen by a roughly 2-fold and almost 4-fold drop in the average total polysaccharide and solids concentrations, respectively, and an accompanying upward shift in the average pH. This upward pH shift accompanies the turnover of volatile fatty acids and a concomitant accumulation ammonia in treated solutions (see Enhanced Ammonia Content in Compost Leachate Processed by Black Soldier Fly Larvae). Inorganic phosphate precipitates out of solution (possibly in the form of struvite or other water insoluble inorganic precipitates). Fully processed liquid has an average NPK ratio of .04:0.04:0.48.

The low N and P levels in the processed leachate fraction rule out the likelihood that the response of plants receiving leachate applications, and subsequently exhibiting a boost in growth rate, is linked with direct addition of either N or P as a nutrient to soil and/or the leaves of treated plants, especially considering that the larval processed liquid exhibits its growth stimulating activity even when diluted in water 10-fold and higher (see Fig. 2). N and P in 10-fold diluted applications, for example, amount to the delivery of no more than negligible 0.004 % N and P to a plant’s roots, and even less is presented to plants at the higher dilutions.

Images of fountain grass and effect of BSF leachate on growth
Fig. 2. Photos comparing the outcome of varying applications of BSF food scrap leachate processed by BSF larvae on the growth rate of Black Fountain grass root and foliage.  Left to right, control (no leachate treatment), 10-fold, 100-fold and 1000-fold dilution of BSF leachate in tap water, respectively. Copyright© 2013, Terry Green, All rights reserved.

DNA probes run on extracts from the leachate fraction show however that about 60% of the microbes in the leachate are of the genus Clostridium (see Turnover of Carbohydrate-Rich Vegetal Matter During Microaerobic Composting and After Amendment in Soil). It is well-known that clostridia, abundant in most soils and present as spores on vegetable matter, are capable of fixing nitrogen under anaerobic conditions (see, for example, Clostridium pasteurianum and Microbial metabolism).  Clostridia have also been isolated as endophytes at the roots of some important agricultural plants such as rice and shown to fix nitrogen in this latter setting when mixed with a diverse consortia of common bacteria (see, for example, Enhanced Rice Seedling Growth by Clostridium and Pseudomonas and Anaerobic nitrogen-fixing consortia consisting of clostridia isolated from gramineous plants).

These observations suggest that some of the growth stimulatory activity seen in applying processed leachate back to soil at the base of plants is likely attributable to enhanced nitrogen fixation induced through inoculation of the soil with clostridia and accompanying soil microbes acting in consortium in creating conditions conducive to the fixation of nitrogen at the root base of plants receiving processed leachate. This explanation is consistent with earlier soil test results posted on this subject in comparing soils treated and untreated with BSF leachate which at the time suggested that the application of processed leachate to soil was inducing fixation of nitrogen (see Black Soldier Flies & Food Scrap |Putting the Leachate to Good Use).

Spent waste (Fig. 3), impregnated with processed leachate, also shows robust plant stimulatory activity on layering it atop soil. Some if not the majority of this activity can be directly traced to processed leachate impregnating the leftover residues that washes off the residues as water percolates through it after amending it into or layering it atop soil (see Fig. 4).

Image of spent waste residuals impregnated with larval processed leachate
Fig. 3. The appearance of food scrap spent waste residuals impregnated with larval processed leachate. Its NPK ratio is approximately 2.7:0.4:1.5. Copyright © 2018, Terry Green, All rights reserved.

Image of larval processed leachate washed free of BSF processed spent waste
Fig. 4. BSF processed leachate, a byproduct in farming BSF larvae off food scrap waste having marked plant stimulatory activity recovered on percolating water over spent food scrap waste. Copyright © 2018, Terry Green, All rights reserved.

Spent waste has a higher average NPK content of approximately 2.7:0.4:1.5 relative to that of the processed leachate fraction. The higher N and P values relative to those in the processed leachate fraction are likely attributable to the accumulation of organic nitrogen in the form of chitin (left behind in the spent waste by larvae molting their exoskeletons while passing through varying instars in their life cycle), and to the formation of insoluble forms of phosphate such as calcium phosphate, brushite, hydroxyapatite, possibly iron phosphates, struvite, etc. (see Table 1 and Phosphate Minerals).

The effect that gibberellins and auxins have on plants mirrors the increased density of root structures, nodal stem elongation, and increases in leaf density seen on treating plants with larval processed leachate (Fig. 2). More work however needs to be done before any definitive conclusion can be drawn regarding what role gibberellins or other auxin-like plant hormones may be conferring to the leachate in augmenting its plant stimulatory activity.

It is however interesting to note that gibberellins have been commercially produced using waste feedstocks derived from a wide array of food scrap and grain wastes (see, for example,  Evaluation of Some Food Industry Wastes for Production of Gibberellic Acid by Fungal Source and Improved Production of Gibberellic Acid by Fusarium moniliforme). While it appears that nobody has yet considered the prospect of combining BSF larval farming operations in processing food scrap wastes with commercial production and recovery of gibberellins, the observation that gibberellins can be produced under conditions similar in many regards to those employed in farming BSF raises the possibility that these two operations might possibly be combined or synchronized in improving the economic viability of both operations.

There could be some additional value in combining these two operations together considering that terpenoid intermediates (see Terpenoid), required in producing insect juvenile hormone, are the same needed for the biosynthesis of gibberellins (see Gibberellin). Because these two regulators share a common metabolic pathway with regard to precursors required in their formation, it would be interesting to know if BSF larvae, feeding and growing off of decaying waste, and fungi present in the waste, might in any manner synergistically through shared terpenoid precursors work together in amplifying the formation of gibberellins in waste on which the larvae and fungi grow.

Regarding environmental and safety matters, it should be noted that if gibberellic acid, itself, is present, and if it is the principal causative agent in stimulating plant growth, that it should pose no danger to the environment on being recycled back into soil. It breaks down in soil according to published reports at a rate of approximately 9% per day, and animal and safety toxicological tests also indicate that it poses no significant risk to animals or the environment (see Gibberellin A3). Furthermore, the NPK ratios in processed leachate are sufficiently low such that it is unlikely that soil applications would pose any problem regarding overloading soil with excess N or P, especially with processed leachate delivered to plants and crops in diluted form.

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