The study, published in the journal SOIL, examined greenhouse soils in two intensive vegetable-growing regions of Liaoning Province, where protected agriculture has flourished since the 1970s and 1980s. The team, led by Boyuan Tan of Shenyang Agricultural University, compared loam soils in one region with sandy loam soils in another. Both soil types had received roughly 41 tonnes of cow and pig manure compost per hectare every year, supplying about 11.6 tonnes of carbon per hectare annually, alongside standard chemical fertilizers and drip irrigation. By sampling plots of different ages, the researchers effectively compressed five decades of soil history into a single snapshot.

The headline finding is striking. In the loam, soil organic carbon content stabilized after about twenty years of manure application, settling between 33.1 and 37.0 grams of carbon per kilogram of soil, with a total stock of roughly 77.5 tonnes of carbon per hectare. In the sandy loam, equilibrium arrived almost immediately: within just two years, carbon content plateaued at 13.3 grams per kilogram, a stock of about 33 tonnes per hectare. Under identical manure inputs, the loam ended up storing two to three times more carbon than its sandier counterpart, and the gap widened rather than narrowed as the years accumulated.

Even more sobering was the fate of the carbon itself. The researchers calculated a sequestration rate, expressing how much of the cumulative manure carbon actually remained in the soil. In the sandy loam, nearly 55 percent of the added carbon was retained after two years, but that figure collapsed to just 4 percent after three decades. The loam fared better, holding a sequestration rate of 16.1 percent at the twenty-year mark, yet even there the rate steadily declined, falling to 7.2 percent after nearly fifty years. In other words, the warmer, more humid greenhouse environment, with temperatures often between 18 and 35 degrees Celsius and humidity around 65 to 80 percent, kept microbes digesting organic matter so vigorously that most of the applied carbon simply returned to the atmosphere as carbon dioxide.

To understand why the two soils behaved so differently, the team dissected soil organic carbon into its component pools. Labile fractions, such as easily oxidizable carbon, dissolved organic carbon, and microbial biomass carbon, turn over quickly and respond to management within seasons. Particulate organic carbon, split into coarse and fine fractions, represents partially decomposed plant and manure residues. Mineral-associated organic carbon, by contrast, is the gold standard of soil carbon storage: organic molecules bound to clay and silt particles, protected from decomposition for years or even centuries. The loam held significantly higher amounts of easily oxidizable carbon, particulate carbon, and mineral-associated carbon than the sandy loam, and its mineral-associated pool kept growing for decades, peaking at 21.83 grams per kilogram after nearly fifty years of manure application.

The sandy loam told a very different story. Its mineral-associated carbon plateaued after only four years, hovering between 6.21 and 7.54 grams per kilogram, and its fine particulate carbon actually declined over time, dropping from a peak of 3.08 grams per kilogram at two years to 1.81 grams per kilogram by year thirty. Correlation analysis revealed why: in the loam, total soil organic carbon was strongly and positively linked to every major fraction, suggesting a cooperative pipeline in which labile and particulate carbon were progressively converted into stable mineral-bound forms. In the sandy loam, carbon was positively correlated only with easily oxidizable and mineral-associated carbon, and negatively with fine particulate carbon, hinting that fresh particulate inputs may even have stimulated the loss of existing stable carbon through priming effects.

The molecular details came from solid-state carbon-13 nuclear magnetic resonance spectroscopy, a technique that reveals the chemical architecture of soil organic matter. In the loam, long-term manure application enriched aromatic and carbonyl carbon, functional groups that resist microbial attack, and raised the aromaticity index by nearly 250 percent compared with unfertilized control fields, an overall gain of about 55 percent in carbon stability. The sandy loam moved in the opposite chemical direction: alkyl carbon increased while aromatic carbon declined, shifting the soil toward hydrophobic, waxy compounds rather than genuinely recalcitrant ones. Structural equation modeling then showed that in the loam, particulate carbon fed the mineral-associated pool, whereas in the sandy loam that pathway was essentially broken, leaving easily oxidizable carbon as the only meaningful precursor of stable carbon.

These results illuminate two distinct stabilization mechanisms operating in the same climate and under the same management. Fine-textured loams, rich in clay and silt, offer abundant mineral surfaces that adsorb organic molecules and shield them from microbes, allowing even metabolically active carbon fractions to contribute to long-term storage. Coarse sandy loams, with limited binding sites, can only protect carbon through hydrophobicity, a weaker defense that depends on the continuous replenishment of fresh labile inputs. The authors conclude that the long-term sequestration rate is not determined by the quantity of external inputs alone but by the efficiency with which labile fractions are transformed into mineral-associated organic carbon, an efficiency that is fundamentally governed by soil composition and mineral properties.

For greenhouse growers, the practical message is that manure is not a one-size-fits-all solution. On sandy soils, piling on more manure year after year yields diminishing returns, because the soil simply cannot hold what it is given. Matching application rates to soil texture, and investing in practices that build mineral protection capacity, may achieve more than sheer volume. The study also carries a caution for carbon accounting: greenhouse agriculture, with its accelerated decomposition, may store far less carbon than open-field experiments suggest, meaning that climate benefits attributed to manure application in protected cultivation could be systematically overestimated. The authors note that future work combining long-term experiments with microbial sequencing and mineralogical analysis will be needed to fully untangle how soil texture, mineral chemistry, and microbial communities jointly decide whether applied carbon becomes a lasting asset or a fleeting guest.

Subject of Research: Long-term soil organic carbon accumulation and stabilization under manure application in greenhouse loam and sandy loam soils

Article Title: Differences in organic carbon fractions and stability explain limited accumulation in loam and sandy loam under greenhouse conditions

Article References: Tan, B., Yang, L., Xiao, X., Li, C., Tan, J., An, N., Meng, L., Yi, Y., Qian, F., Li, N., Liu, X., Li, S., & Han, W. (2026). Differences in organic carbon fractions and stability explain limited accumulation in loam and sandy loam under greenhouse conditions. SOIL, 12(2), 855-869. https://doi.org/10.5194/soil-12-855-2026

Image Credits: AI Generated

DOI: 10.5194/soil-12-855-2026

Keywords: soil organic carbon, greenhouse agriculture, manure application, mineral-associated organic carbon, particulate organic carbon, soil texture, carbon sequestration, 13C NMR spectroscopy, loam, sandy loam, soil fertility, carbon stabilization

The research, published in the open-access journal Discover Soil, examined how sweet pepper (Capsicum annuum L.) responded to biochar at three rates: no biochar as a control, 11.2 megagrams per hectare, and 22.4 megagrams per hectare. Two contrasting soils were tested side by side in a greenhouse pot experiment: a Pacolet sandy loam from the Rodale Institute Southeast Organic Center in Georgia, and a Clymer loam from the Pocono Organic Center in Pennsylvania, a regenerative organic certified site. Both soils came from long-term organically managed fields, giving the team a realistic starting point for evaluating how biochar behaves in systems that already prioritize soil health.

The design was deliberately rigorous. Soils were air-dried, adjusted to a uniform moisture content, and repacked into 18.9-liter pots at a standardized bulk density, with biochar incorporated into the top six centimeters. The biochar itself was a wood-derived product with a strongly alkaline pH of 9.5, an organic carbon content of 83.8 percent, and an enormous surface area of 456 square meters per gram, properties that explain why it can simultaneously alter soil chemistry and provide habitat for microbes. After a stabilization period, young pepper seedlings of the cultivar Sprinter Organic were transplanted, two per pot, and grown through a full season under ambient conditions.

The yield results revealed a striking asymmetry between the two soils. Sandy loam plants outperformed loam plants overall, producing an average of 282 grams of fruit per plant compared with just 146 grams in loam. More importantly, the response to biochar diverged sharply. In sandy loam, yield climbed above 300 grams per plant at the moderate rate of 11.2 megagrams per hectare but plateaued, and even dipped slightly, at the higher dose. In loam, by contrast, the highest rate was transformative: yields that languished below 100 grams per plant in untreated soil surged to roughly 240 grams per plant at 22.4 megagrams per hectare. The pattern supports a growing consensus that biochar delivers its largest gains in lower-performing, less fertile soils.

Plant growth told a subtler story about timing. Early in the season, biochar-amended plants were actually shorter than controls, a transient effect the authors attribute to the alkaline amendment temporarily reshuffling nutrient availability. As the season progressed, those differences faded in sandy loam, while in loam the biochar-treated plants remained consistently taller throughout the growing period. This buffering effect, driven by the release of basic cations such as calcium, potassium, magnesium, and sodium, appears to have been immediately useful in the slightly acidic, lower-fertility loam but less consequential in the already more productive sandy loam.

Beneath the surface, the microbial picture was more complicated. Using phospholipid fatty acid analysis, a technique that quantifies living microbial communities by measuring membrane molecules, the team found that sandy loam harbored roughly twice the microbial biomass of loam: 6,845 versus 3,737 nanograms per gram of soil. Bacteria, fungi, and arbuscular mycorrhizal fungi all followed the same pattern. Biochar itself did not significantly alter these measures, but the trend at the highest application rate was telling: total microbial biomass fell about 17 percent, from 5,628 to 4,682 nanograms per gram, with similar declines across bacterial, fungal, and mycorrhizal markers.

The mycorrhizal fungi proved especially sensitive to dose. Root colonization by these symbiotic fungi, which help plants scavenge phosphorus and other nutrients, peaked at around 45 to 48 percent at the moderate biochar rate, then dropped substantially at the highest rate. The researchers suggest that excessive biochar can push soil pH beyond optimal thresholds, adsorb the chemical signals fungi need to establish symbiosis, or immobilize nutrients within its porous matrix. Intriguingly, reduced mycorrhizal colonization did not translate into reduced yield in loam, implying that when soil nitrogen and phosphorus availability are maintained by the amendment itself, plants may become less dependent on their fungal partners.

Perhaps the most conceptually important finding concerns soil organic carbon. Biochar did raise soil organic carbon levels, as expected, but carbon accumulation bore little relationship to microbial biomass. In sandy loam, the relationship was weak or absent. In loam, it was significantly negative: as soil organic carbon rose from 1.8 to 5.0 percent, microbial biomass fell from roughly 4,200 to 2,800 nanograms per gram. The explanation lies in the chemistry of biochar itself. Its carbon is largely aromatic and recalcitrant, resistant to microbial decomposition, so increases in total soil organic carbon do not necessarily mean increases in the labile carbon that fuels microbial activity. Biochar can even adsorb labile organic compounds, a phenomenon known as negative priming that reduces the food supply for soil microbes.

This decoupling of carbon stocks from biological activity carries real implications for how carbon sequestration is evaluated. A soil can accumulate impressive quantities of organic carbon while its living microbial community stagnates or shrinks, meaning that headline carbon numbers alone are an incomplete measure of soil health. The study also found that fruit nutritional quality was essentially untouched by biochar: nitrogen, phosphorus, and potassium concentrations in harvested peppers remained stable across all treatments, suggesting that yield gains may reflect dilution effects rather than genuine nutritional improvement.

Depth added one final layer of nuance. Microbial biomass peaked not at the surface, where biochar was applied, but in the six-to-twelve-centimeter layer, declining again below that, and treatment effects weakened progressively with depth. The authors are candid about the limitations of their work: it was a single-season pot study with repacked soils, and longer-term field trials across a wider range of soil types are needed to confirm the findings. Still, the practical takeaway is clear. For regenerative organic vegetable systems, the moderate rate of 11.2 megagrams per hectare offered the best balance between productivity and microbial health across both soils, while the highest rate should be reserved for situations where maximizing yield in low-fertility soil is the overriding goal. Biochar, in other words, works best as a precision tool matched to the soil at hand, not as a one-size-fits-all dose.

Subject of Research: Effects of biochar application rate and soil type on sweet pepper yield, soil organic carbon, and microbial communities in regenerative organic systems

Article Title: Effects of soil type and biochar rate on sweet pepper productivity and microbial dynamics in regenerative organic soils

Article References: Effects of soil type and biochar rate on sweet pepper productivity and microbial dynamics in regenerative organic soils. (n.d.). https://doi.org/10.1007/s44378-026-00297-8

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00297-8

Keywords: biochar, soil health, sweet pepper, regenerative agriculture, soil organic carbon, microbial biomass, arbuscular mycorrhizal fungi, PLFA analysis, sandy loam, loam, carbon sequestration, organic farming