Nitrogen and Phosphorus Cycling in an Ombrotrophic Bog: A Foundation for Understanding Ecosystem Responses to Warming

Abstract:

The large pool of carbon stored in peatland soils has accumulated in part because productivity outpaces decomposition under cold, wet, and acidic conditions. Slow decomposition and isolation from groundwater mean ombrotrophic bogs have tightly constrained cycles of nitrogen (N) and phosphorus (P). We used field observations of these limiting nutrients in a forested ombrotrophic bog to build N and P budgets under ambient conditions. We then used this framework as a foundation for understanding changes in the availability of soil inorganic N and P along a gradient of experimental warming (the SPRUCE experiment). Analysis of the nutrient budget shows that under ambient conditions, annual inputs of N and P to this ecosystem are balanced by annual losses. Stoichiometry of whole-plant biomass reveals that plant functional types differ in their N versus P limitation, with trees exhibiting a higher degree of N limitation than shrubs or Sphagnum moss. At this site, the vegetation is recovering from 1974 tree removal and Sphagnum makes up the largest pool of biomass N and P (53% and 43% respectively). Fluxes of N and P in annually produced biomass of trees, shrubs and Sphagnum are similar in scale to one another. With warming, inorganic N and P availability both increased significantly. The most pronounced increases in N and P were in surface peat after 3 years of warming, likely due in part to the loss of ecosystem engineer Sphagnum. However, warming consistently increased N and P availability below 30 cm peat depth, likely because roots of woody plants do not compete for nutrients in waterlogged soils. Inorganic N showed a stronger warming response than P, and our analysis suggests that this ombrotrophic bog system will be subject to profound shifts in ecosystem stoichiometry as it undergoes both real and simulated climate change.