Nitrate affects wood development, not just growth

Nitrogen is a limiting nutrient in many Swedish forests, restricting tree growth and biomass production. Adding nitrogen can boost growth and increase the uptake of carbon dioxide, helping meet the rising demand for wood and wood-based products while contributing to climate-change mitigation. Yet excessive use of fertiliser can disturb ecosystems, highlighting the need for a better understanding of how nitrogen affects trees. 

“We know that nitrogen fertilisation can increase productivity, but we also know it can cause environmental problems, especially when over-used,” says Anna Renström, who conducted the research as part of her doctoral thesis in Hannele Tuominen’s group. “To develop sustainable strategies for improving wood production, we need a clearer picture of how nitrogen affects tree growth, and particularly wood formation.”

Nitrate affects both growth and wood structure

Anna Renström tested four different nitrogen sources on young hybrid aspen trees: ammonium, ammonium nitrate, which is commonly used in fertilisers, the amino acid arginine and nitrate. All of them stimulated growth in similar ways, but nitrate stood out by causing additional, specific changes in both wood structure and chemistry.

“Nitrate increased shoot biomass slightly more than the other nitrogen sources, and it led to larger wood cells and changes in lignin amount and composition,” explains Anna Renström. “Plants use nitrate not only as a nutrient but also as a signal. We believe that this signalling function is responsible for the specific effects we observed.”

Lignin is a key component of wood, influencing strength, rigidity and resistance to pests. Anna Renström and her colleagues found that all nitrogen sources except nitrate reduced the amount of lignin. All sources also changed the lignin composition but the trees responded to nitrate in a different way. Since both lignin amount and composition influence wood properties, these findings suggest that nitrogen fertilisation can change how wood functions in industrial applications such as  pulping and bioprocessing.

Important pieces for a more sustainable nitrogen use

To understand why nitrate has these effects, the researchers examined how it alters gene activity during wood development. Their analysis showed that nitrate triggers rapid responses in developing wood, promoting cell expansion in wood cells, including the water-transporting vessel cells. They also identified a key gene that could mediate this response, potentially providing an important link between nitrate’s molecular effects and the resulting changes in wood structure.

“Larger water-transporting cells can, for example, make trees more vulnerable to drought,” says Anna Renström. “With drought periods expected to become more frequent in the future, these changes may not always be beneficial. That is why it is important to understand how trees respond to nitrogen at the cellular and molecular level, and our findings add some of the missing pieces to that picture.” 

The extensive genetic dataset generated in this project is expected to continue shedding light on how nitrogen influences wood development. This knowledge forms the foundation for identifying traits that breeders could target when developing trees with improved nitrogen efficiency.