Advancing the theory of Progressive Nitrogen Limitation in boreal forests: consequences for the global carbon cycle

Boreal forest growth has increased over the past century; however, it remains uncertain whether growth will continue to increase.

It is hypothesized that higher CO2 may cause nitrogen (N) availability to progressively decline (PNL) over time, constraining growth, and potentially stimulating trees to invest more into acquiring N via mycorrhizal fungi. Some mycorrhizal taxa that dominate under severe N limitation acquire N by decomposing complex soil organic matter.

Thus, PNL may not only constrain tree growth, but it may also constrain carbon (C) accumulation in boreal soils.

Our research will: a) identify whether N availability is declining in different types of boreal forests (wet, mesic, and dry), and, b) determine the consequences of PNL for mycorrhizal communities, soil C quality, and C accumulation rates.

We will utilize samples archived by the Swedish National Forest and Soil Inventories to test whether coordinated shifts are occurring in forest N availability (by analyzing N isotopes), mycorrhizal communities, soil accumulation rates, and soil C quality.

Finally, we will utilize two long-term large-scale field experiments that manipulate CO2, temperature, N supply, and mycorrhizal exclusion, to provide proof that elevated CO2 and N limitation are primary drivers of the coordinated patterns found in inventory data.

This type of data is in critical demand by the scientific community in order to effectively model the global C cycle, and predict the pace of climate change.