Uncertainties and knowledge gaps in the effects of nitrogen fertilisation on tree growth, carbon sequestration, and environmental risks in boreal forest landscapes

Nitrogen fertilisation is a key strategy for enhancing tree growth and increasing carbon sequestration rates in boreal forests. In this review we evaluate the uncertainties of the effects of forest fertilisation for carbon sequestration and highlight important gaps in current knowledge about the efficiency and environmental consequences of nitrogen amendment. Uncertainties in the influence on carbon sequestration rates, along with variation in tree growth effects depending on soil type and climatic conditions, make scaling up of the benefits of increased fertilisation uncertain. Further, the potential benefits of increased forest fertilisation must be weighed against the risks of negative impacts on biodiversity, water quality, and tree physiology. Before expanding forest fertilisation, it is important to consider the balance of benefits, risks, and uncertainties. • Current understanding of N fertilisation effects in boreal forests is reviewed. • Soil carbon gains are short-lived under current forest management. • Tree physiological sensitivity to future climates needs further evaluation. • Higher N dose enhances tree growth but also aggravates environmental impact. • Potential climate benefits must be weighed against risks and uncertainties.