Investigating the Effects of 20 Years of Nitrogen and Phosphorus Fertilization on Boreal Forest Plant and Soil Carbon Pools

The availability of nutrients, such as nitrogen (N) and phosphorus (P), limits plant growth and biomass accumulation in most ecosystems. Internal cycling of nutrients via microbial decomposition of soil organic matter (SOM) regulates the rate of nutrient resupply to plants, so carbon (C) and nutrient cycles are closely linked. This relationship is especially important in boreal forests where cold and wet soil conditions result in slow decomposition and the accumulation of a thick SOM layer. In this ecosystem, C and nutrient pools are influenced by wildfires which cause differential losses of N and P. While it is well established nutrients limit boreal forest plant growth, it is unknown if N or P is the primary limiting nutrient or if they co-limit growth, and how wildfire plays a role in these dynamics.

To investigate how time after wildfire disturbance impacts nutrient limitation in the Alaska boreal forest, we examined the effects of 20 years of N, P, and N & P fertilization on C and nutrient pools in plants and soils across three stages of post-fire succession. We assessed tree, shrub, and understory composition and biomass, and both total and plant available soil C and nutrient pools.

Preliminary analyses indicate that the relationship between nutrient fertilization and aboveground boreal forest plant growth, using DBH as a preliminary proxy, depended on post-fire succession stage. At the youngest site, fertilization did not have an effect on plant growth. At the intermediate age site, plant growth was limited by P only, whereas N and P in combination limited plant growth at the mature site. In contrast, soil organic layer depth, an initial proxy for belowground C pool size, increased the most in response to the combined fertilization treatment at all stages of post-fire succession. These initial results suggest that the effect of fertilization on aboveground plant growth will likely depend on time after wildfire; however, the effect of fertilization on soil carbon pools will be more constant across post-fire succession. Conclusions from our work will contribute to a better understanding of the relationship between nutrient cycling and carbon balance across post-wildfire successional trajectories by furthering the idea that plant and soil carbon pools may have different accumulation responses to nutrient fertilization.