The logic behind the pairing is elegant. Spent mushroom substrate, the fibrous, microbe-rich material remaining after mushrooms such as Agaricus bisporus have been harvested, is produced in enormous volumes worldwide and is packed with decomposed organic matter. In the study, the researchers found that incorporating this substrate into the growing mix tripled its cation exchange capacity, a measure of how well a growing medium can hold onto positively charged nutrients such as potassium, calcium, and magnesium before they leach away. That structural improvement, however, does not by itself guarantee a ready supply of nitrogen, which is where the insect component enters the picture.

Black soldier fly larvae are already commercial stars of waste conversion, devouring food scraps and manure before being processed into animal feed. Their frass, a mixture of larval excrement and shed exoskeleton material, has attracted growing attention as a fertiliser because it delivers nitrogen in forms plants can use quickly. In the experiment, adding a small proportion of frass, just one percent by weight, to the spent mushroom substrate raised the availability of ammonium-nitrogen in the growing medium by 1.44-fold compared with the substrate alone. The researchers framed this as a complementary relationship: the mushroom substrate builds the soil’s capacity to hold nutrients, while the frass supplies a concentrated, fast-acting nitrogen pulse.

The growth responses were remarkable. Wheat plants grown in a mix where spent mushroom substrate replaced half of a commercial potting medium, and with the one percent frass addition, produced shoot fresh biomass 4.38 times greater than control plants grown in standard potting mix. Root biomass climbed 3.27-fold, and leaf relative water content increased 1.34-fold, indicating that the amended plants maintained better hydration. Above ground, the photosynthetic machinery responded in kind: total chlorophyll rose 1.42-fold and carotenoids 1.64-fold, while leaf nitrogen content and leaf protein each increased 1.56-fold. Because leaf nitrogen is a primary determinant of photosynthetic capacity in C3 crops such as wheat, these figures suggest the amendments were feeding the engine of growth directly rather than merely improving soil texture.

One of the study’s most intriguing findings concerns the plants’ antioxidant systems. Enzymes such as catalase and ascorbate peroxidase typically ramp up when plants experience oxidative stress, mopping up the reactive oxygen species that damage membranes and proteins. In the frass-plus-substrate treatment, catalase activity reached 2.89 times the control level and ascorbate peroxidase 3.50 times, which at first glance might suggest stressed plants. Yet the stress markers told the opposite story. Malondialdehyde, a product of lipid peroxidation that signals membrane damage, fell to just 39 percent of control values, and electrolyte leakage, another indicator of cellular damage, dropped to 40 percent. The authors interpret this combination as evidence of a nutritionally supported state rather than a stress-driven one: well-fed plants with vigorous metabolism produce more reactive oxygen species as by-products of photosynthesis, and their enhanced antioxidant machinery keeps those by-products in check before harm occurs.

The yield data translate the physiological gains into agronomic terms. Plants receiving the combined amendments produced 1.74 times greater biological yield and 1.67 times more grains per spike than controls. Grain yield itself rose 1.53-fold, and grain protein content increased 1.41-fold, a meaningful improvement for a staple crop that supplies roughly a fifth of the calories and a substantial share of the protein in the global human diet. For wheat breeders and agronomists, the fact that both quantity and quality moved in the same direction is notable, since yield and protein concentration often trade off against each other when nitrogen supply is managed poorly.

The grain mineral analysis added further nuance. The potassium-to-sodium ratio in the grain improved 3.82-fold, a shift with implications for both plant salt tolerance and human nutrition, given that dietary guidance generally favours higher potassium and lower sodium intake. Grain zinc concentration nearly doubled, rising 1.95-fold, which matters because zinc deficiency affects hundreds of millions of people worldwide and wheat biofortification is an active global research priority. However, the amendments were not uniformly beneficial: grain iron fell to 56 percent of control levels, a decline the authors flag as identifying substrate iron management as a practical consideration for field use. The mechanism behind the iron drop was not resolved in the study, but the finding underscores that even well-designed organic amendments can redistribute micronutrients in unexpected ways.

The broader context makes the results timely. Synthetic nitrogen fertiliser production via the Haber-Bosch process consumes vast amounts of energy and accounts for a significant share of agriculture’s greenhouse gas footprint, while nitrogen mismanagement drives water pollution and soil degradation. Meanwhile, the mushroom industry generates mountains of spent substrate each year, and the rapidly expanding insect farming sector produces frass as a low-value by-product that currently lacks premium markets. Linking these waste streams to cereal production could, in principle, close nutrient loops at industrial scale, reducing both the disposal burden of one sector and the fertiliser demand of another. Previous work has shown insect frass can benefit crops such as maize, lettuce, durum wheat, and tomato, and that spent mushroom substrate can improve degraded soils, but the co-application strategy tested here appears to exploit their complementary chemistry more deliberately.

The authors are careful about the limits of their evidence. The experiment was conducted in pots under polytunnel conditions, which controls the environment but cannot capture the variability of real field soils, weather, and farming economics. Pot size itself can influence plant growth responses, and the researchers acknowledge that field-scale and economic validation is required before the co-amendment strategy can be broadly recommended. Questions about application rates, timing, long-term effects on soil microbial communities, pathogen management, and the consistency of frass composition across insect farming operations all remain open. The iron decline in particular will need targeted follow-up to determine whether adjusting the substrate blend or supplementing iron can preserve the zinc gains without sacrificing iron nutrition.

Even with those caveats, the study offers a compelling proof of concept that the circular bioeconomy can deliver measurable, multi-dimensional benefits to a staple crop rather than incremental ones. A four-fold increase in shoot biomass, a 53 percent grain yield boost, higher protein, doubled zinc, and a sharply improved potassium-to-sodium ratio, all achieved by redirecting two waste streams into the root zone, is the kind of result that gets agronomists and waste managers talking to each other. If field trials replicate these numbers at realistic scales and costs, the humble leftovers of mushroom farms and insect rearing facilities could become a serious tool in the effort to feed a growing population while shrinking agriculture’s environmental footprint. The next harvest of evidence will come from the open field, where weather, soil, and economics will judge whether this laboratory and polytunnel promise can survive contact with the real world.

Subject of Research: Using spent mushroom substrate and black soldier fly frass as circular organic co-amendments to improve wheat productivity and grain quality

Article Title: Spent mushroom substrate and black soldier fly frass as circular co-amendments improve wheat productivity, antioxidant capacity, and grain nutritional quality

Article References: Ali, B., & Imin, N. (2026). Spent mushroom substrate and black soldier fly frass as circular co-amendments improve wheat productivity, antioxidant capacity, and grain nutritional quality. Plant and Soil. https://doi.org/10.1007/s11104-026-09151-w

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09151-w

Keywords: spent mushroom substrate, black soldier fly frass, wheat, organic amendments, circular bioeconomy, grain quality, antioxidant enzymes, zinc biofortification, nitrogen, sustainable agriculture, plant physiology, soil fertility