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In N-polluted, minimally managed soils, recent research has shown that ammonium-N, as well as nitrate-N, may move down through soil profiles. There is a need, therefore, to reassess what controls N species transformations below the rooting depth in such soils. The changes with depth down to 1 m in extractable ammonium-N and nitrate-N concentrations have been studied for two heavily N-impacted acid grassland soils near York, UK, for fresh soils and for soils incubated after either deionised water (d.w.) or ammonium-N spiking. Although in close proximity, the profiles showed marked differences in their relationships of ammonium- and nitrate-N to soil pH, C%, N% and C:N ratio. One profile was slightly more acidic at the surface, which seems to have changed the distribution of organic matter throughout the soil profile. Trends in ammonium production with depth were clearer when ammonium-N concentrations were recalculated on the basis of ammonium-N per unit mass of soil organic C. This then allowed a consistent trend with soil pH to be discerned for both profiles. Ammonium-N spiking showed that ammonium substrate availability was limiting potential net nitrification rate at 20–60 cm depth for both profiles. Potential mineral-N production was considerable at depth, which would facilitate transport of N to surface waters and/or groundwaters. [ABSTRACT FROM AUTHOR]