Biochar has long been promoted as a versatile soil amendment, but a new laboratory study from South Korea’s Jeju Island suggests that its usefulness for managing nitrogen depends heavily on how much organic matter the soil already contains. In a 90-day leaching-incubation experiment, researchers found that adding biochar derived from satsuma mandarin pruning branches reduced inorganic nitrogen availability in soils low in organic matter, while leaving nitrogen dynamics largely unchanged in soils rich in organic matter. The finding, published in Applied Biological Chemistry, helps explain why biochar studies across the world have produced such contradictory results.
The medium and high organic matter soils showed andic properties typical of volcanic ash soils, which are dominated on roughly 80 percent of Jeju Island. These soils are rich in allophane, ferrihydrite and aluminum-humus complexes, giving them high phosphorus retention and cation exchange capacity and, consequently, high fertilizer requirements.
The experimental design was deliberately factorial. Each soil received oil cake, an organic amendment applied at 200 kilograms of nitrogen per hectare, combined with biochar at four rates: 0, 0.2, 1 and 2 percent by weight. A soil-only control was included at each organic matter level, producing fifteen treatments with three replications each. The biochar itself was produced from pruned satsuma mandarin branches, a plentiful by-product on the island, where citrus pruning generates an estimated 27,000 tons of material annually. The branches were pyrolyzed anaerobically at 500 degrees Celsius for 30 minutes in an electric muffle furnace.
Incubation took place at 25 degrees Celsius with 60 percent water-filled pore space under aerobic conditions. After a one-week pre-incubation to stabilize microbial activity, the amendments were mixed in and the soils were leached at eleven time points over 90 days: days 0, 2, 4, 9, 16, 23, 34, 48, 62, 76 and 90. Each leaching event used 0.01 M calcium chloride solution followed by a nitrogen-free nutrient solution to replenish minerals lost during leaching, a procedure adapted from the classic Stanford and Smith method for quantifying temporal inorganic nitrogen production. Ammonium and nitrate in the leachate were measured spectrophotometrically, and in total 495 samples were analyzed.
To interpret the temporal data, the researchers fitted a first-order exponential kinetic model that separates an initial labile pool of mineral nitrogen, released immediately after amendment, from the potentially mineralizable nitrogen pool N0 and the mineralization rate constant k over days 2 to 90. The initial rewetting flush was excluded from curve fitting because air-dried soils release a transient spike of nitrogen upon rewetting that would distort the estimates. The model fit the data well, with adjusted coefficients of determination exceeding 0.95 in most cases.
The results showed a clear hierarchy of nitrogen mineralization potential among the untreated soils. In the controls, the highest N0 appeared in the high organic matter soil at 109.1 milligrams of nitrogen per kilogram, followed by the medium organic matter soil at 77.52 and the low organic matter soil at 49.23 milligrams per kilogram. The mineralization rate constant followed a similar pattern, with the high organic matter soil reaching 0.200 per day, roughly double the 0.107 per day observed in the low organic matter soil. The authors attribute these differences not to organic matter alone but to the combined influence of texture, clay content, cation exchange capacity, total nitrogen and bulk density, all of which co-vary across volcanic ash soils.
Adding oil cake with biochar raised N0 substantially above control levels in every soil, with the largest values reaching 247.2 milligrams per kilogram in the low organic matter soil, 231.0 in the medium soil and 324.5 in the high soil. But the biochar rate mattered. Across all three soils, the 2 percent biochar treatment consistently produced the lowest N0 among the amended treatments, and the mineralization rate constant in biochar-amended treatments remained stable and low, between 0.029 and 0.040 per day, several times lower than in the controls. The authors interpret this as evidence that biochar enhanced nitrogen immobilization and slowed mineralization, effectively acting as a slow-release carrier that moderates the pace at which organic nitrogen becomes plant-available.
The most striking result concerned inorganic nitrogen availability, measured as the sum of nitrate and ammonium nitrogen averaged across the incubation. In the low organic matter soil, biochar reduced inorganic nitrogen availability by 4.7 percent at the 0.2 percent rate, 8.0 percent at 1 percent and 12.9 percent at 2 percent, compared with oil cake alone. A similar but weaker trend appeared in the medium organic matter soil. In the high organic matter soil, however, biochar did not significantly decrease inorganic nitrogen availability at all. The researchers suggest that the abundant organic matter and the organo-mineral matrix of andic soils buffer nitrogen dynamics so strongly that additional biochar makes little difference, whereas low organic matter soils, with less native retention capacity, respond visibly to the added sorption and immobilization surfaces.
The temporal pattern of net nitrogen mineralization rates reinforced this picture. Mineralization proceeded in three phases: an early phase from 0 to 2 days, a mid-phase from 9 to 23 days, and a late phase from 23 to 90 days. The highest rates occurred early, with the unamended control soils significantly exceeding all amended treatments, peaking at roughly 10 milligrams of nitrogen per kilogram in the low organic matter soil, 12 in the medium soil and 26 in the high soil. Soils amended with oil cake and biochar showed markedly lower early mineralization, indicating strong immobilization or priming effects right after amendment. Biochar treatments then enhanced mineralization during the mid-phase, likely as microbes decomposed available substrates, before rates converged and stabilized in the late phase as mineralization and immobilization reached an equilibrium.
The practical stakes are considerable for Jeju, where groundwater supplies more than 90 percent of the island’s drinking water and intensive horticulture has created nitrogen pollution hotspots. Average nitrogen application rates for vegetable crops exceed 480 kilograms per hectare, with nearly 80 percent of applied nitrogen remaining as surplus, and monitoring between 2016 and 2018 found that 60.7 percent of sampled wells exceeded the safe drinking water limit of 10 milligrams of nitrate nitrogen per liter. Because nitrate is highly mobile and poorly retained by soil particles, any amendment that slows its production could reduce leaching. The authors caution, however, that their experiment was a standardized laboratory incubation, and they call for nitrogen-labeled biochar studies to separate amendment-derived nitrogen from native soil nitrogen, along with long-term incubations and field trials under rainfall-driven drainage to test whether the organic-matter-dependent response persists across seasons and management histories. If it does, the message for farmers is straightforward: biochar application should be tailored to soil properties, working hardest where soils are poorest in organic matter.
Subject of Research: Biochar and oil cake effects on nitrogen mineralization in volcanic ash soils of varying organic matter content
Article Title: Soil organic matter controls biochar–oil cake effects on inorganic nitrogen availability and mineralization kinetics in volcanic ash soils
Article References: Aung, S. S., Lee, H., Koo, B.-J., & Park, W.-P. (2026). Soil organic matter controls biochar–oil cake effects on inorganic nitrogen availability and mineralization kinetics in volcanic ash soils. Applied Biological Chemistry, 69(1), Article 58. https://doi.org/10.1186/s13765-026-01125-x
Image Credits: AI Generated
DOI: 10.1186/s13765-026-01125-x
Keywords: biochar, nitrogen mineralization, soil organic matter, volcanic ash soils, Andisols, oil cake, nitrate leaching, Jeju Island, incubation experiment, first-order kinetics, nitrogen use efficiency, soil amendment
