Manure, Mushrooms and More: How Organic Waste Could Power Farms and Feed Their Soil

Researchers found that storing the microbial starter at 35°C preserved greater bacterial diversity than storage at 15°C, helping reactors start under moderate temperatures.

A pile of manure, discarded mushroom substrate, or leftover olive pulp still contains materials that farming can use. Nutrients can return to the soil, organic matter can support growing conditions, and microbes can turn some of that waste into fuel. Finding the right treatment determines how much value survives the journey.

The editorial "A Path for Circular Economy in Agriculture: From Organic Waste to Sustainable Energy and Soil Fertility," published in Agronomy, examines these possibilities through eleven research contributions. Authors Nicolò Montegiove, Alberto Maria Gambelli, and Daniela Pezzolla bring together work on biogas, compost, recycled fertilisers, and soil amendments. The article connects findings from several studies rather than reporting one experiment.

Circular agriculture recovers useful resources from materials that would otherwise need disposal. European policies supporting resource recovery provide a backdrop to this work, which covers agricultural residues, municipal organic waste, sewage sludge, and digestate, the material remaining after biogas production. The evidence shows opportunities for better resource use, with results shaped by processing methods, crop needs, and soil conditions.

Getting more energy from organic leftovers

Anaerobic digestion relies on microbes breaking down organic material without oxygen, making the condition of those microbes an important operating choice. Researchers found that storing the microbial starter at 35°C preserved greater bacterial diversity than storage at 15°C, helping reactors start under moderate temperatures. Starting digestion at 55°C before moving to 35°C encouraged methane-producing microbes, and the accumulation of certain organic acids reduced operating capacity.

Spent mushroom substrate offered a way to improve digestion using enzymes recovered from the waste itself. Treating the substrate with those enzymes increased methane production by 16% and generally raised phosphorus and potassium contents in the remaining liquid and solid digestates. Related research discussed in the editorial explores extracting valuable compounds before digestion, mixing wastes to balance nutrients, and using enzyme-based fuel cells to convert organic substrates directly into electricity.

Bioplastic packaging presented a slower challenge, with polylactic acid films approaching complete breakdown after roughly 100 days of digestion with municipal organic waste at 37°C. The process included a delay of about 60 days before substantial degradation and produced around 380 cubic metres of biogas per tonne of volatile solids, with no evidence of inhibition. Other polymer formulations broke down less readily, making material composition, treatment conditions, and testing in continuously operating facilities essential considerations.

Better compost starts with the right mixture

Cow manure composting improved when researchers adjusted the particle sizes of spent mushroom substrate added to the mixture. A treatment containing 15% coarse particles and 5% fine particles extended the hot composting phase, met the study's maturity criteria, and increased available phosphorus and potassium by 4.8% to 59.1%. The finding gives practical importance to material preparation, since additives, particle size, and process control influence whether the finished compost is stable and suitable for plants.

Matching recycled inputs to crops and soils

A two-year maize–Italian ryegrass rotation showed why organic fertilisers need to be assessed crop by crop. Raw digestate, composted digestate, and composted sewage sludge supported maize yields and forage quality comparable to mineral fertiliser, and composted sewage sludge increased average plant height; ryegrass produced lower yields under organic fertilisation. Another contribution improved a double-disc fertiliser spreader through computer modelling and prototype checks under headwinds and tailwinds, addressing the practical need to distribute nutrients evenly.

Biochar, a carbon-rich material produced by heating feedstocks with limited oxygen, gave different results depending on its source and use. A tea plantation treatment combining 2% pig-manure biochar produced at 350°C with biogas slurry supplying all fertiliser nitrogen recorded the highest levels of two measured soil carbon fractions and increased tea polyphenols and amino acids. Antioxidant activity rose by 3.25% and 5.97% in two laboratory assays compared with chemical fertilisation alone, alongside increased activity of an enzyme involved in carbon cycling.

Water-stressed lettuce responded strongly to biochar mixed into compost, with yield reaching 88% of the unstressed comparison after adding 10% biochar, compared with 30% using compost alone. Incorporating biochar into compost reduced carbon losses by up to 48% and nitrogen losses by up to 67% in the dryland research. A separate laboratory study found that adding 5% "plastichar," made by heating hazel pruning residues with polyethene waste, improved plant-available water in degraded soil, leaving questions about its long-term stability.

Tomatoes grown without soil showed improvements in some fruit quality measures when biochar replaced part of the cocopeat growing medium and was combined with wood distillate. The treatment containing 5% biochar and wood distillate increased antioxidant activity and phenolic content without a clear change in vegetative growth or yield, and treatments altered the growing medium's microbial community. These results illustrate why better fruit quality should be considered separately from producing more fruit.

Recycled residues offered ways to influence nutrient supply more directly. Artificial humic acid made from corn stalks increased a measure of available phosphorus by up to 15.4% in one soil particle-size fraction during a 60-day experiment. Related studies described fertiliser capsules containing yerba mate processing waste and an olive-residue formulation that released nutrients gradually over 90 days, increasing nutrient availability with relatively low carbon dioxide emissions compared with the other fertilisers tested.

What farms need before wider adoption

Recovering nutrients does not automatically produce a suitable fertiliser, as research on olive-pomace digestates makes clear. Those materials increased soil carbon and nitrogen availability during a 90-day incubation, raised carbon dioxide emissions, caused plant toxicity at concentrations above 10%, and exceeded the cited mercury limits. Microplastics detected in composted municipal organic waste add another reason to examine the quality and safety of materials intended for agricultural use.

Researchers identify processing control, useful combinations of materials, and crop-specific responses as recurring lessons across the collection. Much of the evidence comes from laboratory experiments and soil incubations, leaving a need for longer field trials and fuller assessments of costs and environmental impacts across each product's life cycle. Successful reuse depends on understanding the whole journey from discarded material to finished input, including how it performs in soil and what contaminants it may carry.