Why Manure Alone Cannot Lock Carbon Into Greenhouse Soils

Every year, greenhouse farmers across the world spread enormous quantities of animal manure onto their soils, expecting the dark, carbon-rich material to build fertility that will last for decades. A new long-term study from northeast China suggests that this expectation is only half right. After tracking greenhouse plots that had received manure for anywhere between two and fifty years, researchers report that soil organic carbon did not keep climbing with each additional tonne of manure. Instead, the soils hit a ceiling, and the height of that ceiling depended less on how much carbon farmers added than on the texture of the soil itself.

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