Long-term nitrogen fertilization boosts soil carbon retention in Finnish boreal forest

New study from the Natural Resources Institute Finland (Luke) and European collaborators reveals that long-term nitrogen (N) fertilization can make boreal forest soils more resistant to carbon loss by changing how microbes respond to temperature and moisture. The findings suggest that N fertilized forests could store more carbon both in trees and soil compared to their N-limited counterparts, helping to mitigate climate change.

The study, led by Dr. Boris Ťupek from Luke, examined how decades of nitrogen fertilization have affected soil greenhouse gas (GHG) fluxes in a Scots pine (Pinus sylvestris) forest in central Finland. The international research team monitored carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) emissions over three growing seasons (2021–2023), alongside soil temperature, moisture, and organic carbon stocks.
 

Microbial respiration under N fertilized conditions

Soil microbes drive the decomposition of organic matter, releasing CO₂ as they respire. In this study, researchers modeled the microbes’ temperature and moisture responses and discovered that nitrogen altered these relationships.

In control soils, respiration peaked at around 15.8°C, while in fertilized soils, the peak shifted slightly higher to 16.8°C. Moreover, fertilized soils showed a weaker response to soil moisture changes — indicating that nitrogen availability made microbial respiration less sensitive to fluctuations in wetness or dryness.

“We found that nitrogen fertilization not only increased tree growth but also changed microbial response to soil water limitation,” explains Ťupek. “The microbes became less sensitive to changes in soil moisture, which on annual scale slowed down the overall release of soil carbon due to lower CO2 peaks after soil rewetting which were frequent during the summer after periods with low precipitation.”

Although nitrogen fertilization slightly reduced methane uptake and increased nitrous oxide emissions, the overall effect was a reduction in net soil CO2 emissions. Fertilized soils released around 10.3% of their stored carbon per year, compared to 12.2% in unfertilized plots — a difference that equates to 345 grams less CO2 per square meter per year on average.
 

Climate mitigation potential

Overall, the results suggest that long-term nitrogen fertilization enhances both tree biomass carbon storage and soil carbon retention, leading to a net decrease in greenhouse gas emissions from the forest ecosystem.

“Our findings show that long-term nitrogen addition can influence how boreal forest soils respond to warming and moisture changes,” says Ťupek. “In our case, this could mean more stable carbon storage and lower climate warming potential.”
 

An international collaboration

The research was led by Raisa Mäkipää and Aleksi Lehtonen and conducted in collaboration with Stockholm University, École Normale Supérieure (France), and the Czech Academy of Sciences as part of the EU Horizon 2020 HoliSoils project (Holistic management practices, modelling and monitoring for European forest soils) funded by the European Union’s Horizon 2020 program (Grant Agreement No. 101000289).