The roots of trees are integrating organs over temporal variations reporting on various properties of the surrounding rhizosphere. In a field experiment the reactions of Norway spruce fine roots to wood ash and liquid fertilisation was investigated for 2 years. The liquid complete fertilisation contained 70–100 kg N and was meant, as with the wood ash (mainly Ca, K, Mg, P), to improve the nutritional balance of the trees. The nitrate reductase activity (NRA) of the fine roots was enhanced by the liquid fertilisation and through the ash treatment. A direct correlation between nitrate concentrations in the soil solution and the NRA in the fine roots could not be proven, but the NRA was significantly correlated with the additional N input on the liquid fertilised plots. The elevated NRA with the wood ash treatment could be caused by a change in the nitrate availability but also by other direct consequences of an upshift of the soil pH. The NRA as an indicator for N status on the ecosystem level and in the long term view is confirmed as a valuable additional parameter.