Mixing Mineral Fertilizer With Compost Cuts Net Warming From Korean Rice Paddies

A field experiment on a rice paddy in Miryang, South Korea, has found that combining conventional mineral fertilizer with composted animal manure produced the lowest net global warming impact of four fertilization regimes tested, even though the combined treatment emitted more methane and nitrous oxide than any single-fertilizer approach. The key, the researchers report, lies in accounting for the full carbon balance of the field rather than greenhouse gas emissions alone.

The study, conducted at the experimental farm of Pusan National University and published in Applied Biological Chemistry, compared a no-fertilizer control, conventional NPK fertilization, composted animal manure alone, and the combination of both. Rice of the dongjinchal cultivar was transplanted on June 9, 2023, and harvested on October 17, 2023. Plots received either 90 kilograms of nitrogen per hectare as urea with phosphorus and potassium, two megagrams per hectare of mixed livestock manure compost, or both amendments together, arranged in a randomized complete block design with three replications.

Weekly gas sampling with closed chambers throughout the year showed that cumulative methane emissions during the growing season were highest under the combined treatment at 108 kilograms per hectare, followed by compost alone at 64.5, mineral fertilizer at 46.6, and the control at 28.0 kilograms per hectare. Nitrous oxide emissions followed a similar pattern, peaking at 1.98 kilograms per hectare under combined fertilization, while carbon dioxide emissions, ranging from 712 to 1,220 kilograms per hectare, did not differ significantly among treatments. Taken at face value, these numbers suggest the combined treatment warms the climate most.

But the picture changed once the team calculated the net ecosystem carbon budget, which integrates all carbon entering and leaving the field: carbon fixed by the growing rice plants, carbon added in compost and urea, carbon lost through heterotrophic respiration, harvested biomass removal, and methane emissions. The combined treatment achieved the highest net ecosystem carbon budget because its carbon inputs from fertilization and plant production exceeded its carbon losses. Compost alone received a similar amount of carbon in fertilizer inputs, but its lower gross primary production meant a reduced carbon budget compared with the combined treatment.

Soil organic carbon measurements taken before the study in May 2023 and at harvest confirmed this pattern. Annual soil organic carbon change was negative under the control, mineral fertilizer, and compost treatments, but positive only where mineral fertilizer and compost were applied together. Carbon inputs ranged from 39 kilograms of carbon per hectare under mineral fertilizer to 438 under compost and 477 under the combined treatment, and the authors estimate that roughly 438 to 477 kilograms of carbon per hectare must be added annually to maintain soil carbon in this temperate soil, where organic matter decomposes rapidly.

When greenhouse gas emissions were weighted by their 100-year warming potentials, with methane 27 times and nitrous oxide 273 times that of carbon dioxide, and soil carbon sequestration was subtracted, the combined treatment showed the lowest average net global warming potential, although the difference was not statistically significant. The warming effect of its higher emissions was offset by greater carbon accumulation in the soil and ecosystem. The authors caution that their net ecosystem carbon budget estimates relied on a fixed ratio of net primary production to gross primary production of 0.61, derived from MODIS satellite products, which can vary with crop type and environmental conditions.

Water management emerged as a strong influence on emissions in its own right. Mid-season drainage for three weeks in July and August reduced soil water-filled pore space from 92 to 31 percent, sharply increasing carbon dioxide and nitrous oxide fluxes while suppressing methane. Emissions of all three gases were higher during the drainage period than during flooding, rising by 63.3 percent for carbon dioxide, 65.0 percent for nitrous oxide, and 58.8 percent for methane, the latter apparently sustained by aerenchyma development in rice plants at the tillering stage that enhanced methane transport from the anaerobic soil to the atmosphere.

Productivity also favored the combined approach. Rice grain yield was highest and greenhouse gas intensity, defined as net global warming potential per kilogram of grain, was lowest under combined fertilization, indicating a reduced climate impact per unit of rice produced. The authors note that differences in greenhouse gas intensity among the fertilized treatments were relatively small because higher emissions were generally accompanied by higher yields, and that the yield advantage cannot be explained by nutrient quantity alone, since nutrient inputs were not fully equalized among treatments. They attribute it instead to combined nutrient and soil-conditioning effects, with mineral fertilizer supplying readily available nutrients during peak demand and compost releasing nutrients gradually while improving nutrient retention and root conditions.

The central message of the study is methodological: evaluating fertilization practices on emissions alone can misrepresent their overall climate impact. An integrated assessment incorporating the net ecosystem carbon budget, net global warming potential, and greenhouse gas intensity provides a more complete framework for identifying fertilization strategies that sustain crop productivity while limiting warming. The authors emphasize that their findings come from a single site over one growing season, and that long-term, multi-site experiments across diverse paddy soils and different fertilizer combinations are needed to establish how generalizable the result is.

Subject of Research: Effects of combined inorganic and organic fertilization on greenhouse gas emissions, soil carbon, and net global warming potential in a South Korean rice paddy

Article Title: Combined application of inorganic and organic fertilizers reduces global warming potential from rice paddy soil

Article References: Kim, M. S., Lee, H. H., Lee, G. H., Chung, S. U., Kim, D. H., Jeon, S. H., Park, S. M., Kulesza, S., & Hong, C. O. (2026). Combined application of inorganic and organic fertilizers reduces global warming potential from rice paddy soil. Applied Biological Chemistry, 69(1), Article 81. https://doi.org/10.1186/s13765-026-01143-9

Image Credits: AI Generated

DOI: 10.1186/s13765-026-01143-9

Keywords: rice paddy, greenhouse gases, methane, nitrous oxide, soil organic carbon, compost, NPK fertilizer, net ecosystem carbon budget, global warming potential, greenhouse gas intensity, South Korea, carbon sequestration