A 47-year field experiment on a calcareous chernozem soil in Hungary has revealed that nitrogen fertiliser acts as a powerful shield against carbon loss under arable farming while simultaneously slowing carbon recovery when land is converted to grassland, as published in the journal Plant and Soil.
The 1973 Starting Point at Nagyhörcsök Research Station
The long-term study began in autumn 1973 at the Nagyhörcsök Research Station of the HUN-REN Centre for Agricultural Research. Test plots were set up by scientists on calcareous chernozem land boasting an initial top 20-centimetre organic carbon level of 3.04 percent, alongside a cation exchange capacity of 28.3 meq per 100 grams and a water pH of 8.24. Calcium ammonium nitrate served as the source for four annual nitrogen rates administered to the plots—none, 100, 200, or 300 kilograms per hectare—divided equally between autumn and spring applications.
For the initial decades, the plots grew a rotating sequence of field crops under conventional ploughing, with stalks and residues ploughed back into the soil after harvest. Then, in autumn 2000, the entire experimental area was sown with a mixture of eight grass species. The sward was mown once or twice a year, and all above-ground biomass was removed.
Soil Sampling Milestones Across Four Decades
Soil monitoring was carried out by the investigation team at three critical checkpoints: after 17 years of farming in 1990, following 27 years of crop cultivation in 2000, and at the twenty-year mark of grassland oversight in 2020. Total soil organic carbon (SOC) was determined using the modified Walkley–Black method. Labile carbon, known as permanganate-oxidizable carbon (POXC), was measured by shaking one gram of soil with 20 millimolar potassium permanganate solution and reading absorbance at 550 nanometres.
During the cropping phase, the unfertilised control plots suffered severe degradation. SOC content fell significantly from 1.91 percent in 1990 to 1.74 percent in 2000, which amounted to an annual loss of 471 kilograms of carbon per hectare. Nitrogen fertiliser altered this trajectory. Across the fertilised plots, the yearly reduction in SOC stock slowed to a mere 41 kilograms per hectare annually, which represents a decline in depletion speed of approximately 90 percent. A dose of 100 kilograms of nitrogen per hectare was enough to essentially halt the decline.
Grassland Conversion and the Nitrogen Reversal
Following the 2000 conversion to grassland, the plots experienced a reversal of fortune. Twenty years of mown grassland lifted SOC content to an average of 2.24 percent. In the unfertilised plots, carbon accumulated at a rate of 425 kilograms per hectare per year.
Nitrogen fertilisation worked against soil recovery during this phase. At the 200-kilogram dose, the carbon sequestration rate fell to 164 kilograms per hectare per year. Even after twenty years under grass cultivation, the maximum organic carbon concentration found in the highest-performing plots reached only 2.30 percent, remaining well short of the 3.04 percent benchmark recorded in 1973.
Future Carbon Management Strategies for Chernozem Soils
The study concludes that land use remains the most effective carbon strategy, with nitrogen fertiliser playing a supporting role that requires careful tuning. For the world’s black soils, maintaining stable carbon levels depends on moderate fertilisation and leaving residues on the field during arable farming, followed by low-nutrient grassland management during restoration phases.
Frequently Asked Questions About the Hungarian Soil Study
How did nitrogen fertiliser affect plant biodiversity during the grassland phase?
Plant biodiversity dropped by roughly 20 percent under nitrogen treatment, which completely wiped out legumes everywhere except on the control plots and caused the vegetation to transition toward perennials that thrive on high nutrient levels.
What happened to the labile carbon fraction known as POXC?
When calculated as a mean across the three observation dates, permanganate-oxidizable carbon climbed from a baseline of 352 milligrams per kilogram in untreated controls to reach 399 milligrams per kilogram at maximum nitrogen application, though management type had a greater impact than nutrient inputs.
What depth did the researchers sample for their soil measurements?
The researchers sampled the topsoil at a depth of 0 to 20 centimetres, which captured the plough layer but missed deeper carbon dynamics.
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