Nitrogen dynamics 35 years after stand-replacing fire: Aboveground pools grow as surface soil pools remain unchanged.

Natural disturbances induce variable recovery pathways, and long-term studies are key to understanding how that variability affects ecosystem function over time and across space. In subalpine forests, high-severity wildfires produce high variability in postfire tree density, but how this variability relates to nitrogen (N) cycling remains unclear. We resampled lodgepole pine (Pinus contorta var. latifolia) stands 35 years after stand-replacing fire and asked (1) how N concentrations, pools, and mineralization changed with increasing lodgepole pine biomass from 15 to 35 years postfire; (2) how N concentrations, pools, and rates of change varied across the gradient of early postfire stand density; and (3) whether among-stand structure and function were converging over time. Vegetation, forest floor litter, downed coarse wood, surface mineral soil (0-15 cm), and resin-sorbed N were measured in fourteen 0.25-ha plots in Yellowstone National Park (Wyoming, USA). From 25 to 35 years postfire, average total aboveground N pools increased from 252 to 405 kg N ha-1. Over the same 10-year period, soil N pools averaged 986 kg N ha-1 and did not change, resin-sorbed soil N remained low, and the only common symbiotic N-fixing plant in the plots (silvery lupine, Lupinus argenteus) remained sparse. Aboveground lodgepole pine biomass increased at similar rates from 15 to 25 and 25 to 35 years postfire (2.5 and 2.2 Mg ha-1 year-1, respectively) along with N pools (4.6 and 3.8 kg N ha-1 year1) and forest floor litter N (1.2 and 2.2 kg N ha-1 year1). Among stands 35 years postfire, total aboveground N varied threefold (212-638 kg N ha-1) and was positively correlated with lodgepole pine aboveground net primary productivity and biomass but unrelated to stem density. Soil N pools ranged from 570 to 1668 kg N ha-1 and were negatively correlated with lodgepole pine stem density but unrelated to tree productivity or biomass. Among-stand variation in stem density remained high but declined over time while N pools became less variable even as pools increased and variability in tree productivity increased from 25 to 35 years postfire. Rapid tree growth and biomass accumulation strongly regulate the N cycle in 35-year-old lodgepole pine stands, but sources of recovered N remain unresolved.