18. Overall approach

The surplus of nutrients is defined as the difference between total nutrient inputs by different sources and nutrient removal by crop harvest. In case of carbon, there is no uptake from soil since all carbon is derived from CO₂ in the air by photosynthesis. There we define the surplus as the input of carbon minus the CO₂-C emission due to decomposition of residue and manure input during the year of application.

The fate of the surplus is soil accumulation and soil losses to air (carbon and nitrogen) and to water (all nutrients). In principle there are also carbon losses to water but those losses are neglected compared to air emissions due to decomposition of SOC. A target surplus aims to bring (mostly increase) carbon or nutrient contents in soil to a desired value in view of soil health and crop production, while a critical surplus aims to reduce carbon or nutrient losses from soil below a critical value. .

A target surplus is calculated as the sum of a target soil accumulation, bringing soils up to a desired carbon or nutrient soil content in a given time period, and the losses (emissions or leaching) from soil at the desired soil content. When the soil has reached the desired carbon or nutrient soil content (an equilibrium after a defined time period), the target surplus only includes the losses (emissions or leaching) from soil at the desired soil content, which may differ from the current losses.. The target surpluses before and after equilibrium after equilibrium are both calculated. If a target soil content does not exist (in case of sulphur), the target surplus is set at the current surplus. A target input is set equal at a target uptake and a target surplus.

A critical surplus is calculated as the critical loss of carbon or nutrients to air and/or water. Those critical losses are only relevant for N, P and metals for which criteria exist with respect to air emissions (NH3) and dissolved concentrations (NO3, P and metals). The critical input is calculated as the target (or current) uptake and the critical surplus (loss). Unlike target surpluses and inputs, we neglect soil accumulation or release, in line with the calculation of critical loads. The reason for this assumption is that after a delay time, during which soils reach equilibrium, the calculated critical surplus leads to dissolved element concentrations that are just at the critical level.