Peanut Shells and Mushroom Waste Could Rescue Soil Sickened by Decades of Carrot Monoculture

On a research farm in Shandong Province, China, a field that has grown nothing but carrots for twenty years has become an unlikely laboratory for one of agriculture’s most stubborn problems. Continuous cropping, the practice of planting the same crop season after season on the same land, gradually poisons the soil beneath it. Yields fall, root-knot nematodes and Fusarium wilt multiply, and the microbial communities that keep soil healthy collapse into imbalanced shadows of themselves. A two-year field experiment conducted by researchers at Qingdao Agricultural University, published in the Journal of Agriculture and Food Research, now offers a detailed mechanistic picture of how two humble waste products, crushed peanut shells and spent mushroom residue, can begin to reverse that decline when they partially replace chemical fertilizers.

The team set up four treatments in a greenhouse facility on Shajiang black soil, a Calcic Vertisol that had been under continuous carrot rotation since 2000. One plot received conventional chemical fertilizer at rates typical of local farmers, which the researchers had previously shown to over-apply nitrogen and phosphorus while shortchanging potassium. A second received an optimized chemical regime calibrated by soil testing, moisture sensors, and target yields. The third and fourth treatments kept the same total nitrogen, phosphorus, and potassium inputs as the optimized regime but substituted part of the chemical fertilizer with peanut shells, incorporated as compressed pellets at 22.5 to 30 tonnes per hectare, or with air-dried, sieved mushroom residue at 22.5 tonnes per hectare. Carrots of two varieties were grown twice a year, in spring and autumn, across two full crop cycles, allowing the researchers to track changes through germination, rosette, fleshy root expansion, and harvest.

The most striking result concerned soil organic carbon, the backbone of soil fertility. Under both chemical fertilizer treatments, total organic carbon remained essentially flat across all four growth periods, confirming that conventional fertilization merely maintains the existing carbon pool without expanding it. In the organically amended plots, total organic carbon climbed steadily, rising by 35.92 percent under peanut shells and 42.33 percent under mushroom residue by the 2021 harvest compared with the 2020 expansion stage. The labile fractions told an equally dramatic story. Water-soluble substances, extractable humus, and humic acid all surged between the 2020 and 2021 harvests, with increases ranging from roughly 87 percent to more than 150 percent, and in every sampling period their contents followed the same hierarchy: mushroom residue first, peanut shells second, then optimized chemical fertilizer, and finally conventional practice.

To probe the chemistry behind these gains, the researchers turned to Fourier transform infrared spectroscopy, reading the molecular fingerprints of the humic fractions in the soil. The ratio of aliphatic carbon absorption peaks at 2920 and 2850 wavenumbers to the aromatic peak at 1630 wavenumbers serves as a gauge of how complex and aromatized organic matter has become. Across fulvic acid, humic acid, and humin, this ratio rose by anywhere from about 1.6 percent to nearly 57 percent in the amended treatments relative to conventional fertilization. In practical terms, the organic amendments increased the share of aliphatic, aromatic, and carbohydrate carbon while reducing carboxylic, phenolic, and alcohol functional groups, simplifying the molecular structure of the humic substances and lowering their degree of aromatization. Simpler, less aromatized humic material is generally more accessible to microbes, and mushroom residue outperformed peanut shells on this measure as well.

Soil enzymes, the catalytic workhorses of nutrient cycling, responded in kind. The researchers assayed six enzymes spanning the carbon, nitrogen, and phosphorus cycles: dehydrogenase, beta-glucosidase, invertase, urease, protease, and phosphatase. Both organic treatments elicited markedly higher enzyme activities than either chemical regime, with only a marginal advantage of optimized over conventional chemical fertilization. Between the two harvests, invertase activity rose by 12.72 to 59.34 percent, beta-glucosidase by 16.86 to 88.58 percent, and dehydrogenase by 11.31 to 15.76 percent depending on treatment. Mushroom residue produced the highest invertase and dehydrogenase activities, while peanut shells drove the strongest beta-glucosidase response. Protease and phosphatase followed similar upward trajectories under organic amendment, climbing roughly 15 percent between harvests.

Correlation analysis revealed a tightly coupled system beneath the surface. Every organic carbon fraction except water-soluble substances correlated positively and significantly with every enzyme except urease, and the strongest relationships linked urease activity to humin and total organic carbon, with correlation coefficients of at least 0.70. This finding carries a subtle but important implication: the recalcitrant humin fraction, which barely changed during the first year before accumulating steadily in the second, appears to serve as the stable carbon reservoir that sustains enzyme synthesis over the long term, rather than the fleeting pulse of water-soluble carbon released immediately after amendment.

The microbial census added another layer. High-throughput sequencing of bacterial 16S rRNA genes and fungal internal transcribed spacer regions showed no significant differences in alpha diversity among treatments, yet principal coordinates analysis separated the communities into two clear clusters: the organically amended plots on one side, the chemically fertilized plots on the other. Organic substitution, in other words, reshaped the community by shifting the relative abundances of dominant taxa rather than by adding species. Bacterial biomass carbon rose by 44.42 percent under peanut shells and 49.71 percent under mushroom residue compared with conventional fertilization, while microbial biomass nitrogen climbed even more steeply, by 79.94 and 85.24 percent respectively.

The taxonomic shifts read like a recruitment notice for beneficial organisms. The organically amended soils enriched Proteobacteria and Ascomycota, phyla associated with organic matter decomposition, nitrogen fixation, and plant growth promotion, while suppressing Firmicutes, Basidiomycota, and Olpidiomycota. At the genus level, the organic plots became strongholds of Pseudomonas, Sphingomonas, Streptomyces, Myceliophthora, and Trichoderma, well-known biocontrol and plant-growth-promoting taxa that were nearly absent under conventional chemical fertilization. Peanut shells preferentially favored Chaetomium, an efficient decomposer of straw, while mushroom residue recruited Myceliophthora, a thermophilic fungus with a powerful cellulolytic enzyme arsenal. Most strikingly, the conventional chemical treatment was strongly associated with Olpidium, a fungal genus flagged by the authors as an indicator of soil-borne disease risk and degraded root health. Its sharp decline under organic substitution, alongside the enrichment of beneficial biocontrol genera, suggests a microbial mechanism by which waste-derived amendments could suppress the pathogen build-up that defines continuous cropping obstacles.

Why did mushroom residue consistently outperform peanut shells? The authors attribute the gap to substrate chemistry. Mushroom cultivation has already partially mineralized the residue, leaving it with a lower carbon-to-nitrogen ratio and higher contents of readily available nitrogen, phosphorus, potassium, and trace elements. It also contains residual mycelium, extracellular enzymes, and partially decomposed lignocellulose that break down faster than the densely lignified, compressed peanut shell pellets. Both materials, however, avoid the pitfalls of the alternatives: unlike raw crop straw, they do not trigger temporary nitrogen immobilization, and unlike livestock manure, they carry lower risks of pathogen, heavy metal, or antibiotic contamination, an advantage particularly relevant for root vegetables grown in direct contact with soil.

The researchers are careful to frame the caveats. Two years is a short window for detecting slow-moving changes in carbon pools and microbial assemblages, the results come from a single site with one soil type under a specific climate, and the study did not measure how the soil improvements translate into yield or disease suppression. Longer-term trials at multiple locations, coupled with yield monitoring and disease surveys, will be needed to confirm the persistence of the effects. Still, the message from the Shandong carrot fields is clear and increasingly well supported across agricultural science: keeping total nutrient inputs constant while swapping a slice of the synthetic fertilizer bill for agricultural waste can expand the soil carbon pool, sharpen its biological availability, reawaken its enzymes, and tilt its microbial ecology back toward the beneficial. For farmers locked into monoculture cycles, the waste stream may be the cheapest soil medicine available.

Subject of Research: Effects of organic amendments substituting chemical fertilizers on soil biochemical properties and microbial communities in continuously cropped carrot systems

Article Title: Effects of peanut shells and mushroom residues substitution for chemical fertilizers on soil biochemical properties and microbial communities in continuously cropped carrot system

Article References: Zhu, L., Kong, Y., Guo, M., Li, Y., Zhang, P., Tang, W. H., & Chen, Y. (2026). Effects of peanut shells and mushroom residues substitution for chemical fertilizers on soil biochemical properties and microbial communities in continuously cropped carrot system. Journal of Agriculture and Food Research, 31, Article 103342. https://doi.org/10.1016/j.jafr.2026.103342

Image Credits: AI Generated

DOI: 10.1016/j.jafr.2026.103342

Keywords: soil organic carbon, continuous cropping obstacles, carrot monoculture, peanut shells, mushroom residue, soil enzymes, soil microbiome, organic fertilization, humic substances, high-throughput sequencing, soil fertility, sustainable agriculture