According to the OECD-Eurostat methodology (2013), the nitrogen (N) budget in agriculture represents the difference between the input of N on agricultural land and the output of N from agricultural land with crop yields. We distinguish between gross and net nitrogen budget.

Gross nitrogen surplus is the difference between total input and output of nitrogen. Gross nitrogen input is represented by the following sources: N excreted by farm animals, N in mineral fertilizers, biological fixation of N by legumes, deposition of atmospheric N, N from other organic fertilizers (composts, digestates, sewage sludges ...) and N from seeds and planting material. Output represents nitrogen uptake of harvested crops.

The net nitrogen surplus refers mainly to the available N for crop growth (net input). Net nitrogen surplus is calculated by subtracting N that is released into the atmosphere (from animal houses, manure stores, during grazing, during and as a result of fertilising with mineral, animal and other organic fertilisers, during the decomposition of crop residues in soil and as a result of cultivation of histosols) from the gross nitrogen budget. This is N that is released into the atmosphere through emissions of ammonia (NH₃) nitrous oxide (N₂O) and nitrogen oxides (NO_X).

The gross and net nitrogen surpluses were expressed in tonnes N at the national level and in kilograms N per hectare of utilized agricultural area. Data for the period 1992–2019 are presented. The indicator also presents the structure of input and output of N and comparison with EU countries (EU-28).

The main sources of nitrogen input in agriculture are livestock manures (48%) and mineral fertilizers (39%). Atmospheric deposition contributes 9% of nitrogen input, biological fixation by legumes 3%, seeds and seedlings 0.4% and other organic fertilizers 0.2%. In the period 1992 and 2019, total nitrogen input decreased by 15%. The main reason is a 27% reduction in nitrogen consumption from mineral fertilizers. Nitrogen input from livestock manure decreased by 10%. Due to the reduced areas of utilized agricultural area, the input of atmospheric nitrogen decreased by 12% and the input by seeds and seedlings by 13%. Due to the increased extent of legume production, biological nitrogen fixation increased by 129%. Nitrogen input per hectare of utilized agricultural area decreased by 4%, mainly due to a 17% decrease in nitrogen input from mineral fertilizers per hectare of utilized agricultural area.

In the period 1992–2019, the annual nitrogen output by crops varied from 52 to 116 kg per hectare. The variation was mainly due to weather conditions, as nitrogen output was notably lower in dry years. Nitrogen output per hectare of utilized agricultural area increased by 46% during this period. The main source of nitrogen output was permanent grassland (64%), followed by cereals (18%) and forage crops (12%). Other crops (roots, tubers, fruits, vegetables, legumes and industrial plants) together contributed 5% of total nitrogen output.

In the period 1992–2019 the gross nitrogen surplus varied between 42 and 112 kg per hectare. The highest surpluses (more than 80 kg N/ha) were recorded in the dry years 1992, 1993, 2000, 2001 and 2003, when outputs with crop yields were reduced. The trend analysis clearly revealed that gross nitrogen surplus in the period 1995–2019 was decreasing by 1.6 kg per year or by 50% over the entire period, and net nitrogen surplus was decreasing by 1.5 kg per year or by 81%. The lower nitrogen surplus was mainly due to a 46% increase in nitrogen uptake by crops and a 4% decrease in nitrogen input per hectare of utilized agricultural area. Lower nitrogen budgets indicate better nitrogen management and, consequently, lower emissions of nitrogen compounds into the environment. While 51% of total nitrogen input was taken from agricultural land by crops in the period 1992–2003, the percentage increased to 67% in the period 2004–2019.

In the period 2006-2015, Slovenia had a slightly higher average gross nitrogen budget (54 kg N/ha) than EU Member States (50 kg N/ha). This is, among others, also a consequence of the large percentage of grassland in the structure of agricultural land and the associated livestock production, characterized by extensive emissions of nitrogen compounds into the air.