A network of 18 wildfire chronosequences reveals key drivers of the boreal nitrogen balance

Ecosystem productivity and carbon uptake in the circumpolar boreal forest are contingent on available nitrogen, which ultimately originates from inputs via deposition and biological nitrogen fixation. Nitrogen deposition rates in boreal forests are relatively small compared to other biomes, and most biological nitrogen fixation research has focused on moss-diazotroph associations. However, the relative contributions of these two primary nitrogen inputs to ecosystem nitrogen stocks have not been widely investigated. In this study, we combined a mass balance approach and literature synthesis to estimate rates of nitrogen accumulation and nitrogen inputs across a network of 18 wildfire chronosequences spanning the boreal biome. We found that nitrogen accumulation rates were strongly linked with fire regime (stand-replacing versus surface fires) and canopy dominance (deciduous versus evergreen canopies). Furthermore, a considerable amount of accumulating nitrogen in these boreal forests was unexplained by the known inputs estimated from the literature synthesis, particularly in forests with stand-replacing fire regimes and more deciduous tree cover that together had the highest nitrogen accumulation rates. This unexplained fraction of nitrogen inputs in some forests may originate from poorly quantified niches of biological nitrogen fixation. Exploring this research frontier will help improve predictions of boreal forest nitrogen cycling and carbon uptake in changing climate and wildfire regimes.